GreenPAK Universal Dev Board 1

This page describes the USB protocol used by the GreenPAK Universal Dev Board 1, as shipped in the GreenPAK 4 Development Kit and probably others as well. Protocol RE is done by @whitequark with board revision v1.3.1 and firmware v2.4.

Protocol

The protocol is based on USBHID. It uses interrupt URBs, always with a 63 byte payload. All numbers below are in hexadecimal.

Overall URB payload format:

Offset	Length	Meaning
00	01	HID report number, always 01
01	02	Packet type
03	01	Packet sequence number

The packet sequence number only makes sense in the case where a single command has to be split between several packets because of the URB length limit. For write-type commands, the WRITE packet contains the amount of packets to be written after this one and its READ counterpart contains 00. For read-type commands, the WRITE packet contains 00 and its READ counterpart contains the amount of packets to be read after this one.

Every byte not specified in the individual packet format was always observed as 00.

"Read back command for ..." means that the returned payload for this READ command is exactly the same as what was provided for the WRITE command it refers to.

Packets

WRITE 0500

This packet (likely) resets the board controller.

READ 053f

Read back for WRITE 0500.

WRITE 2104

This packet turns the STATUS LED on and off.

Offset	Length	Meaning
04	01	STATUS LED enabled

READ 213f

Read back for WRITE 2104.

WRITE 0439

This packet configures the I/O buffers, LEDs, and the expansion connector.

Format:

Offset	Length	Meaning
04	02	TP2 output configuration
06	02	TP3 output configuration
08	02	TP4 output configuration
0a	02	TP5 output configuration
0c	02	TP6 output configuration
0e	02	TP7 output configuration
10	02	TP8 output configuration
12	02	TP9 output configuration
14	02	TP10 output configuration
16	02	TP12 output configuration
18	02	TP13 output configuration
1a	02	TP14 output configuration
1c	02	TP15 output configuration
1e	02	TP16 output configuration
20	02	TP17 output configuration
22	02	TP18 output configuration
24	02	TP19 output configuration
26	02	TP20 output configuration
2f	03	Expansion connector configuration
32	01	TP3..TP6 LED configuration
33	01	TP7..TP10 LED configuration
34	01	TP12..TP15 LED configuration
35	02	TP16..TP20 LED configuration
37	01	"POWER" LED status? Observed as 01

TPx output configuration:

Value	Meaning
0200	N/C
0c01	VDD
0c00	GND
0e01	Pull-up
0e00	Pull-down
0c03	Logic generator, push-pull
0403	Logic generator, open-drain + pull-up
0603	Logic generator, open-drain + pull-down
0a03	Logic generator, open-drain, drives HI
0803	Logic generator, open-drain, drives LO
0e03	Logic generator, pull-up/down

TP3..TP6, TP7..TP10 and TP12..TP15 LED configuration (a bit mask):

Value	Meaning
01	TP(n+0) LED enabled
02	TP(n+1) LED enabled
04	TP(n+2) LED enabled
08	TP(n+3) LED enabled
10	TP(n+0) LED inverted
20	TP(n+1) LED inverted
40	TP(n+2) LED inverted
80	TP(n+3) LED inverted

TP16..TP20 LED configuration (a bit mask):

Value	Meaning
0100	TP(n+0) LED enabled
0200	TP(n+1) LED enabled
0400	TP(n+2) LED enabled
0800	TP(n+3) LED enabled
1000	TP(n+4) LED enabled
0001	TP(n+0) LED inverted
0002	TP(n+1) LED inverted
0004	TP(n+2) LED inverted
0008	TP(n+3) LED inverted
0010	TP(n+4) LED inverted

Expansion connector configuration (a bit mask):

Value	Meaning
000100	VDD
000004	TP2
000001	TP3
000010	TP4
000040	TP5
010000	TP6
040000	TP7
100000	TP8
400000	TP9
800000	TP10
200000	TP12
000008	TP13
000002	TP14
008000	TP15
000020	TP16
020000	TP17
002000	TP18
000800	TP19
080000	TP20

WRITE 490b

This command trims the oscillator.

Offset	Length	Meaning
05	01	Trim value
06	06	Unknown; observed as 05040a110201

READ 493f

Read back for WRITE 490b. Note that the payload is not identical.

Offset	Length	Meaning
06	06	Unknown; observed as 05040a110201

WRITE 4204

This command starts measuring the oscillator frequency.

READ 423f

This command reads back the oscillator frequency after WRITE 4204.

Offset	Length	Meaning
06	02	Oscillator frequency

WRITE 033f / WRITE 0318

This command transfers the bitstream with the purpose of verification.

With sequence number 04:

Offset	Length	Meaning
04	01	Unknown; observed as 08
07	02	The number of SCL cycles to use, 0898 (including the trailing cycles)
09	38	reg<0:447>

With sequence number 03:

Offset	Length	Meaning
04	3d	reg<448:935>

With sequence number 02:

Offset	Length	Meaning
04	3d	reg<936:1423>

With sequence number 01:

Offset	Length	Meaning
04	3d	reg<1424:1911>

With sequence number 00:

Offset	Length	Meaning
04	15	reg<1912:X>, 32 bits of unknown purpose, reg<X:2047>

READ 073f / READ 1a3f

Read back for WRITE 033f / WRITE 0318, respectively.

Command sequences

Trimming

In a loop, searching by changing the trim value between ff to 00:

1. WRITE 490b
2. READ 493f
3. WRITE 4204
4. READ 423f

Verifying

1. WRITE 033f SEQ 04
2. READ 073f
3. WRITE 033f SEQ 03
4. READ 073f
5. WRITE 033f SEQ 02
6. READ 073f
7. WRITE 033f SEQ 01
8. READ 073f
9. WRITE 0318 SEQ 00
10. READ 1a3f

It is unknown how to determine verification status.