Analog-to-Digital Converter - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 612

• Stop AD0 when in wait mode (set bits 5 to 1)
• Disable external trigger on channel 7 (set bits 4, 3, and 2 to 0)
• Disable AD0 interrupt (set bit 1 to 0)
Write the value $E0 into this control register.
The setting of the ATD control register 3 is as follows:
• Perform four conversions
• Disable FIFO mode
• When BDM becomes active, complete the current conversion then freeze
Write the value $22 into ATD0CTL3.
The setting of the ATD control register 4 is as follows:
• Select 10-bit operation (set bit 7 to 0)
• Set sample time to two A/D clock periods (set bits 6 and 5 to 00)
• Set the value of PRS4,PRS0 to 00101
Write the value $05 to this control register.
The setting of the ATD control register 5 is as follows:
• Result register right-justified (set bit 7 to 1)
• Result is unsigned (set bit 6 to 0)
• Nonscan mode (set bit 5 to 0)
• Single-channel mode (set bit 4 to 0)
• Select channel 7 (set bits 2..0 to 111)
Write the value $87 to this control register
The following subroutine will perform the desired ATD confi guration:
#include “c:\miniide\hcs12.inc”

openAD0 movb #$E0,ATD0CTL2

jsr wait20us ; wait for 20 μ s

movb #$22,ATD0CTL3

movb #$05,ATD0CTL4

rts

wait20us movb #$90,TSCR1 ; enable TCNT and fast timer flag clear

movb #0,TSCR2 ; set TCNT prescaler to 1

bset TIOS,$01 ; enable OC0

ldd TCNT ; start an OC0 operation

addd #480 ; “

std TC0 ; “

brclr TFLG1,C0F,* ; wait for 20 μ s

rts
This routine does not write into the ATD0CTL5 register because that would start the conver-
sion. We should write into the ATD0CTL5 register only when we want to perform the conversion.
The C language version of the subroutine is as follows:
void wait20us (void);
void openAD0 (void)
{

ATD0CTL2 5 0xE0;

wait20us();