Ethernet packets

This page details the packets we will send across our internal network to define the standards of communication between the digital boards and the controlling PC. Both the FPGA designer and the PC software programmer will need to make reference to this page in order to coordinate their respective designs.

Transmitting
To the right is a diagram depicting the structure of an Ethernet packet. On the left side is marked the blocks that must be defined by the FPGA to be passed to the CP2200/1. The first 8 bytes ("Preamble and Start Frame Delimiter") and last 4 bytes ("CRC") will be generated by the CP2200/1, so the FPGA need not even be aware of them. The first significant block is the 6-byte destination MAC address. The FPGA will have to store this to know the address of its conversation partnet. The next block is the 6-byte source MAC address. Each CP2200/1 comes with a factory-set unique MAC address stored in the last page of Flash memory that will be used as the source MAC address. This address must be retrieved at start-up, inserted in the MAC interface of the CP2200/1 as well as stored by the FPGA for packet addressing. The next block is the 2-byte length block. This is the number of bytes of data, i.e. everything between the length block and the CRC. Then finally comes the data block, which must be padded to a minimum of 46 bytes but cannot exceed 1,500 bytes. In practice, out packets are padded to 47 bytes of payload: the letter of the CP2200/1 manual seems to indicate although this is likely a mistake. (Their likely error reduces to the fact that an N-member array with first index 0 ends with index N-1, not N.)

Receiving
The CP2200/1 appears to give access to the entire 64-byte (minimum) packet, which would include everything on the above diagram except for the "Preamble and Start Frame Delimiter". The CP2200/1 can be programmed to filter out any packets not addressed to it, but it only verifies the first 5 bytes of the MAC address, which is useless, given that our system would likely be instrumented from a consecutive factory batch of controllers, perhaps differing only in the last octet of their MAC address. Manual filtering is necessary to create a proper filter. For good, standards-compliant packets the FPGA can strip away and ignore the first 14 bytes (e.g. it does not need the length field, as we define our own standard for the packets below, using the first two data bytes to specify intended physical address and packet type.) From there the FPGA can parse the data field according to the specifications outlined below.

Packets from the PC to the FPGA
We will use six types of packets in our communications, paired into three "conversations" or "cycles": a reset cycle, a query cycle, a programming cycle. Each packet's data section will begin with a two-byte code to identify the relevant board and packet type. As a mnemonic, these bytes will use ASCII codes to represent a single-letter shorthand for each packet.

The reset cycle - "R" packets


The reset cycle is a conversation whose purpose is to reset the digital control board. On each power-on, the various chips on the digital board need to be re-initialized. This includes the Ethernet chip itself, so the reset functionality needs to be built into the FPGA logic by default and needs to execute on start-up with no external stimulus in order to obtain Ethernet control. However it may also be necessary to instigate a reset externally for some reason. This cycle allows the external PC to initiate a reset and will notify the PC when the system is fully initialized.

Additionally, the reset cycle gives the tagger boards and PC a chance to build the proper address maps. The boards acquire the MAC address of the PC and the PC builds a MAC-Location table, where the "Location" is a hard-coded 8-bit slot identifier hard-coded into the bus-board. This organization step is necessary for the PC to be able to pinpoint SiPM channel groups.

The "R group" of packets sent from the PC to the card to initiate a reset process in various forms. Two possible reset packets (corresponding to two degrees of reset) are possible
 * Full or "Hard" reset: This resets all chips on the board. This will contain no data as no further instructions can be remembered after a board reset.
 * Selective or "Soft" reset: This will have flags to reset the ADC and the DAC. A Soft Reset command also finalizes a Hard Reset cycle, as it allows the control board to learn the PC's MAC address. (Subsequent unicast communication makes for a cleaner system compared to an all-broadcast scheme)

R-packet - hard reset
The packet type byte (2nd in packet payload) will be an ASCII R: 0x52, 0101 0010. This packet orders control passed to the Reset_hard module which resets the Ethernet Controller chip (EC), loads its own MAC address and transmits an S-packet to confirm completion of this stage and allow the PC to pair the source MAC address with the included slot Location address (always in the 1st byte of payload).

