Table of Contents
Advertisement
1.5.6.1
SSP frame transmission and reception
During an SSP connection, SSP frames are preceded by SOF and followed by
EOF. The last data dword after the SOF prior to the EOF always contains a CRC.
An SSP phy checks the frame length and validity of the CRC.
Receiving phys must acknowledge ACK or NAK within 1 ms if there is no
condition for discarding frames. The ACK primitive indicates that SSP frames
have been received into a frame buffer without errors. The NAK (CRC ERROR)
primitive indicates that SSP frames have been received with a CRC error.
1.5.6.2
SSP flow control
An SSP phy uses RRDY to grant credit for permission for the other SSP phy in
the connection to transmit frames. Each RRDY increments credit by one frame.
Frame transmission decrements credit by one frame. Credit of zero frames is
established at the beginning of each connection.
SSP phys shall not increment credit past 255 frames.
To prevent deadlocks where an SSP INIT port and SSP TARG port are both
waiting on each other to provide credit, an SSP INIT port shall never refuse to
provide credit by withholding RRDY because it needs to transmit a frame itself.
It may refuse to provide credit for other reasons (e.g., temporary buffer full
conditions).
When the drive sends an OPEN frame and, then, receives the OPEN_ACCEPT
primitive from the initiator due to abort processing in the drive (e.g., when
receiving the TASK frame), the drive sends the CREDIT_BLOCKED primitive
instead of an RRDY primitive. In this way, the drive may refuse to offer credits.
If credit is zero, SSP phys that are going to be unable to provide credit for 1 ms
may send CREDIT_BLOCKED. The other phy may use this to avoid waiting
1 ms to transmit DONE (CREDIT TIMEOUT).
If credit is nonzero, SSP phys that are going to be unable to provide additional
credit for 1 ms, even if they receive frames per the existing credit, may transmit
CREDIT_BLOCKED.
After sending CREDIT_BLOCKED, an SSP phy shall not transmit any additional
RRDYs in the connection.
1.5.6.3
Interlocked frames
Table 1.31 shows which SSP frames shall be interlocked and which are non-
interlocked.
One approach for successively processing frames and corresponding ACK or
NAK (responses to these frames) in a single processing unit consists of ensuring
that no new frame is sent until the response to the preceding frame is returned.
A frame processed in this way is called an interlocked frame.
C141-C012
1.5 Address frames
1-53
Advertisement
Table of Contents
