Use of the HC12 A/D Converter

USING THE HC12 A/D CONVERTER

Power up A/D Converter (ADPU = 1 in ATDCTL2)
Set ATDCTL4 = 0x01 (Gives 2 MHz AD clock with 8 MHz E-clock, 8-bit mode)
Select 8-channel mode (S8CM = 1 in ATDCTL5)
Set CD = 0 in ATDCTL5 (CD = 1 for factory test only)
Select MULT in ATDCTL5:
- MULT = 0: Convert one channel eight times
  - Choose channel to convert with CC, CB, CA of ATDCTL5.
- MULT = 1: Convert eight channels
Select SCAN in ATDCTL5:
- SCAN = 0: Convert eight samples, then stop
- SCAN = 1: Convert continuously
After writing to ATDCTL5, the A/D converter starts, and the SCF bit is cleared. After eight conversions are complete, the SCF flag in ATDSTAT is set. You can read the results of the conversions in ADR[0-7]H.
If SCAN = 0, you need to write to ATDCTL5 to start a new sequence. If SCAN = 1, the conversions continue automatically, and you can read new values in ADR[0-7]H.
To get interrupt after eight conversions completed, set ASCIE bit of ATDCTL2. After eight conversions, ASCIF bit in ATDCTL2 will be set, and an interrupt will be generated.
With 8 MHz E-clock and ATDCTL4 = 0x01, it takes 9 $\mu$ s to make one conversion, 72 $\mu$ s to make eight conversions.
On HC12 EVBU, AD channels 0 and 1 are used to determine start-up program (D-Bug12, EEPROM or bootloader). Do not use AD channels 0 or 1 unless absolutely necessary (you need 7 or 8 channels). If you do need AD channels 0 and/or 1, power up EVBU, then remove jumpers which select start-up program.
$\begin{displaymath}{\tt ADRx[15..6]} = \frac{V_{in} - V_{RL}}{V_{RH}-V_{RL}} \times 1024\end{displaymath}$

Normally, $V_{RL}$ = 0 V, and $V_{RH}$ = 5 V, so

$\begin{displaymath}{\tt ADRx[15..6]} = \frac{V_{in}}{\rm 5~V} \times 1024\end{displaymath}$

Example: ADR0[15..6] = 448 => $V_{in}$ = 2.19 V
To use 10-bit result, set ATDCTL4 = 0x81 (Gives 2 MHz AD clock with 8 MHz E-clock, 10-bit mode),and add the following to hc12.h:
```
#define ADR0   (* (volatile unsigned int *)(_BASE+0x70)) 
#define ADR1   (* (volatile unsigned int *)(_BASE+0x72)) 
.
.
```

You can get more accuracy by averaging multiple conversions. If you need only one channel, set MULT = 0, then average all eight result registers:

   int avg;

   avg = ((ADR0>>6) + (ADR1>>6) 
        + (ADR2>>6) + (ADR3>>6)
        + (ADR4>>6) + (ADR5>>6) 
        + (ADR6>>6) + (ADR7>>6)) >> 3;

Bill Rison
2001-04-09