Difference between revisions of "Programming the Ethernet controller"

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== Interface ==
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The modules involved in communication with the Ethernet Controller chip (EC) serve as the core of the FPGA. The tasks that need to be performed by these modules include
 +
* executing the complex board reset and address lookup sequence
 +
* polling for new packets and switching execution accordingly
 +
* collecting sensor information upon a status report request
 +
* programming the DAC upon a program packet receipt
 +
* building return packets
  
:''See also: [[Ethernet packets]]''
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In  our design, the modules involved with packet handling and interfacing with sensor chip controllers are organized by ''state''. A state register specifies the current stage of the process and only the corresponding module is allowed to act during that stage. Aside from defining a process sequence, the state serves as a complex "enable" signal for the modules, ensuring that only one module is driving the communication bus used to query the EC.
  
== The eight states ==
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= Controller =
  
There are eight major states to the module.
+
== Foreword on Timing ==
  
=== State list ===
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With a controller this complex, the timing of signals must be inspected even more scrupulously than usual. Simultaneity of the rising edge of the "Done" signal and the corresponding output data may not be guaranteed. This is solved by either
 +
# a one-cycle-delayed "Done" signal to ensure that the data lines have been stabilized. (All delays are implemented via the <tt>c_delay</tt> module which postpones the signal by one cycle via two sequential flip-flops, each shifting by half-cycle). 
 +
# coding the target component to read the data lines on the ''falling edge'' of the clock during the a "Go" signal.
 +
The latter method turned out to be the dominant one in this design.
  
; (000) Reset Cycle
+
== The eight states ==
: The reset cycle resets the selected chips.  There are flags for resetting all 4 chips: DAC, ADC, temperature sensor, Ethernet controller.  This is the entry state from a power-on.  From this state, proceed unconditionally to 001.
 
; (001) Transmit "I"
 
: This state transmits an "I" packet to acknowledge that a reset occurred but the board is now prepared to accept new packets and function normally. From this state, proceed unconditionally to 010.
 
; (010) Idle
 
: This is the idle state where the state machine awaits a new packet's arrival.  If the Receive FIFO is empty, it loops back on itself and continues checking the FIFO.  If the FIFO is not empty, proceed to 011.
 
; (011) Read Packet
 
: This state accesses the first packet.  If packets need to be filtered by the FPGA, this state will filter and return to 010 on a bad packet.  The MAC data is discarded, as it is unimportant to the FPGA, and the first data byte is read.  This byte is used as a switch: an ASCII "R" loops to 000.  A "Q" proceeds to 100.  A "P" proceeds to 110.  Any other value is a bad packet and the state machine returns to 010.
 
; (100) Poll Status
 
: This state polls the status chips (the ADC and the temperature sensor) so that recent data is ready for transmission.
 
; (101) Transmit "S"
 
: This state packages and transmits an "S" packet over the Ethernet to report back the status of the board in response to a "Q" packet.
 
; (110) Program DAC
 
: This state programs the DAC with new values according to the mask and data in the "P" packet.
 
; (111) Transmit "D"
 
: This state packages and transmits a "D" packet over the Ethernet to report back the DAC voltages in response to a "P" packet.
 
  
=== State interconnect ===
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=== State-Module organization ===
  
These states will form the outline of the functional block diagram.  Within each state will be a smaller process or set of processes, possibly broken into substates.  Additionally we will have a central ''state'' register.  Each block reads the state value in the register and enables itself upon seeing its own value. After completion of its function, a block will write a new value to the state register to enable the next block.
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The core of the the FPGA is divided roughly into eight modules enabled by by the 3-bit state value. Below is an index of the states and their corresponding modules. In this discussion of states, 'X' is a binary wild card the values are immediately explained.
  
