User guide
12.2 Hardware Multiplier Controls
The selection of operation to be performed by the multiplier is determined by four control bits in the MCNT register: SUS, MSUB, MMAC,
and SQU. The number of operands that must be loaded to trigger the specified operation is dictated by the OPCS bit setting, except
when the square function is enabled (SQU = 1). Enabling the square function implicitly defines that only a single operand (either MA or
MB) needs to be loaded to trigger the square operation, independent of the OPCS bit setting. The MCNT register bits must be config-
ured to select the desired operation and operand count prior to loading the operand(s) to trigger the multiplier operation. Any write to
MCNT automatically resets the operand load counter of the multiplier, but does not affect the operand registers, unless such action is
requested using the Clear Data Registers (CLD) control bit. Once the desired operation has been specified via the MCNT register bits,
loading the prescribed number of operands triggers the respective multiply, multiply-accumulate/subtract or multiply-negate operation.
12.3 Register Output Selection
The Hardware Multiplier implements the MC Register Write Select (MCW) control bit so that writing of the result to the MC2:MC0 reg-
isters can be blocked to preserve the MC registers (accumulator). When the MCW bit is configured to logic 1, the result for the given
operation is not written to the MC registers. When the MCW bit is configured to logic 0, the MC registers are updated with the result of
the operation. The MC1R, MC0R read-only register pair are updated independent of the MCW bit setting. This register pair always
reflect the output that would normally be placed in MC1:MC0, given that MCW = 1 or MMAC = 0. When MCW = 0 and MMAC = 1, the
MC1R:MC0R content may not match the MC1:MC0 register content, but it will be predictable and may be useful in certain situations.
See Table 12-1 for details.
12.3.1 Signed-Unsigned Operand Selection
The operands can be either signed or unsigned numbers, but the data type must be defined by the user software via the Signed-
Unsigned (SUS) bit prior to triggering the operation. For an unsigned operation, the Signed-Unsigned bit (SUS) in the MCNT register
must be set to 1; for a signed operation, the SUS bit must be cleared to 0. The multiplier treats unsigned numbers as absolute magni-
tude. For a 16-bit positional binary number, this represents a value in the range 0 to 2
16
- 1 (xFFFFh). The signed number representa-
tion is a two's-complement value, where the most significant bit is defined as a sign bit. The range of a 16-bit two's-complement num-
ber is -2(
16-1)
(x8000h) to +2
(16-1)
- 1 (x7FFF). The product of any signed operation will be sign extended before being stored or accu-
mulated/subtracted into the MC registers. The SUS bit should always be configured to logic 0 (i.e., signed operands) for the multiply-
negate operation. Attempting an unsigned multiply-negate operation results in incorrect results and setting of the OF bit. Modifying the
operand data type selection via the SUS bit does not alter the contents of the MC registers. The MC registers are read/write accessi-
ble and can be modified by user code when necessary.
12.3.2 Operand Count Selection
The OPCS bit allows selection of single operand or two operands operation for the multiply and multiply-accumulate/subtract opera-
tions. When the OPCS bit is cleared to 0, the multiply or multiply-accumulate/subtract operation established by the SUS, MSUB, and
MMAC bits is triggered once two operands are loaded, one to each of the MA and MB registers. When OPCS is set to 1, the opera-
tion commences once data is loaded to either MA or MB. The OPCS bit is ignored when the square operation is enabled (SQU), since
loading of data to the MA or MB register actually writes to both registers.
12.4 Hardware Multiplier Operations
The control bits, which specify data type (SUS), operand count (OPCS or SQU), and destination control (MCW), have already been
described. However, there are two additional MCNT register bits that serve to define the Hardware Multiplier operation. The multiply-
accumulate/subtract and multiply-negate operations are enabled by the Multiply-Accumulate Enable (MMAC) and Multiply Negate
(MSUB) bits in the MCNT register. When the MMAC bit is set to 1, the multiplier performs a multiply-accumulate (if MSUB = 0) or a mul-
tiply-subtract (if MSUB = 1). If MMAC is configured to 0, the multiplier result is not accumulated or subtracted, but can be stored direct-
ly (if MSUB = 0) or negated (if MSUB = 1) before storage. The multiply-negate operation (MMAC = 0, MSUB = 1) is only allowable for
signed data operands (SUS = 0). For unsigned multiply-accumulate/subtract operations, the OF bit is set when a carry-out/borrow-in
from the most significant bit of the MC register occurs. For a signed two’s-complement multiply-accumulate/subtract operations, the
OF bit is set when the carry-out/borrow-in from the most significant magnitude position of the MC register is different from the carry-
out/borrow-in of the sign position of the MC register. Since there is no overflow condition for multiply and multiply-negate operations,
the OF bit is always cleared for these operations with one exception. The OF bit will be set to logic 1 if an unsigned multiply-negate
(invalid operation) is requested. Table 12-1 shows the operations supported by the multiplier and associated MCNT control bit settings.
12-3
MAXQ Family User’s Guide
Maxim Integrated