Page 211 - Programming Microcontrollers in C

196 Chapter 4 Small 8-Bit Systems

The following function will provide a reading of a single channel of
the ADC input:
unsigned int read_adc(int k)
{
AD_CTST &=~0X7;
AD_CTST |=k;
while(AD_CTST.COCO==0)
; /* wait here til COCO is set */
return AD_DATA;
}
The argument k is the channel that is to be read, and k can have
a value of 0 to 7 to read the external channels.
The first two lines of code in the above function will place the
channel number to be read in the channel bits of AD_CTST. These
bits must be cleared by an instruction sequence that will not alter the
upper bits of AD_CTST because the ADON bit is in the upper portion
of AD_CTST. This bit cannot be reset while the ADC operation is
continuing. The first line of code clears the least significant three
bits, and the second line places the channel number in these bits.
Writing to AD_CTSTwill cause the ADC conversion to start. There­
fore, all that must be done is to wait until the conversion is completed
to read the data into the program. The code

while(AD_CTST.COCO==0)
; /* wait here til COCO is set */
will keep control of the microcontroller in that instruction sequence
until the COCO bit, which is the conversion completion bit, is set. At
that time the value found in AD_DATA will be the result of the latest
conversion.
Often, the ADC results must be subjected to some processing to
remove unwanted characteristics of the signal being measured. Here
is a case where careful use of assembly language procedures can
make a big difference in the execution speed as well as the amount of
code needed. An example that is often used is to average the past
values of the data. A reasonably simple approach is to allow the lat­
est ADC reading to have a 50% weight and all of the past readings to
have a 50% weight. The following example code will accomplish
this task in three different ways: