24.4.5.2.3 Virtual Target Transfers
HDR-DDR Flow Control elements provide a mechanism for the Target to terminate an HDR-DDR Write transfer with or without CRC and also generate and send a CRC word in cases where the Controller has terminated an HDR-DDR read transfer.
A broadcast and directed CCC 'ENDXFER' provides the framework for Controllers and Targets to exchange setup parameters for ending data transfer in HDR-DDR Mode. Flow control parameters supported for HDR-DDR transfer are as follows:
- Enable the ACK/NACK capability for HDR-DDR write command.
- Enable write early termination request.
- Enable CRC word indicator for write/read early termination request.
By default, HDR-DDR Abort CRC and HDR-DDR Write Abort capabilities are enabled. The Controller's application, in turn, is expected to negotiate the flow control capabilities with the Target by issuing the ENDXFER CCC.
The Controller is expected to send a broadcast or directed ENDXFER CCC with the defining byte 0xF7, with the additional data byte programmed as per the flow control requirements. The ENDXFER CCC with the defining byte 0xF7 needs to be followed by another broadcast/directed ENDXFER CCC with the defining byte 0xAA. This is used by the Target to confirm and configure its flow control capabilities as per the previous ENDXFER CCC (defining byte 0xF7), whose data byte structure is shown in Table 24-26.
| Bits | Description | Values |
|---|---|---|
| [7:6] | CRC word indicator | 2'b11: No CRC word follows early termination request |
2'b01: CRC word follows early termination request Other: Reserved | ||
| [5] | Enable WRITE early termination request | 1'b0: Enable 1'b1: Disable |
| [4] | Enable ACK/NACK capability for the WRITE command | 1'b0: Enable 1'b1: Disable |
| [3:0] | Reserved for future use |
Following the configuration of flow control capabilities using the ENDXFER CCCs, the Target functions as follows for each of the flow control capabilities.
- Command level ACK/NACK Capability: If this capability is enabled by the Active Controller, then the Target decides to either ACK or NACK the write or read command based on the following
- Address match (dynamic address, group address, Virtual Target Address)
- Availability of data and response queues
- Detection of parity error in the Command Word
- Write Data Early Termination Request: If this capability is enabled by the Active Controller, the Target might terminate a write transfer in the following cases:
- Rx FIFO / Response FIFO Overflow
- Parity error in the received data word
CRC word follows Early Termination Request: If this capability is enabled by the Active Controller, then the Target drives or receives the CRC word after HDR-DDR Read or Write Early Termination scenarios.