A lockup can occur when there is a valid checksum that cannot be cleared. Different UCD3138 devices have different numbers of checksum.

A UCD3138(A) device has one single block. There are two locations for checksum and each checksum is 4 bytes.

• 0x7fc – boot checksum, sum of the pflash bytes from address 0 to 0x7fB
• 0x7ffc – overall checksum for pflash, sum of the pflash bytes from address 0 to 0x7ffB

Figure 2-4 is a flowchart showing how UCD3138(A) ROM handles the checksum verification. Regardless of which two checksums is valid, a jump occurs to address 0 to execute code in pflash.

A UCD3138064(A) device has block 1 and block 2. There are four locations for checksum and each checksum is 4 bytes.

• 0x7fc – boot checksum for block 1, sum of the pflash bytes from address 0 to 0x7fB
• 0x7ffc – overall checksum for block 1, sum of the pflash bytes from address 0 to 0x7ffB
• 0x87fc – boot checksum for block 2, sum of the pflash bytes from address 0x8000 to 0x87fB
• 0xfffc – overall checksum for the 64K program combining block 1 and 2, or for block 2 alone

Figure 1-2 is a flowchart showing how UCD3138064(A) ROM handles the checksum verification.

A UCD3138128(A) has block0, block1, block2, and block3. There are four locations for checksum and each checksum is 8 bytes.

• 0x7f8 – boot checksum for block 0, sum of the pflash words from address 0 to 0x7f7
• 0x7ff8 – overall checksum for block 0, sum of the pflash words from address 0 to 0x7ff7
• 0xfff8 – overall checksum for the 64K program combining block 0 and 1, sum of the pflash words from address 0 to 0xfff7
• 0x1fff8 – overall checksum for the 128K program, or the 64K program combining block 2 and 3

Figure 1-3 is a flowchart showing how UCD3138128(A) ROM handles the checksum verification.