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MPC563XM Reference Manual, Rev. 1
300
Freescale Semiconductor
Preliminary—Subject to Change Without Notice
12.5
External
Signal Description
V
standby
RAM Power Supply pin.
12.6
Functional Description
The Amba-AHB bus is a two stage pipelined bus which would require the SRAM controller to insert a wait
state when a read follows a write. The SRAM controller implements a late-write-buffer to enable zero wait
state write-read combinations.
ECC handling on this device is done on 32-bit boundary. Because the e200z335 core on this device is a
cacheless processor, the platform RAM is organized in on a 32-bit boundary versus the 64-bit organization
used on eSys MCUs based on the e200z6 core. This organization was driven by performance reasons: for
the cacheless e200z335 processor, most RAM writes will be 32 bits or smaller in size. It was estimated
that implementation of a 32-bit RAM ECC provides 7-8% overall performance improvement over a 64-bit
organization that would force read-modify-write cycles to write 32-bit data. Each 32-bit word requires a
7-bit ECC code, so the RAM array is organized in two banks of 39 bits each.
The ECC code performs single bit corrections and indicate a multiple bit error on all double-bit read errors.
Multiple bit errors will assert an error indication in the bus cycle, as well as setting the PRNCE bit in the
ECSM’s ESR.
During a write operation for 8-bit and 16-bit data, a read of 32-bit data will be checked for ECC, prior to
merging in the write data. If a correction is required, it will be corrected prior to merging in the write data.
Then a new ECC code word is generated and written to the RAM. If a multiple bit error occurs during the
read portion of the write operation, then the write will not be performed.
It is essential for the ECC check bits to be initialized after Power On Reset. The write transfer must be 32
or 64 bits in size because a less than 32 bit write transfer will generate a read / modify /write operation
which will check the ECC value upon the read.
12.6.1
Access Timing
The Amba-AHB bus is a two stage pipelined bus, which makes the timing of any access dependent on the
access during the previous clock.
shows the wait states for accesses, current is the access being
measured,
previous
is the RAM access during the previous clock.
Table 12-1. Wait States During RAM Access
Current
Previous
Wait States
Read
Idle
0
Read
0
32/64-bit Write
0
8/16-bit Write
1
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