=== State variable ===
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{| class="wikitable" border="1" align="center" style="text-align:center" cellspacing="0" cellpadding="4"
 +
|+ State-Module Index
 +
|-
 +
! State !! Module Name !! Description !! Succeeding State
 +
|-
 +
| align="center" | 000 || [[FPGA_Reset|Reset]]_hard || align="left" |Coordinates the reset and start-up of the EC. || 100
 +
|-
 +
| align="center" | 001 || [[FPGA_Reset|Reset]]_soft || align="left" | Extends the reset to the PC-requested chips and records PC's MAC for later communication. || 100
 +
|-
 +
| align="center" | 010 || [[FPGA_Idler|Idler]] || align="left" | This is the active module during the FPGA's default idle state. It awaits the "Receive FIFO buffer not empty" interrupt and passes control to the Reader || 011
 +
|-
 +
| align="center" | 011 || [Packet] [[FPGA_Reader|Reader]] || align="left" | Skips the packet header and reads the first two bytes ("location" and "type") of the packet payload. It rejects misdirected or invalid-type bytes. Control is passed according to packet type to Query, Program or Reset Modules || 100, 110, 00X
 +
|-
 +
| align="center" | 100 || [[FPGA_Querier|Querier]] || align="left" | Queries the values of the Temperature sensor and ADC, stores them in their respective registers and passes control to the Transmitter for delivery || 101 
 +
|-
 +
| align="center" | 110 || [DAC] [[FPGA_Programmer|Programmer]] || align="left" | Programs the DAC based on instructions in packet and stores the values in the DAC register. || 111
 +
|-
 +
| align="center" | 1X1 || [Packet] [[FPGA_Transmitter|Transmitter]] || align="left" | Composes and sends a packet of either [[Ethernet_packets|'S' or 'D' type]]. These correspond respectively to "'''S'''tatus" values reported by the sensor chips (state=101) and current '''D'''AC values (state=111) || 010
 +
|-
 +
|}
  
Based on the assignments of state values to the various states, certain interpretations of the state's bits arise.  They may or may not prove useful in coding the system.  They are given here in the case that they do become useful.
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=== State interconnect ===
* S<sub>2:1</sub>:
 
** S<sub>2</sub> = 0: "Core cycle" that executes regularly and forms the spine of the state diagram.
 
*** S<sub>2:1</sub> = 00: "Initialization cycle" that prepares the state diagram for normal functioning.
 
*** S<sub>2:1</sub> = 01: "Standard cycle" that the state machine returns to most frequently to process appropriate switching to branches.
 
** S<sub>2</sub> = 1: "Branch cycles" that execute selectively based on received packets.
 
*** S<sub>2:1</sub> = 10: "Query cycle" as described in the page on [[Ethernet_packets#The_query_cycle|Ethernet packets]].
 
*** S<sub>2:1</sub> = 11: "Programming cycle" as described in the page on [[Ethernet_packets#The_programming_cycle|Ethernet packets]].
 
* S<sub>0</sub>:
 
** S<sub>0</sub> = 0: "Setup state" to prepare internal workings for a communication.
 
** S<sub>0</sub> = 1: "Transfer state" to communicate with the external PC.
 
  
== Emulator ==
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As described above, these states form the outline of the functional block diagram. This implementation calls for a central ''state register''.  Each block reads the state value in the register and enables itself upon seeing its own value.  After completion of its function, a block will write a new value to the state register to enable the next block. With several modules writing to the register, usual precautions must be taken to avoid more than one drivers forcing a line simultaneously. All modules must be designed to go to high impedance on their output lines when they are not active.
  
