Datasheet

PICkit™ 3 Debug Express

#5: To begin an ADC conversion, set bit 1 of ADCON0, the GO/DONE bit. When the

conversion is done the hardware will clear that bit, so the GO/DONE

may then be polled

to wait for the conversion to complete. Once the conversion is complete and GO/DONE

= 0, the ADC conversion result may be read from ADRESH and ADRESL.

3.7.3 Exploring the Lesson 7 Source Code

Open the lesson source files 07 ADC.c and 07 ADC.h in an MPLAB editor window if

they are not already open.

Of note is that the Timer0 setup code has been moved into a function and replaced with

a function call. Two new functions were added to support the ADC.

void Timer0_Init(void)

void ADC_Init(void) unsigned char

ADC_Convert(void)

The function prototypes have also been added to the header file, 07 ADC.h.

In main() before getting to the while(1) loop, the program makes two function calls

to set up the Timer0 and ADC peripherals using Timer0_Init() and ADC_Init(),

respectively.

To change the LED rotation speed based on the potentiometer, the ADC conversion

value is used to set Timer0 just after it overflows. The higher the value written to Timer0,

the less time it takes to overflow again, as the timer counts up from the written value.

This is accomplished with two new statements at the bottom of the while(1) loop:

The TMR0H buffer is written with the 8 Most Significant bits of the ADC conversion, and

then is written with Timer0 with a ‘0’ in the low byte on the TMR0L assignment

statement. Recall from Lesson 5 that since TMR0H is actually a buffer and not the

upper byte of the timer, and is written to the timer when TMR0L is written. Thus, it must

be written first as it is here.

We can calculate the amount of delay for a given ADC value, knowing that

Timer0_Init() sets the TMR0 prescaler to 1:4, and our oscillator is 1MHz. Timer0

will count at 4 * the instruction rate, or 4 * 1/(F

OSC/4) = 4 * 1/(1MHz/4) = 4 * 1/250kHz

= 16 us. The number of counts until overflow occurs is 0x10000 – (start count) where

(start count) is the value written to TMR0 – The ADC result in the upper byte and 0x00

in the lower. The total delay is then the number of counts times the count rate. For an

ADC result of 0x81, the delay is (0x10000 – 0x8100) * 16 us = 0x7F00 * 16 us = 32512

* 16 us = 0.52 seconds.

3.7.4 Build and Run the Lesson 7 Code with PICkit 3 Debug Express

Build and program the Lesson 7 project, then Run the application in the debugger.

Turning the demo board potentiometer will affect the rotation speed of the LEDs. The

switch may be pressed to reverse the rotation.

Halt the Lesson 7 program. Note that several SFRs and variables have already been

added to a Watch window. Use Breakpoints and Step commands to explore the code.

Observe how the ADC result in ADRESH is affected by the potentiometer voltage, and

how this result is copied into TMR0.

TMR0H = ADC_Convert(); // MSB from ADC

TMR0L = 0; // LSB = 0