NXP Semiconductors MPC5605BK, Reference Manual

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Chapter 31 Static RAM (SRAM)
MPC5606BK Microcontroller Reference Manual, Rev. 2
890 Freescale Semiconductor
31.4 SRAM ECC mechanism
The SRAM ECC detects the following conditions and produces the following results:
• Detects and corrects all 1-bit errors
• Detects and flags all 2-bit errors as non-correctable errors
• Detects 39-bit reads (32-bit data bus plus the 7-bit ECC) that return all zeros or all ones, asserts an
error indicator on the bus cycle, and sets the error flag
SRAM does not detect all errors greater than 2 bits.
Internal SRAM write operations are performed on the following byte boundaries:
• 1 byte (0:7 bits)
• 2 bytes (0:15 bits)
• 4 bytes or 1 word (0:31 bits)
If the entire 32 data bits are written to SRAM, no read operation is performed and the ECC is calculated
across the 32-bit data bus. The 8-bit ECC is appended to the data segment and written to SRAM.
If the write operation is less than the entire 32-bit data width (1 or 2-byte segment), the following occurs:
1. The ECC mechanism checks the entire 32-bit data bus for errors, detecting and either correcting or
flagging errors.
2. The write data bytes (1 or 2-byte segment) are merged with the corrected 32 bits on the data bus.
3. The ECC is then calculated on the resulting 32 bits formed in the previous step.
4. The 7-bit ECC result is appended to the 32 bits from the data bus, and the 39-bit value is then
written to SRAM.
31.4.1 Access timing
The system bus is a two-stage pipelined bus, which makes the timing of any access dependent on the access
during the previous clock cycle.
Table 31-4 lists the various combinations of read and write operations to
SRAM and the number of wait states used for the each operation. The table columns contain the following
information:
• Current operation — Lists the type of SRAM operation currently executing
• Previous operation — Lists the valid types of SRAM operations that can precede the current
SRAM operation (valid operation during the preceding clock)
• Wait states — Lists the number of wait states (bus clocks) the operation requires, which depends
on the combination of the current and previous operation