== Controller ==
 
  
=== Non-State Components ===
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== Miscellaneous non-state-based components ==
  
* '''transceiver'''
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Please refer to the individual design detail pages for:  
** This block provides a level of abstraction between the precise CP2200/1 interface and a generalized interface seen by the internal blocks of the FPGA.  It takes information on the next transfer to carry out and returns information on the last transfer completed.  It has a pulse signal to begin a transfer and a pulse signal to notify of a completed transfer.  This eliminates a need for the internal workings of the FPGA to be aware of the timing of the interface; it simply begins a transfer and waits for notification of the transfer's completion.
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* [[FPGA_Registers|Registers]]
** inputs
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* [[FPGA_Reusables|Miscellaneous Reusable Components]]
*** ''CLK'': clock
 
*** ''/Rst'': asynchronous, active-low reset
 
*** ''Go'': pulse to begin a transmission
 
*** ''R/W_in'': read/write toggle: active-high read, active-low write
 
*** ''A_in'': 8-bit bus for address to read to/write from
 
*** ''D_in'': 8-bit bus for data to write; ignored during a read
 
** outputs to internals
 
*** ''Done'': pulse to signal completion of a transmission
 
*** ''R/W_out'': read/write flag: active-high read, active-low write
 
*** ''A_out'': 8-bit bus for address of last read/write
 
*** ''D_out'': 8-bit bus for data of last read; internal systems should ignore for a write
 
** outputs to CP2200/1
 
*** ''/CS'': active-low chip select
 
*** ''MotEn'': Motorola/Intel format toggle: active-high Motorola, active-low Intel
 
*** ''MuxEn'': Multiplexed flag; not used for CP2201
 
*** ''ALE'': ALE strobe
 
*** ''/Wr'': Active-low write flag
 
*** ''/Rd'': Active-low read flag
 
** inouts
 
*** ''AD'': 8-bit address and data bus
 
* '''state register'''
 
** A three-bit register to store the current state.
 
** inputs
 
*** ''Clk'': clock
 
*** ''/Rst'': asynchronous, active-low reset to zero the register (puts system into reset state)
 
*** ''En'': write enable
 
*** ''D'': three-bit data-in bus
 
** outputs
 
*** ''Q'': three-bit data-out bus
 
* '''temperature register'''
 
** A ten-bit register to store the most recent temperature data.
 
** inputs
 
*** ''Clk'': clock
 
*** ''/Rst'': asynchronous, active-low reset to zero the register (puts system into reset state)
 
*** ''En'': write enable
 
*** ''D'': ten-bit data-in bus
 
** outputs
 
*** ''Q'': ten-bit data-out bus
 
* '''ADC registers'''
 
** A set of eight 12-bit registers to store the most recent ADC data.  Also includes a demultiplexer to select which register to write to.
 
** inputs
 
*** ''Clk'': clock
 
*** ''/Rst'': asynchronous, active-low reset to zero the register (puts system into reset state)
 
*** ''En'': write enable
 
*** ''S'': 3-bit select bus
 
*** ''D'': 12-bit data-in bus
 
** outputs
 
*** ''Q'': 12-bit data-out bus; outputs data of register chosen by S bus
 
  
=== (000) Reset Cycle ===
 
  
Block 000 will have four functional blocks: one each for the DAC, ADC, and Ethernet controller, and one to coordinate their completion.  The temperature sensor lacks an external reset function; it self-initializes on startup.  The "R" packet will supply flags as to whether or not to enable the various blocks.  A power-on reset will default to resetting all components.  Using the enable flags like a mask on the done lines, the fourth functional block will update the state register.  For information on the reset procedures, see [[Reset and Initialization]].
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== Interface ==
 
 
inputs
 
* ''Clk'': clock
 
* ''/Rst'': asynchronous, active-low reset
 
* ''State'': 3-bit state value
 
* ''D_En'': DAC enable, assume held high/low by previous block
 
* ''A_En'': ADC enable, assume held high/low by previous block
 
* ''E_En'': Ethernet enable, assume held high/low by previous block
 
  
internal signals
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[[Image:MuxIntelTiming.png|frame|Multiplexed Intel Bus and internal I/O timing scheme enforced by FPGA clocks]]
* ''S_En'': state enable, ''S_En'' <= not (''St(2)'' or ''St(1)'' or ''St(0)'')
 
