I²C - Half-Duplex Transfer; 32-39 Bytes Out, 0+ Bytes In

Report Type: Interrupt IN / OUT
Report Size: 44 Bytes
Acknowledged By: Transactional reports in the range [0x80, 0xff] or Command Acknowledgement for USB failures

Report ID	Flags	Count LSB	Count MSB	Address	Payload
10000100	rrr 0 R c ff	cccccccc	cccccccc	0 aaaaaaa	39 bytes

Report ID

HID Report ID. Always 0x84.

This is effectively the seven most significant bits of the payload size (in bytes). The total number of bytes in the payload can be found by: <pre>(Report ID - 128) * 8 + rrr</pre>

Flags

Bits	Direction	Meaning
ff	IN	For IN Reports this is the Fault Code; 00 is OK, 01 is Address NACK, 10 is Data NACK, 11 is Timeout. For OUT Reports this is unused so it should be set to 0.
c	IN	For IN Reports this is the Collision flag; 1 if there was a bus collision and the device lost arbitration, 0 if the transaction was successful. For OUT Reports this is unused so it should be set to 0.
R	IN	For IN Reports this is the Read flag; 1 if the Slave Read stage is taking place (or was taking place when an error occurred), 0 if the Slave Write stage was not completed. For OUT Reports this is unused so it should be set to 0.
rrr	IN / OUT	The number of payload bytes contained in the last packet of the report (the ‘residual’). The total number of bytes in the payload can be found by: <pre>(Report ID - 128) * 8 + rrr</pre>

Count

For an OUT Report this is the number of bytes that will be shifted from the Slave and returned to the Host after the Master-to-Slave payload has been delivered. This can be 0 if the transaction is write-only.

For an IN Report where the bytes are being shifted from the Master to the Slave (R == 0) this is the number of bytes remaining to be shifted from the Master to the Slave.

For an IN Report where the bytes are being shifted from the Slave to the Master (R == 1) this is the number of bytes remaining to be shifted from the Slave to the Master.

This field is Little Endian, ie. the Least Significant Byte of the word appears first. Bit numbering within the constituent bytes is unaffected.

Payload

A block of 39 bytes that will be shifted onto the bus for an OUT Report, or the Slave’s response for an IN Report. Only the first 32 + rrr bytes are meaningful, the rest are ignored or undefined.

Notes

The IN Reports (Slave-to-Master-to-Host transfers) will be sent back in chunks to allow a smaller buffer to be used and to reduce latency. The Host software must therefore expect any number of reports in the range [0x80, 0xff] until the transfer is completed. Transfers are considered completed once the Count field of the IN Report reaches 0, the ff flag is not 00 (indicating a fault) or the c flag is 1 (also a fault).

The internal buffer size for I²C OUT Reports is limited to 1028 bytes. It is a circular buffer and OUT Reports are always accepted by the USB stack, which means that data at the beginning of the buffer can be overwritten without warning if the buffer size is exceeded. Therefore it is important that the Host software always waits for the previous I²C transfer to complete before sending more data.