The PACKET_FC method allocates blocks of credit in finer granularities than LIMIT_FC, using the receive completion space more efficiently with a small increase in user logic.
Start with two registers, CPLH_PENDING and CPLD_PENDING, (loaded with zero at reset), and then perform these steps:
1. When the user application needs to send an NP request, determine the potential number of CplH and CplD credits it might require:
NP_CplH = ceiling[((Start_Address mod RCB) + Request_Size) / RCB]
NP_CplD = ceiling[((Start_Address mod 16 bytes) + Request_Size) /16 bytes]
(except I/O Write, which returns zero data)
[(req_size + 15)/16]
The modulo and ceiling functions ensure that any fractional RCB or credit blocks are rounded up. For example, if a memory read requests 8 bytes of data from address 7Ch , the returned data can potentially be returned over two completion packets ( 7Ch - 7Fh , followed by 80h - 83h ). This would require two RCB blocks and two data credits.
2. Check these:
CPLH_PENDING + NP_CplH < Total_CplH
CPLD_PENDING + NP_CplD < Total_CplD
3. If both inequalities are true, transmit the non-posted request, and increase CPLH_PENDING by NP_CplH and CPLD_PENDING by NP_CplD. For each non-posted request transmitted, keep NP_CplH and NP_CplD for later use.
4. When all completion data is returned for an non-posted request, decrease CPLH_PENDING and CPLD_PENDING accordingly.
This method is less wasteful than LIMIT_FC but still ties up all of an non-posted request completion space until the entire request is satisfied. RCB_FC and DATA_FC provide finer de-allocation granularity at the expense of more logic.