* ''Go'': when ''S_En'' goes high ''Go'' pulses for one cycle
 
* ''D_Go'': DAC reset go pulse, ''D_Go'' <= ''Go'' and ''D_En''
 
* ''A_Go'': ADC reset go pulse, ''A_Go'' <= ''Go'' and ''A_En''
 
* ''E_Go'': Ethernet reset go pulse, ''E_Go'' <= ''Go'' and ''E_En''
 
  
blocks
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The compact approach to wire and poll the EC is via the Multiplexed Intel bus format. Aside from some use of the interrupt and reset pins, all communication is done via this bus. (The control architecture of the chip uses 8-bit registers selected with 8-bit addresses.) A communication over this bus begins with an pulse on the "ALE" pin, by the fall of which a valid address is expected on the 8-bit inout "AD" bus. After this /RD (/WR) signal falls, executing the read (write) process, and then rises accompanied with valid input from (to) the chip. The adjacent figure illustrates this process along with the approach to timing the conversation taken in this design. Please refer to Section 16.2 in the manual for details on the timing tolerances on this bus.
* '''DAC Reset'''
 
** Resets and initializes the DAC.
 
** inputs
 
*** ''Clk'': clock
 
*** ''/Rst'': asynchronous, active-low reset
 
*** ''D_Go'': go pulse to begin reset/initialization process
 
** outputs
 
*** - All DAC reset/initialization control lines -
 
*** ''D_Done'': goes high when reset/initialization process is complete, falls on ''D_Go'' pulse
 
* '''ADC Reset'''
 
** Resets and initializes the ADC.
 
** inputs
 
*** ''Clk'': clock
 
*** ''/Rst'': asynchronous, active-low reset
 
*** ''A_Go'': go pulse to begin reset/initialization process
 
** outputs
 
*** - All ADC reset/initialization control lines -
 
*** ''A_Done'': goes high when reset/initialization process is complete, falls on ''A_Go'' pulse
 
* '''Ethernet Reset'''
 
** Resets and initializes the Ethernet controller.
 
** inputs
 
*** ''Clk'': clock
 
*** ''/Rst'': asynchronous, active-low reset
 
*** ''E_Go'': go pulse to begin reset/initialization process
 
** outputs
 
*** - All Ethernet reset/initialization control lines -
 
*** ''E_Done'': goes high when reset/initialization process is complete, falls on ''E_Go'' pulse
 
* '''Coordinator'''
 
** Coordinates the completion of each reset cycle and notifies other blocks that the reset process is complete.
 
** inputs
 
*** ''Clk'': clock
 
*** ''/Rst'': asynchronous, active-low reset
 
*** ''D_En'': high when DAC is to be reset
 
*** ''D_Done'': high when DAC is done resetting
 
*** ''A_En'': high when ADC is to be reset
 
*** ''A_Done'': high when ADC is done resetting
 
*** ''E_En'': high when Ethernet controller is to be reset
 
*** ''E_Done'': high when Ethernet controller is done resetting
 
** internal signals
 
*** ''Flag'' <= (''D_Done'' or not ''D_En'') and (''A_Done'' or not ''A_En'') and (''E_Done'' or not ''E_En'')
 
** outputs
 
*** ''Done'': when ''Flag'' goes high, ''Done'' pulses for one cycle; connects to state register as an enable
 
*** ''New_St'': new state to be written to the state register; goes to 001 while ''Done'' is high
 
  
=== (001) Transmit "I" ===
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As shown in the figure, the 20&nbsp;MHz clock is very convenient for metering this conversation. The communication delay for the FPGA (most of which runs essentially on 5&nbsp;MHz) turns out to be only two clock cycles.
  
