EE 308 Lab 11

EE 308 - LAB 11

Final version for 2001

The 68HC12 Analog to Digital Converter

Note: Do this lab without your memory expansion board.

The analog to digital converter is described in Section 17 of the M68HC12B Family Manual. The analog to digital converter can have up to eight inputs on pins Port AD0 through Port AD7. (Ports AD0 and AD1 are used by DBug-12 at startup to determine whether to execute DBug12 or run code from EEPROM or the bootlaoder. You should not use Ports AD0 or AD1 as A/D inputs unless all other six inputs are being used.) The A/D converter also uses two dedicated pins V_RH and V_RLfor high and low voltage references respectively. On your HC12 board, V_RHis connected to V_CC, and V_RL is connected to GND. An input voltage of V_RL gives an output of 0x00, and an input of V_RH gives an output of 0xFF. If we measure a voltage between V_RL and V_RH, we can compute the value by simple ratios

$\begin{displaymath}voltage = {\frac{measurement * (V_{RH} - V_{RL})}{1023}} + V_{RL}\end{displaymath}$

For example, if V_RH = 5 volts, and V_RL = 0 volts, and the measurement is 0x2B0, then the measured voltage is

$\begin{displaymath}\frac{172 * (5 - 0)}{1023} + 0 = 3.363 \,\,\, {\rm volts.}\end{displaymath}$

Note that the above equations apply when the A/D converter is used in 10-bit mode. When used in 8-bit mode, replace the 1023 by 255.

To make sure that you do not damage the A/D converter on your HC12 you should be sure to do the following:

You should have a 1 to 10 KOhm resistor in series with the A/D input pin to prevent damage if your input voltage rises above V_RH.
You should never allow the voltage at the A/D pin to go below V_RL.

1.: Write a program to take a 0 to 5 Volt analog input from PAD3, convert it to an 8-bit digital number, and output the 8 bits of the converted digital value on Port A of your HC12. Set the A/D converter to do continuous conversion of the single channel AD3. Thus, all of the result registers will contain converted values from channel 3. Use your LED's to display one of the result registers. Display the A/D output once every 64 ms. Use the RTI function of the HC12 to do this.
2.: Connect a voltage from a pot to PAD3, so you can vary PAD3 from 0 to 5 Volts. Compare the value displayed on the LEDs with multimeter measurements for several different input voltages.
3.: The A/D conversion measurements can be improved by averaging the values in the registers ADR0H through ADR7H. Display the average of the 8 result registers on Port B. Is the value more stable than it was when you displayed the unaveraged value?
4.: Change your program to do the conversions in 10-bit mode. With an input voltage of about 2.5 volts, record the values in the eight results register ADR0 through ADR7. How much do they vary?

Bill Rison
2000-04-14