MAC7200 datasheet(464/976 Pages) FREESCALE

464 / 976 page

Platform RAM Array Controller (PRAM_CTL)

MAC7200 Microcontroller Family Reference Manual, Rev. 2

386

Freescale Semiconductor

Figure 19-3. Block Diagram: 39-bit ECC Decode

The encoder logic is included in the write data path to the RAM and generates the 7-bit ECC code which

is written to the RAM with the data. Together, the data bus and ECC bits are called the code word. Six of

the ECC bits are calculated using a modified Hamming code. The seventh and final ECC bit is calculated

so that the parity of the entire code word is odd. Again, this process is done twice in parallel for the 64-bit

implementation.

The decoder logic is included in the read data path from the RAM back to the AHB bus. The decoder uses

the same algorithm as the encoder and generates 6 unique ECC bits, or the syndrome[5:0], from the 39-bit

code word (7 ECC and 32 data bits). This syndrome is used to determine which bit within the 39-bit data

bus caused the single-bit correctable error. See Chapter 13, “Miscellaneous Control Module (MCM)” for

more detail on which syndromes map to which bits. A seventh ECC bit representing the overall parity of

the entire code word is also generated.

Table 19-6 shows the relationship of the overall parity bit, the syndrome and error detection and correction.

Odd parity is shown as a value of 1. In this table, the term "valid codeword" refers to the codewords

defined in Table 19-11. All other codewords are "invalid."

It is important to note that the first entry in this table, overall even parity and a syndrome of zero, could be

generated by the failure of the either the 7th ECC bit or the entire 39 bits of data. For this reason, an odd

parity scheme was chosen for the 7th ECC bit, and a check is performed to generate a mulitple bit error

when the entire 39-bit codeword is zero.

Table 19-6. Parity vs. Syndrome

Overall Parity

Syndrome[5:0]

Result

single-bit correction

valid codeword

single-bit correction

invalid codeword

multiple-bit error

non-zero

multiple-bit error

no error

dec_bus_out[31:0]

ecc_syndrome[5:0]

ecc_parity

ecc_single_detect

ecc_multiple_detect

pram_rdata[39:0]

39 bit codeword

32 data bits

Bit Correction Muxing

Syndrome

Decode

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