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CHAPTER 4 INTERFACES
88
(Multi-receive cell transfer)
The
μ
PD98405 supports multi-receive cell transfer. Whether to enable multi-cell transfer is set in the
MBL field of the receive VC table. The maximum burst size for multi-cell transfer is 2 to 5 cells, and can
be set in cell units. (1-cell transfer is always enabled.)
The following table lists each setting of the MBL field and the corresponding multi-cell transfer size.
MBL field
Maximum burst size for multi-cell transfer
000
Disabled.
001
Disabled.
010
2 cells
011
3 cells
100
4 cells
101
5 cells
110
5 cells
111
5 cells
When the
μ
PD98405 successively receives cells from the same VC (for which multi-cell transfer is
enabled), that is, when cells from the same VC are contiguously stored into the receive FIFO, it writes
the receive data into system memory by performing multi-cell transfer. When the
μ
PD98405 does not
successively receive cells from the same VC, it performs normal 1-cell burst transfer, regardless of
whether multi-cell transfer is enabled for the VC. For example, when 5-cell transfer is enabled for VC1
and the
μ
PD98405 receives a cell from another VC after successively receiving 2 cells from VC1, the
μ
PD98405 performs 2-cell transfer for VC1.
Because the
μ
PD98405 performs multi-cell transfer only when it successively receives cells from the
same VC, multi-cell transfer for other VCs is not affected when multi-cell transfer is enabled for multiple
VCs.
(3) Burst size split function
The
μ
PD98405 performs 12-word burst transfer for normal DMA transfer and 1- to 5-cell burst transfer for
multi-cell DMA transfer. During DMA transfer, the
μ
PD98405 supports a function for automatically splitting
the burst size with PCI bus cache boundaries considered.
When the AD bit of the GMR register is set to 1, this function is disabled. The
μ
PD98405 attempts to
perform DMA transfer in 12-word burst or multi-cell transfer mode without considering the DMA transfer
start address or cache boundaries. When the AD bit is set to 0, the
μ
PD98405 splits the burst size with the
DMA transfer start address and cache boundaries considered. When this function is enabled, burst
transfer is not disconnected at any cache boundary. For multi-cell transfer, the burst size is also split
automatically. This function is also supported for non-cell data (such as transmission or reception
indication) DMA transfer. By default, the AD bit is set to 0 to enable this function.