This is a simple state. It loads to the CP2200/1 a packet containing an ASCII "I" in the first byte and padding (any value) in all remaining bytes (minimum size of data is 46 bytes, so there needs to be 45 bytes of padding).  After the transmission is complete, the block writes a 010 to the state register.
+
This communication standard calls for a bridge module that communicates with the EC upon request from other modules. A "[[FPGA_Transceiver|Transceiver]]" was designed for this purpose. It abstracts the communication with the EC as well as the clock frequency difference. This module in fact subdivides the main 20&nbsp;MHz; clock to generate the "slow" 5&nbsp;MHz clock for the rest of the FPGA. Please refer to the [[FPGA_Transceiver|detailed page]] on the Transceiver for more information.
  
=== (010) Idle ===
 
  
Block 010 continuously polls the interrupt registers on the CP2200/1 until the Receive FIFO Empty flag comes back as a zero.  On this condition it transitions to state 011.
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== Combined control flow ==
  
inputs:
+
[[Image:OperationCourse.png|frame|Operation course between the digital board and the controller PC]]
* ''Clk'': clock
 
* ''/Rst'': asynchronous, active-low reset
 
* ''State'': 3-bit state value
 
* ''TxRx_A'': 8-bit address from transceiver
 
* ''TxRx_D'': 8-bit data from transceiver
 
* ''TxRx_Done'': pulse from transceiver to signal transfer complete
 
 
 
internal signals
 
* ''S_En'': state enable, ''S_En'' <= not ''St(2)'' and ''St(1)'' and not ''St(0)''
 
* ''Go'': when ''S_En'' goes high ''Go'' pulses for one cycle
 
 
 
blocks
 
* '''Fetcher'''
 
** Reads register 0x76 (INT0RD) by sending a read request to the transceiver
 
** inputs
 
*** ''F_Go'': pulse to start request, ''F_Go'' <= ''Go'' or ''Loop''
 
** outputs
 
*** ''TxRx_Go'': ''Go'' line on transceiver
 
*** ''TxRx_R/W'': ''R/W'' line on transceiver, tied high (read)
 
*** ''TxRx_A'': ''A_in'' bus on transceiver, tied to 0x76 (0111 0110)
 
**** Note: ''D_in'' on transceiver is not used; assume this line defaults to zero
 
* '''Looper'''
 
** Switch to determine if this state should loop on itself or continue to the next state.
 
** inputs
 
*** ''S_En'': state enable
 
*** ''TxRx_Done'': ''Done'' pulse from transceiver
 
*** ''TxRx_Data'': ''D_out'' bus from transceiver
 
** outputs
 
*** ''Loop'': pulse to repeat fetch cycle; ''Loop'' <= ''S_En'' and ''TxRx_Done'' and ''TxRx_Data(6)''
 
*** ''Done'': pulse to finish state; connects to state counter as an enable in order to increment the state to 011; ''Done'' <= ''S_En'' and ''TxRx_Done'' and not ''TxRx_Data(6)''
 
 
 
=== (011) Read Packet ===
 
 
 
Block 011 is the first block with switching between following states.  Its first task is to load the MAC data of the packet.  If filtering inside of the FPGA is required, this block is responsible for it.  The only MAC filtering likely to be needed is a check to see if the destination MAC address matches the MAC address of the CP2200/1, but the CP2200/1 should filter this itself (verify in the data sheet).  If the packet is bad, this block tells the CP2200/1 to skip the packet and returns to block 010.  The MAC filter would be the first substate.
 
 
 
After MAC filtering comes the second substate (or the only substate if MAC filtering is found to be unnecessary).  This reads the first byte of the packet's data and uses it as a switch to determine the transition to the next state:
 
{| align="center" cellpadding="4" border="0" cellspacing="0" style="text-align:center"
 
! ASCII
 
|  
 
! Next State
 
|-
 
| R ||      || 000
 
|-
 
| Q ||      || 100
 
|-
 
| P ||      || 110
 
|-
 
| other ||      || 010
 
|}
 
Anything other than the three defined packets is considered a bad packet and is discarded in the CP2200/1 before transitioning back to state 010.  An "R" packet obtains the reset mask from the second byte then discards the packet before transitioning to state 000.  State 000 assumes that the reset mask is held, so this output needs to be latched in place until the next time this block activates.  A query has no switches or masks on it, so a "Q" packet will be discarded from the CP2200/1 before continuing on.  A "P" packet has a large amount of data attached to it, so that is the only packet not discarded before continuing to the next state.  This allows later states to access the packet data, but also puts on those later states the responsibility for discarding the packet once finished with it.
 
