2. PCI Interface
50
PowerSpan II User Manual
80A1010_MA001_09
Integrated Device Technology
www.idt.com
2.3.3
Data Phase
The data phase deals with the control of burst length.
2.3.3.1
Writes
For non-DMA writes, the length of the PCI write transaction is dependent on the length of the
transaction delivered from the source bus. Writes originating from the processor bus can be either
single cycle writes or burst writes. Burst writes from the processor bus are always 32 bytes in length.
This burst is converted to an 8 byte burst on a 32-bit PCI bus. Either PCI-1 or PCI-2 can be configured
as 32-bit. Single cycle writes from the 64-bit processor bus are translated into two 8 byte burst writes
on the 32-bit PCI bus. This information is summarized in
The PB Master can also generate extended cycles. Extended cycles are either 16 byte or 24 byte
transactions. These cycles are enabled by setting the Extended Cycle (EXTCYC) bit to 1 in the
“Processor Bus Miscellaneous Control and Status Register” on page 304
When the Dual PCI PowerSpan II is used, incoming PCI writes are executed as similar writes on the
alternate PCI interface. For example, a 64-byte burst write to memory space from the PCI-1bus is
executed as a 64-byte burst write to the memory space on the PCI-2 bus, provided the target on PCI-2
does not disconnect.
DMA Writes
The PowerSpan II DMA channels always attempt to perform the longest possible burst
—
up to
128-bytes
—
on the PCI bus.
2.3.3.2
Reads
The minimum memory read prefetch quantity is 8 bytes (default). Setting the MEM_IO bit in the
“PCI-1 Target Image x Control Register” on page 268
enables 1,2,3, or 4 byte memory reads on the
PCI bus.
Table 8: PB Writes and Their Corresponding PCI Writes
PB Write
64-bit PCI Write
32-bit PCI Write
32-byte Line Write
4-beat 32-byte Burst Write
8-beat 32-byte Burst Write
8-byte Single Write
Single Beat 8-byte Write
2-beat 8-byte Burst Write
Write transactions intended for I/O space on the alternate PCI bus must be single
beat writes. Bursting is not supported for a target image programmed to generate
an I/O access on the alternate PCI bus. A burst write directed at such a target
image results in a Target-Disconnect after every data beat.