C8051F360/1/2/3/4/5/6/7/8/9
11.6. Rounding and Saturation
A Rounding Engine is included, which can be used to provide a rounded result when operating on frac-
tional numbers. MAC0 uses an unbiased rounding algorithm to round the data stored in bits 31–16 of the
accumulator, as shown in Table 11.1. Rounding occurs during the third stage of the MAC0 pipeline, after
any shift operation, or on a write to the LSB of the accumulator. The rounded results are stored in the
rounding registers: MAC0RNDH (SFR Definition 11.12) and MAC0RNDL (SFR Definition 11.13). The accu-
mulator registers are not affected by the rounding engine. Although rounding is primarily used for fractional
data, the data in the rounding registers is updated in the same way when operating in integer mode.
Table 11.1. MAC0 Rounding (MAC0SAT = 0)
Accumulator Bits 15–0
(MAC0ACC1:MAC0ACC0)
Greater Than 0x8000
Less Than 0x8000
Equal To 0x8000
Equal To 0x8000
Accumulator Bits 31–16
(MAC0ACC3:MAC0ACC2)
Anything
Anything
Odd (LSB = 1)
Even (LSB = 0)
Rounding
Direction
Up
Down
Up
Down
Rounded Results
(MAC0RNDH:MAC0RNDL)
(MAC0ACC3:MAC0ACC2) + 1
(MAC0ACC3:MAC0ACC2)
(MAC0ACC3:MAC0ACC2) + 1
(MAC0ACC3:MAC0ACC2)
The rounding engine can also be used to saturate the results stored in the rounding registers. If the
MAC0SAT bit is set to ‘1’ and the rounding register overflows, the rounding registers will saturate. When a
positive overflow occurs, the rounding registers will show a value of 0x7FFF when saturated. For a nega-
tive overflow, the rounding registers will show a value of 0x8000 when saturated. If the MAC0SAT bit is
cleared to ‘0’, the rounding registers will not saturate.
11.7. Usage Examples
This section details some software examples for using MAC0. Section 11.7.1 shows a series of two MAC
operations using fractional numbers. Section 11.7.2 shows a single operation in Multiply Only mode with
integer numbers. The last example, shown in Section 11.7.3, demonstrates how the left-shift and right-shift
operations can be used to modify the accumulator. All of the examples assume that all of the flags in the
MAC0STA register are initially set to ‘0’.
11.7.1. Multiply and Accumulate Example
The example below implements the equation:
(
0.5
×
0.25
)
+
(
0.5
×
– 0.25
)
= 0.125 – 0.125 = 0.0
MOV
MOV
MOV
MOV
MOV
MOV
MOV
NOP
NOP
NOP
MAC0CF,
MAC0AH,
MAC0AL,
MAC0BH,
MAC0BL,
MAC0BH,
MAC0BL,
#0Ah
#40h
#00h
#20h
#00h
#E0h
#00h
; Set to Clear Accumulator, Use fractional numbers
; Load MAC0A register with 4000 hex = 0.5 decimal
;
;
;
;
Load
This
Load
This
MAC0B register
line initiates
MAC0B register
line initiates
with 2000 hex = 0.25 decimal
the first MAC operation
with E000 hex = -0.25 decimal
the second MAC operation
; After this instruction, the Accumulator should be equal to 0,
; and the MAC0STA register should be 0x04, indicating a zero
; After this instruction, the Rounding register is updated
120
Rev. 1.0
SILABS [ SILICON LABORATORIES ]