  
=== (100) Poll Status ===
+
Conceptually, the operation course must proceed as outlined in the adjacent figure. The main concern in the tagger control is maintaining a map between board/channel addresses and actual energy bins. For this purpose, the diagrammed two-stage reset plan was devised in the course of which the FPGA learns the PC's MAC address and the PC builds a MAC-Location lookup table. (The "Location" is an 8-byte slot identifier which allows the PC to pinpoint the SiPM channel group.)
  
This block will have at least two functional blocks: one to poll the temperature sensor and one to poll the ADC.  Its job is to update all status values from the status chips in preparation for transmission.  Once all status values have been updated it transitions to state 101.
+
The internal FPGA operation course that supports this scheme (and the general control board functionality requirements) is outlined below.
  
Currently we assume that data values will be stored in the FPGA.  If data values will be stored on CP2200/1 Flash memory or other storage device, the interfaces and blocks will have to be adjusted appropriately.
+
[[Image:DigBoardScheme.png|center]]
  
inputs
 
* ''Clk'': clock
 
* ''/Rst'': asynchronous, active-low reset
 
* ''State'': 3-bit state value
 
  
internal signals
+
= Emulator =
* ''S_En'': state enable, ''S_En'' <= not (''St(2)'' or ''St(1)'' or ''St(0)'')
 
* ''Go'': when ''S_En'' goes high ''Go'' pulses for one cycle
 
  
blocks
+
Since the modules described here represent the core of the FPGA, their combined simulation calls for inclusion of all other fringe modules.  Essentially the whole FPGA will have to be tested to ensure this scheme for the core is acting properly. The challenge is in the complexity of the stimulus for such simulation: whole packets will need to be sent in and possibly inter-packet Ethernet Controller Chip states tested.
* '''ADC Poll'''
 
** Updates values stored in the FPGA from the ADC.
 
** inputs
 
*** ''Clk'': clock
 
*** ''/Rst'': asynchronous, active-low reset
 
*** ''Go'': go pulse to begin
 
** outputs
 
*** - All ADC control lines -
 
*** ''A_Done'': goes high when reset/initialization process is complete, falls on ''Go'' pulse
 
*** ''En'': enable line for writing to the internal registers
 
*** ''Sel'': 3-bit select bus to specify which ADC channel data is available
 
*** ''Data'': 12-bit data bus to carry data to FPGA internal registers
 
* '''Temp Poll'''
 
** Updates value stored in the FPGA from the temperature sensor.
 
** inputs
 
*** ''Clk'': clock
 
*** ''/Rst'': asynchronous, active-low reset
 
*** ''Go'': go pulse to begin
 
** outputs
 
*** - All temperature sensor control lines -
 
*** ''En'': enable line for writing to the internal register
 
*** ''Data'': 10-bit data bus to carry data to FPGA internal registers
 
* '''Coordinator'''
 
** Coordinates the completion of each polling cycle and notifies other blocks that the polling process is complete.
 
** inputs
 
*** ''Clk'': clock
 
*** ''/Rst'': asynchronous, active-low reset
 
*** ''A_Done'': high when ADC is done polling
 
** outputs
 
*** ''Done'': when ''A_Done'' goes high, ''Done'' pulses for one cycle; connects to state register as an enable
 
**: Note that the temperature sensor does not signal completion.  That is because the temperature sensor need only update one value, while the ADC must update eight values.  Thus it is known ahead of time that the temperature sensor will already be done by the time the ADC is done.
 
