Teapot Project

100Mbps Ethernet wrapper for hardware accelerators.

This project creates an ethernet interface for a generic hardware accelerator applications and handles ethernet frame streamin and out of input and output data.

Ethernet packets

This wrapper workes with layer 2 ethernet packets, opperating at the level of the ethernet frame. It support two types of packets:

• application packets whos payload is passed to the accelerator
• configuration packets used to set the ASICs MAC address, Vlan IDentifier, and the TX phase selection

All packets whos destination MAC do not match the ASIC's current MAC address will be filtered out. Unless otherwise specified the ASIC's MAC address is 00:90:CF:00:BE:EF (read as Nortel:BEEF).

Application packets

The application packet payload is streamed to the accelerator though the application wrapper. In the case of this design, the accelerator is a custom bfloat16 multiplier, so the payload will contain the two 16 bit values A and B we wish to multiply.

Request

[ dst mac (6 Bytes) ][ src mac (6 Bytes) ][ ethtype = 0x88B5 (2 Bytes) ][ A (2 Bytes) ][ B (2 Bytes) ][ padding (42 Bytes) ][ FCS (4 Bytes) ] 
0

The result of the accelerator's computation, here C=A*B, will be contained in the payload of the response packet addressed to the src mac of the original request packet.

Response

[ dst mac (6 Bytes) ][ src mac (6 Bytes) ][ ethtype = 0x88B5 (2 Bytes) ][ C (2 Bytes) ][ padding (44 Bytes) ][ FCS (4 Bytes) ] 
0

Response packet streamout will start as soon as the last bits of B are received, for area reasons, the ASIC will not check for corrupted FCS or if the ethernet frame length complies with the 802.3 spec.

Unless otherwise configured application packets use ethertype 0x88B5, the first IEEE 802.3 specified "Local Experimental Ethertype".

Configuration packets

Configuration packets are used to set the ASIC's current:

• MAC address
• Vlan ID
• TX data to clock phase offset

These packets are not forwarded to the accelerator, do not provide any acknoledgement and due to our area limitation are not store and forwarded. Any corrupted packets will result in a corrupted configuration.

Packet:

[ dst mac (6 Bytes) ][ src mac (6 Bytes) ][ ethtype = 0x88B6 (2 Bytes) ][ New MAC (6 Bytes) ][ padding (3 bits)][ VID (12 bits)][ padding (38 Bytes) ][ FCS (4 Bytes) ]
0

Unless otherwise configured application packets use ethertype 0x88B6, the second IEEE 802.3 specified "Local Experimental Ethertype", whos existence linux networking libraries are unaware of. So trust me: it's real.

Configuration parameters

MAC address

ASIC's current MAC address, all packets not addressed (dst mac) to this address will be filtered out, and all responses will use this as the source address.

Default MAC: 00:90:CF:00:BE:EF (read as Nortel:BEEF)

VLAN ID

In the event a packet is vlan tagged, packets not matching the VLAN ID will be filtered out. If a packet isn't vlan tagged, it is assumed to belong to our current VLAN.

Default VLAN ID: 0xDAD

Configuration pins

TX data to reference clock phase offset

To comphensate for the output data to reference clock offset induced by the delay on the path from the clock input pin, to the tiny tapeout design's data out flip-flop and back to the output pin, the reference clock for the data out flip flop is selectable, allowing us to use a 180 degree dephased reference clock.

This dephasing configuration is captured during reset depending on the state of the tx_phase pin.

Values:

• 0 no phase shift
• 1 180 degree phase shift

Assumptions

Desgin will be using the official microchip LAN8720A daughter board for testing.

This ASIC will be supporting 100Mbps ethernet using the following assumptions:

• PHY will be implemented used the microchip LAN8720A(I) chips
• External network is full duplex - dropping CRS support in asic
• No 10Mbps support
  • dissabling auto-negociation
• LAN8720A clk is provided by an external clocks that is shared with the ASIC.
• Stream from LAN8720A is assumed gapeless, any packet gaps will trigger the ongoing packet to be tagged as containing an error

MAC behavior

This MAC supports:

• jumbo frames upto 9000 bytes
• VLAN tagging (802.21Q)

The MAC will filter out all unicast packets not matching the configured destination MAC address. This implies that all broadcast and multicast packets will be forwarded.

Improvements

List of potenciel future improvements :

• 10Mbps support with dynamic switching between 100Mbps and 10Mbps
• Adding VLAN tagging to responses from VLAN tagged packets
• Make the wrapper easy to import into a third party project
• Add perf counters and expose said counters over JTAG
• Management data to JTAG bridge ? (MDIO/MDC)

Coffee-shop Chip family

This ASIC is part of a larger family of open-source Ethernet connected IP featuring:

• coffeepot first generation switch.
• teapot Ethernet wrapper for building network connected accelerators (this repo).
• coldbrew Ethernet connected beacon for broadcasting an ethernet frame with an uptime count until the heat death of the universe (this repo).

Credits

Thanks to the Tiny Tapeout project, its contributors, and all the community working on open source silicon tools for making this possible.