BIP AbstractJTAGProtocol

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  BIP: ?
  Title: Abstract JTAG Protocol
  Author: Luke Dashjr <luke+bipmhbp@dashjr.org>
  Status: Draft
  Type: Standards Track
  Created: 14-07-2012

Specification

This protocol is designed to be spoken only over a reliable (including error correction) lower-level protocol, such as USB-Serial or USB-CDC, to a local device.

Packet Format

• 4 bytes: magic: always fd414a50
• 1 byte: packet length/type
• N bytes: packet data
• 2 bytes: packet data BSD "sum1" checksum

Packet length/types:

• 00 -- Abort
• 01 to ef -- Data packet length (1 to 239 bytes)
• f0 to fc -- Reserved
• fd -- Bad packet (command aborted)
• fe -- ACK
• ff -- End of command

ACK should be sent in response to each 239-byte packet. Bulk data transfers, such as bitstream uploads, should wait until ACK is received before sending more packets.

Checksum must include the complete packet up until the checksum itself; this is not used to prevent corruption (USB already guarantees that), but rather to avoid acting when something foreign sends stuff (possibly intermixed with a real AJP packet). If the checksum does not match, the device should send a "fd" packet back and ignore the content.

Note that while Abort packets abort the entire command, but do not guarantee it had no effect.

Anything up to the initial magic is ignored.

Command Format

• 1 byte: request id
• 1 byte: command id
  • Cx -- JTAG interface commands
  • Dx -- Replies to JTAG interface commands
  • Ex -- JAP Controller commands
  • Fx -- Replies to JAP Controller commands
• 1 byte: device number (ignored for some commands)
  • 00 is the controller
  • 01 to fe are JTAG devices
  • ff for "all JTAG devices"
• 1 byte: status (should always be 00 for requests)
  • 00 -- success
  • 80 -- failure
• N bytes: data

Commands less than 4 bytes should be ignored. Additional data beyond what is expected should be ignored.

Commands

c0 -- clock

Request Data:

• 1 byte: number of clocks to ignore at the end (only 1-3 are valid)
• N bytes: two bits per clock
  • 1 bit: TMS high
  • 1 bit: TDO high

Reply Data:

• N bytes: one bit per clock
  • 1 bit: TDI high

c1 -- read/write register

Note that data is written/read in big endian bit order. This means that the first bit written/read is 80, then 40, and so on. If you are not using a multiple of 8 bits, this is still the case; for example, if you read 7 bits all high, you will get fe, not 7f.

Request Data:

• 1 byte: bit values (bitwise or'd)
  • 80 -- Use IR register (default is DR)
  • 40 -- Read+Write (default is Write only)
  • 01 to 07 -- Don't do last N bits of data
• N bytes: data to write

Reply Data (may be parallel with request):

• N bytes: data read (only if Read+Write set in request)

c2 -- reset

(No Request or Reply Data)

c3 -- run

(No Request or Reply Data)

e0 -- ping

Device number ignored. Replies with same data as request

e1 -- get device count

Device number ignored. May be required before device access.

Reply Data:

• 1 byte: number of devices
• N bytes:
  • 1 byte: device id number

e2 -- hardware version

Note: Only expected to work for controller

Reply Data:

• 4 bytes: hardware version (big endian)
• 2 bytes: hardware vendor id authority
  • 0000 -- USB
  • ffff -- Ad-hoc
• 1 byte: length of hardware vendor id
• N bytes: hardware vendor id (binary)
• 1 byte: length of hardware device id
• N bytes: hardware device id (binary)
• 1 byte: length of hardware serial number
• N bytes: hardware serial number (UTF-8)
• 1 byte: length of human-readable hardware model
• N bytes: human-readable hardware model/version string (UTF-8)

e3 -- software version

Note: Only expected to work for controller

Reply Data:

• 4 bytes: software version (big endian)
  • ffffffff -- Unknown
• 1 byte: length of software-specific unique identifier (git commit?)
• N bytes: software-specific unique identifier
• 1 byte: length of human-readable software name
• N bytes: human-readable hardware model/version string (UTF-8)
• 2 byte: number of AJP features supported

e4 -- device capabilities

Reply Data:

• 2*N bytes: supported capabilities
  • c000 -- JTAG protocol commands
  • e500 -- watch/poll command

e5 -- watch/poll

Request Data:

• 1 byte: how long to watch, in seconds
  • 00 -- cancel watch
  • ff -- never expire
• N bytes: packets to send

Reply (immediately for all, and whenever results change individually):

• Same as if the packets were received directly

Examples

Reset the command queue

To reset the command queue, send 243 zero bytes followed by the Abort packet (fd414a50 00 7052); this will pad-out any "open" packet, and abort any open command.