*** ''New_St'': new state to be written to the state register; goes to 101 while ''Done'' is high
 
  
=== (101) Transmit "S" ===
+
A stripped down emulator for the Ethernet Controller has been written. It is a essentially a set of registers with a Multiplexed Intel bus communication layer and packet file read/write layers. These registers, however, are not passive memory banks but include "events" that are triggered by particular register states. For instance writing to registers designated to make up a receive buffer pointer actually delivers the requested byte from the buffer to the appropriate control register to be available for a subsequent request. A simple interrupt system (stimulated externally by the simulation layer) has also been included.
  
This block compiles the status values into a single packet by loading them into the CP2200/1 in a defined order and format, including padding/converting any values that need it.  Once the packet has been sent, the block transitions to state 010.
 
  
=== (110) Program DAC ===
+
= See Also =
  
This block will have a substate to obtain the programming mask.  It then loops 32 (or 24 or 16) times on a second substate that obtains the next programming value and, if the corresponding mask bit is high, programs that channel of the DAC.  A mux may be needed to select the appropriate bit from the programming mask.  It also updates the locally stored DAC channel values (which may be stored on the FPGA, the CP2200/1 Flash, or other) in preparation for a "D" response packet.  Then it transitions to state 111.
+
* State Modules
 +
** [[FPGA_Reset|Reset]]
 +
** [[FPGA_Idler|Idler]]
 +
** [[FPGA_Reader|Reader]]
 +
** [[FPGA_Querier|Querier]]
 +
** [[FPGA_Programmer|Programmer]]
 +
** [[FPGA_Transmitter|Trasmitter]]
  
=== (111) Transmit "D" ===
+
* Non-state Modules
 +
** [[FPGA_Transceiver|Transceiver]]
 +
** [[FPGA_Interrupt_Catcher|Interrupt Catcher]]
 +
** [[FPGA_Registers|Registers]]
  
This block loads a "D" to the transmit buffer then loops 32 (or 24 or 16) times to load the locally stored DAC channel values to the transmit buffer.  Once the full packet has been loaded, it sends the packet, then transitions to state 010.
+
* [[Ethernet_packets|Ethernet Packet formatting]]

Latest revision as of 06:33, 5 November 2009

The modules involved in communication with the Ethernet Controller chip (EC) serve as the core of the FPGA. The tasks that need to be performed by these modules include

  • executing the complex board reset and address lookup sequence
  • polling for new packets and switching execution accordingly
  • collecting sensor information upon a status report request
  • programming the DAC upon a program packet receipt
  • building return packets

In our design, the modules involved with packet handling and interfacing with sensor chip controllers are organized by state. A state register specifies the current stage of the process and only the corresponding module is allowed to act during that stage. Aside from defining a process sequence, the state serves as a complex "enable" signal for the modules, ensuring that only one module is driving the communication bus used to query the EC.

Controller

Foreword on Timing

With a controller this complex, the timing of signals must be inspected even more scrupulously than usual. Simultaneity of the rising edge of the "Done" signal and the corresponding output data may not be guaranteed. This is solved by either

  1. a one-cycle-delayed "Done" signal to ensure that the data lines have been stabilized. (All delays are implemented via the c_delay module which postpones the signal by one cycle via two sequential flip-flops, each shifting by half-cycle).
  2. coding the target component to read the data lines on the falling edge of the clock during the a "Go" signal.

The latter method turned out to be the dominant one in this design.

The eight states

State-Module organization

The core of the the FPGA is divided roughly into eight modules enabled by by the 3-bit state value. Below is an index of the states and their corresponding modules. In this discussion of states, 'X' is a binary wild card the values are immediately explained.

