Computer Hardware User Manual

4-4 Computer Group Literature Center Web Site
MCECC Functions
4
Note The table is not complete because it cannot account for the effects
of a write posting operation. If the Petra MCECC sector is idle
and a write cycle is initiated on the local bus, the cycle is write-
posted and the local bus is acknowleged in two clock ticks. If
another bus cycle is initiated while the write post operation is in
progress, the new cycle is stalled until the write posting is
complete. The Read cycles are extended by one clock cycle if the
the NCEBEN bit is set in the SDRAM Control register.
Since the bandwidth between the SDRAM and the processor local
bus is generally higher than that of the logic it replaces (the
MCECC pair and EDO DRAMs), software will take less time to
execute. This could change the behavior of certain applications.
Cache Coherency
The MCECC sector supports the MC680x0 caching scheme on the local
bus by always providing 32 bits of valid data during DRAM read cycles
regardless of the number of bytes requested by the local bus master for the
cycle. It also supports cache coherency by monitoring the memory inhibit
(MI∗) signal.
For a write or read cycle, the MCECC sector always waits for MI∗ to be
negated before it begins a read or write cycle to the DRAM. If another
local bus slave asserts TA∗ or TEA∗ before MI∗ is negated, then the
MCECC sector never begins the DRAM write cycle.
Table 4-2. Memory System Cycle Timing
Access
Description
Memory States
Idle Active Hit Active Miss
Read Single 4 clock cycles 3 clock cycles 5 clock cycles
Read Burst 4-1-1-1 clock cycles 3-1-1-1 clock cycles 5-1-1-1 clock cycles
Write Burst 2-1-1-1 clock cycles 2-1-1-1 clock cycles 2-1-1-1 clock cycles
Write Longword 2 clock cycles 2 clock cycles 2 clock cycles
Write 1 or 2 Bytes 9 clock cycles 8 clock cycles 10 clock cycles