Texas Instruments MSC1210 - Manual (Page 154)

Summation/Shifter Register
12-18
12.13.1 Manual Summation Mode
The first mode of operation, manual summation, allows you to quickly add
32-bit values. In this mode, your program simply write the values to be added
to the SUMR0, SUMR1, SUMR2, and SUMR3 SFRs. When a value is written
to SUMR0, the current value of SUMR0-3 will be added to the summation
register. For example, the following code will add 0x00123456 to
0x0051AB04:
SSCON = 0x00; // Clear summation register, manual summation
SUMR3 = 0x00; // High byte of 0x00123456
SUMR2 = 0x12; // Next byte of 0x00123456
SUMR1 = 0x034; // Next byte of 0x00123456
SUMR0 = 0x56; // Next byte of 0x0012345 – Perform addition
SUMR3 = 0x00; // High byte of 0x0051AB04
SUMR2 = 0x51; // Next byte of 0x0051AB04
SUMR1 = 0xAB; // Next byte of 0x0051AB04
SUMR0 = 0x04; // Next byte of 0x0051AB04 – Performs addition
ANSWER = (SUMR3 << 24) + (SUMR2 << 16) + (SUMR1 << 8) + SUMR0;
The previous code, although certainly more verbose than a simple
ANSWER = 0x00123456 + 0x0051AB04 instruction in ‘C’, is much, much
faster when analyzed in assembly language. In assembly language, the above
solution requires just four MOV instructions for each summation, whereas the
simple addition approach (which does not take advantage of the MSC1210
summation register) takes at least 8 MOV instructions and 4 ADD instructions.
12.13.2 ADC Summation Mode
The ADC summation mode functions very similarly to the manual summation
mode, but instead of your program writing values to the SUMRx registers, the
ADC writes values to the SUMRx registers.
In this mode, the CNT bits of SSCON are set to indicate how many ADC
conversions should be summed in the summation register. The ADC will then
deliver the requested number of results to the summation register and trigger
a summation auxiliary interrupt, if enabled (see Chapter 10, Interrupts).
SSCON = 0x00; // Clear summation register, manual summation
SSCON = 0x50; // ADC summation, 8 samples from ADC
while(! (AISTAT & 0x40)); // Wait for 8 samples to be added
SUM = (SUMR3 << 24) + (SUMR2 << 16) + (SUMR1 << 8) + SUMR0;
The previous code first clears the summation registers by setting SSCON to
0, and then sets SSCON to ADC summation and requests that eight samples
from the ADC be summed. The while() loop then waits for the summation auxil-
iary interrupt flag to be set, which indicates the requested operation was com-
plete. The final line then takes the four individual SFRs and calculates the total
summation value.