CN8478 Datasheet, PDF (60/221 Pages) Conexant Systems, Inc – Multichannel Synchronous Communications Controller (MUSYCC)

2.0 Host Interface
2.2 PCI Configuration Registers
CN8478/CN8474A/CN8472A/CN8471A
Multichannel Synchronous Communications Controller (MUSYCC™)
2.2.6.3 Latency
Computation—Burst
Access
When the following is assumed:
• MUSYCC has enough internal buffering to buffer up to 4-dwords worth of
information per channel before performing a 2-dword burst cycle for every
access.
• MUSYCC has a granularity of 8 for its latency timer (that is, MUSYCC is
always configured to give up the bus in equal to or less than the desired
time-out).
• The system will support MUSYCC burst writes and reads.
• MUSYCC, with all 256 receive and transmit channels active, needs to
move 1,024 kdwords/s, or one dword every 0.98 µs, or 4-dword bursts
every 3.92 µs. That is, 130 clock cycles between bursts for a 33-MHz PCI
bus rate, and 260 clock cycles for a 66-MHz PCI bus rate.
The following can be seen:
• The worst case time it would take each burst cycle to finish is 16 cycles (16
clock rule) + 8 cycles, target latency = 24 clock cycles to finish, worst
case.
• With one MUSYCC and one host operating at a PCI bus rate of 33 MHz:
The host has 130 cycles between bursts – 24 cycles to finish, worst case =
106 clock cycles. The host's T must be programmed to 106 cycles –
8 cycles, target latency = 98 cycles. Rounding for granularity yields
96 cycles.
• With one MUSYCC and one host operating at a PCI bus rate of 66 MHz:
The host has 260 cycles between bursts – 24 cycles to finish, worst case =
236 clock cycles. The host's T must be programmed to 236 cycles –
8 cycles, target latency = 228 cycles. Rounding for granularity yields
224 cycles.
• For n MUSYCC and one host operating at a PCI bus rate of 33 MHz:
The host has 130 cycles between bursts – (n x 24 cycles, worst case) –
8 clock cycles, target latency = T cycles. Therefore, for two MUSYCC's
and one host, a host's T of 24 would be sufficient; that is, 130 cycles –
(2 x 24) – 8 cycles = 74 clock cycles. Rounding for granularity equals
72 cycles.
• For n MUSYCC and one host operating at a PCI bus rate of 66 MHz:
The host has 260 cycles between bursts – (n x 24 cycles, worst case) –
8 clock cycles, target latency = T cycles. Therefore, for two MUSYCC's
and one host, a host's T of 24 would be sufficient; that is, 260 cycles –
(2 x 24) – 8 cycles = 204 clock cycles. Rounding for granularity equals
200 cycles.
On reset, the value of the latency timers are reset to 0.
2-22
Conexant
100660E