DisplayPort
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DisplayPort is a protocol (link) and bus (physical) standard for transporting video, audio and other data from a "source" device to a "sink" device.
Link Layer
The link layer consists of a Main Link, which is used for transporting video and audio, and an Auxiliary Channel (AUX CH) which is used for configuration & retrieving information such as the EDID.
Auxiliary Channel
The AUX CH is a packet-based protocol that can transport both I²C and Native AUX CH packets. The source initiates a request and the sink always sends a response within a timeout. A packet is at most 16 (20?) bytes large.
Each packet starts with a single byte header. Bit 7 indicates whether the packet is an I²C packet or Native AUX CH packet.
Native AUX CH
Request header format
| Bit(s) | Description |
|---|---|
| 7 | 0 (Native AUX CH transaction) |
| 6:4 | Request type (000 = write, 001 = read) |
| 3:0 | Padding (must be all zeroes) |
Reply header format
| Bit(s) | Description |
|---|---|
| 7:6 | Must be zero / ignored |
| 5:4 | Native AUX CH reply (00 = ACK, 01 = NACK, 10 = defer) |
| 3:0 | Padding (must be all zeroes) |
I²C
A I²C transaction is initiated by sending an initial packet with the address. After receiving an ACK data can be read or written. The source must always specify the address and the amount of data to be sent or read by adding a byte that denotes the length minus 1.
The last packet must have MOT = 0. All other packets must have MOT = 1.
For example, an I²C transaction to retrieve the EDID may go as follows (> denotes source to sink, < denotes sink to source):
> 0b0_1_00_0000, 0, 0x50 (I2C, MOT = 1, write, address 0x50) < 0b00_00_0000 (AUX_ACK, I2C_ACK) > 0b0_1_00_0000, 0, 0x50, 0, 0 (I2C, MOT = 1, write, address 0x50, length 1, data 0) < 0b00_00_0000 (AUX_ACK, I2C_ACK) > 0b0_1_01_0000, 0, 0x50 (I2C, MOT = 1, read, address 0x50) < 0b00_00_0000 (AUX_ACK, I2C_ACK) loop 128 times: > 0b0_1_01_0000, 0, 0x50, 0 (I2C, MOT = 1, read, address 0x50, length 1) < 0b00_00_0000, x (AUX_ACK, I2C_ACK, data x) repeat loop > 0b0_0_01_0000, 0, 0x50 (I2C, MOT = 0, read, address 0x50) < 0b00_00_0000 (AUX_ACK, I2C_ACK)
It is possible to read/write multiple bytes of data in one request which is faster, though this depends on the I²C bus speed. E.g. a 100KHz device can send 1 bit every 10µs, which allows it to send up to 3 bytes per reply (300µs / 10µs/bit = 30 bits > 3 bytes).
A DEFER reply may be sent if the sink fails to deliver a response within 300µs. This may happen with e.g. slow I²C devices. When receiving a DEFER reply a STATUS_REQUEST should be sent at a later point.
Request header format
| Bit(s) | Description |
|---|---|
| 7 | 1 (I²C transaction) |
| 6 | Middle-of-transaction bit. |
| 5:4 | Request type (00 = write, 01 = read, 10 = write status_request) |
| 3:0 | Padding (must be all zeroes) |
Reply header format
| Bit(s) | Description |
|---|---|
| 7:6 | I²C-over-AUX reply (00 = ACK, 01 = NACK, 10 = defer) |
| 5:4 | Native AUX CH reply (00 = ACK, 01 = NACK, 10 = defer) |
| 3:0 | Padding (must be all zeroes) |