R'-packet - soft reset
The packet type byte (2nd in packet payload) will be a 0xD2, 1101 0010 - essentially an ASCII R with the MSB flipped to '1'. This packet orders a more customized "soft" reset. After the mandatory Location and Packet Type bytes, this packet carries the 6-byte PC MAC address (to allow the FPGA to record the address of its conversation partner) as well as a reset mask for the on-board chips. Currently the two least significant bits of this byte order reset of the ADC (bit 1) and DAC (bit 0). Another S-packet is returned after this stage. As with the S-packet at the end of the hard reset stage, this packet is meant for notification. An explicit query request (Q-packet) is recommended for reliable voltage values on board. (Certainly the validity of the highest ADC channel 7 is not guaranteed after an ADC reset request in the R'-packet)

The query cycle - "Q" packet
The query cycle is a conversation regarding the status of the digital board. It polls the sensor devices and reports back their most recent data.

The Q-packet is sent from the PC to the FPGA to request a status report from its sensor chips. There is no data attached to a query packet: the FPGA triggers the query module upon recognizing a Q-type packet, which is identified as ASCII Q: 0x51, 0101 0001 in the packet type field (2nd byte of packet payload). The boards reports back in response to the query via the S-packet.

The programming cycle - "P" Packet
The programming cycle is a conversation intended to set the values of the DAC channels. It sends programming data to the board and receives confirmation of the programming in the form of a D-packet

The identifying (second) byte of this packet's payload is ASCII P: 0x50, 0101 0000. The next four bytes form a programming mask. Any channel that is to be reprogrammed will have a 1 in the corresponding location, and any channel that is to be left alone will have a 0 in the corresponding location. The MSB of the first byte refers to channel 0 and the LSB of the fourth byte will be channel 31. Thus, if only channels 5 through 17 are to be programmed, the packet would contain: Following this are 64 bytes of programming data listing the values for channels 0-31 (in that order) in 2-byte words. Each channel needs 14 bits, so the format is two leading zeros and 6 MSB of data is the first byte, then 8 LSB of data in the second byte. The nominal transfer function of the DAC chip is:

$$V=50 V_{ref}\frac{D}{2^{14}}$$

where D is the 14-bit binary representation of the voltage value and Vref=3.3V in the current board design, yielding a 0-165V range, assuming the recommended 10V+Vmax=175V power supply is used.

Place holders for all channels must be present in the packet, but only those marked in the mask will be programmed; all other bytes will be ignored and can take on any value. Regardless of the mask, the word corresponding to the last channel must be followed but another byte as padding. The packet format is shown in detail in the adjacent figure.

"S" packet: status report
This is the packet sent from the control board to the PC to report on the current status of the board. The first two bytes of the packet will be the usual Location Stamp byte and the packet identifier: an ASCII S: 0x53, 0101 0011. After that will come the status data. This totals 20 bytes: one for Location, one for Packet Type, two of temperature, sixteen of voltages. In an effort to pad the packet to the minimum size required by Ethernet standards, the temperature and ADC data values in the S-packet may be repeated again in the same sequence.
 * The first two bytes of data will be the temperature. The temperature sensor returns 10 bits of data. The first byte will contain six leading zeros, then the two MSB of data.  The second byte will contain the 8 LSB of data.
 * The next 16 bytes of data will be the ADC channels. The 12-bit values returned by the ADC are padded into 2-byte words.  The first byte for each channel will have four leading zeros, then the 4 MSB of the data.  The second byte for each channel will contain the 8 LSB of the data.  The channels will be reported from channel seven down to channel zero.

"D" packet: DAC setup complete
This is the packet sent from the FPGA to the PC to confirm that the DAC has been programmed to specifications. The first byte of this packet is an ASCII D: 0x44, 0100 0100. The next 64 bytes are the values of the DAC channels printed in 2-byte words. As before, the format is two leading zeros and 6 MSB of data in the first byte and 8 LSB of data in the second byte. The channel values are listed from 0 to 31. This confirms to the PC that the data was programmed according to specification and helps synchronize the control board and the PC. All channels are reported back, not just those that were reprogrammed during this conversation. Thus PC software needing to rebuild its records of voltage values can issue a program (P) packet with the mask bits set to zero to read back the SiPM bias voltages from the control board.

Note that the DAC chip has a write-only interface: state of particular channels cannot be queried. The returned voltages are those stored by the FPGA for programming. The values reported are the nominal programmed values simply confirming that the correct values were received. However, the returned values do reflect the corrections made by the FPGA in the values that would have exceeded the voltage limits.

For real feedback, the "DAC health channel" may be used instead. The current control board hardware design specifies that DAC channel 31 be used as a DAC-health monitoring level. The line is connected with ADC channel 7. Assignment and reading back of values on this channel will be of great help in calibrating the DAC chip.

Channel 18 also has a special allocation: it controls the gain switch on the amplifier board.