State-Module Index
State Module Name Description Succeeding State
000 Reset_hard Coordinates the reset and start-up of the EC. 100
001 Reset_soft Extends the reset to the PC-requested chips and records PC's MAC for later communication. 100
010 Idler This is the active module during the FPGA's default idle state. It awaits the "Receive FIFO buffer not empty" interrupt and passes control to the Reader 011
011 [Packet] Reader Skips the packet header and reads the first two bytes ("location" and "type") of the packet payload. It rejects misdirected or invalid-type bytes. Control is passed according to packet type to Query, Program or Reset Modules 100, 110, 00X
100 Querier Queries the values of the Temperature sensor and ADC, stores them in their respective registers and passes control to the Transmitter for delivery 101
110 [DAC] Programmer Programs the DAC based on instructions in packet and stores the values in the DAC register. 111
1X1 [Packet] Transmitter Composes and sends a packet of either 'S' or 'D' type. These correspond respectively to "Status" values reported by the sensor chips (state=101) and current DAC values (state=111) 010

State interconnect

As described above, these states form the outline of the functional block diagram. This implementation calls for a central state register. Each block reads the state value in the register and enables itself upon seeing its own value. After completion of its function, a block will write a new value to the state register to enable the next block. With several modules writing to the register, usual precautions must be taken to avoid more than one drivers forcing a line simultaneously. All modules must be designed to go to high impedance on their output lines when they are not active.


Miscellaneous non-state-based components

Please refer to the individual design detail pages for:


Interface

Multiplexed Intel Bus and internal I/O timing scheme enforced by FPGA clocks

The compact approach to wire and poll the EC is via the Multiplexed Intel bus format. Aside from some use of the interrupt and reset pins, all communication is done via this bus. (The control architecture of the chip uses 8-bit registers selected with 8-bit addresses.) A communication over this bus begins with an pulse on the "ALE" pin, by the fall of which a valid address is expected on the 8-bit inout "AD" bus. After this /RD (/WR) signal falls, executing the read (write) process, and then rises accompanied with valid input from (to) the chip. The adjacent figure illustrates this process along with the approach to timing the conversation taken in this design. Please refer to Section 16.2 in the manual for details on the timing tolerances on this bus.

As shown in the figure, the 20 MHz clock is very convenient for metering this conversation. The communication delay for the FPGA (most of which runs essentially on 5 MHz) turns out to be only two clock cycles.

This communication standard calls for a bridge module that communicates with the EC upon request from other modules. A "Transceiver" was designed for this purpose. It abstracts the communication with the EC as well as the clock frequency difference. This module in fact subdivides the main 20 MHz; clock to generate the "slow" 5 MHz clock for the rest of the FPGA. Please refer to the detailed page on the Transceiver for more information.


Combined control flow

Operation course between the digital board and the controller PC

Conceptually, the operation course must proceed as outlined in the adjacent figure. The main concern in the tagger control is maintaining a map between board/channel addresses and actual energy bins. For this purpose, the diagrammed two-stage reset plan was devised in the course of which the FPGA learns the PC's MAC address and the PC builds a MAC-Location lookup table. (The "Location" is an 8-byte slot identifier which allows the PC to pinpoint the SiPM channel group.)

The internal FPGA operation course that supports this scheme (and the general control board functionality requirements) is outlined below.

DigBoardScheme.png


Emulator

Since the modules described here represent the core of the FPGA, their combined simulation calls for inclusion of all other fringe modules. Essentially the whole FPGA will have to be tested to ensure this scheme for the core is acting properly. The challenge is in the complexity of the stimulus for such simulation: whole packets will need to be sent in and possibly inter-packet Ethernet Controller Chip states tested.

A stripped down emulator for the Ethernet Controller has been written. It is a essentially a set of registers with a Multiplexed Intel bus communication layer and packet file read/write layers. These registers, however, are not passive memory banks but include "events" that are triggered by particular register states. For instance writing to registers designated to make up a receive buffer pointer actually delivers the requested byte from the buffer to the appropriate control register to be available for a subsequent request. A simple interrupt system (stimulated externally by the simulation layer) has also been included.


See Also