Overflow and Underflow Logic

The following discussion of overflow and underflow applies to sequential accumulators (MACC or Adder-Accumulator) implemented in a single DSP58. The accumulator must have at least one guard bit. When the pattern detector is set to detect a pattern equal to 00000…0 with a mask of 0011111 …1 (default settings), DSP58 flags overflow beyond 00111 … 1 or underflow beyond 11000… 0. The USE_PATTERN_DETECT attribute is set to PATDET to enable the use of the pattern detect logic. This overflow/underflow implementation uses a redundant sign bit and reduces the output bit width to 57 bits.

By setting the mask to other values like 0000111 …1, the bit value P[N] at which overflow is detected can be changed. This logic supports saturation to a positive number of 2^N –1 and a negative number of 2^N in two’s complement where N is the number of 1s in the mask field.

To check overflow/underflow condition for N = 2, the following example is used:

• Mask is set to 0...11.
• The (N) LSB bits are not considered for the comparison.
• For N = 2, the legal values (patterns) are 2²–1 to –2², or 3 to –4.

See the following figures for overflow and underflow examples. The pattern detect logic asserts the overflow/underflow signal for only one clock cycle in the same cycle in which the P output that caused the overflow/underflow is produced.

• PATTERNDETECT is 1 if P = = pattern or mask
• PATTERNBDETECT is a 1 if P = = patternb or mask

Overflow is caused by addition when the value at the output of the adder/subtracter/logic unit goes over 3. Adding 1 to the final value of 0..0011 gives 0..0100 as the result, which causes the PATTERNDETECT output to go to 0. When the PATTERNDETECT output goes from 1 to 0, an overflow is flagged.

Underflow is caused by subtraction when the value goes below –4. Subtracting 1 from 1..1100 yields 1..1011 (–5), which causes the PATTERNBDETECT output to go to 0. When the PATTERNBDETECT output goes from 1 to 0, an underflow is flagged.