12-22
ColdFire CF4e Core User’s Manual
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BIST
5. Part 5
a) Begin at last address location (address max)
b) Read (data)—write (complement)—read (complement)
c) Decrement address and repeat for entire address space
6. Part 6
a) Begin at last address location (address max)
b) Read (data)
c) Decrement address and repeat for entire address space
The March C+ algorithm is a 14N per data background with 14 operations per data word.
A data background is a bit stream pair, such as 5-A, 3-C, and 0-F. Motorola prefers using
three backgrounds to provide coverage of the memory array independent of physical
organization. The March C+ algorithm provides a fault coverage of more than 99.9% of the
memory defect classes.
12.3.6 ROM BIST Algorithm
A memory BIST for a read-only memory (ROM) has two purposes: to verify data in the
ROM and to ensure no defects affect that data when a read operation is conducted (a
destructive read). ROM verification uses a compression scheme in which the compressed
data (or signature) is compared against a golden value created when the ROM data is
programmed. The scheme is based on cyclic-redundancy code (CRC) methodology, which
uses a linear feedback shift register (LFSR). An LFSR that has inputs as the output data bus
from memory is generally called a multiple input signature-analysis register (MISR).
MISR-LFSR is a shift register in which the last (right-most) bit is brought back to various
bits along the length of the shift register through XOR gates. The memory data bus is also
brought into the LFSR through XOR gates. The bits that receive the feedback are chosen
from polynomial tables. The initial LFSR state before it captures any data is called the seed.
The MISR-LFSR operation is similar to division by a prime number, as follows:
The input data stream = the dividend
The polynomial = the prime divisor
The state of the MISR-LFSR after each capture cycle = the remainder
The ROM BIST has two read passes on the memory, as shown in the following steps:
1. Begin at first address location (address 0)
2. Read (data)—compress (data)
3. Increment address and repeat for entire address space
4. Conduct self-pause
5. Release self-pause
6. Begin at first address location (address 0)
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