$24
Lab 6: AVR timer and synchSMs (Reflex game) (1 Day) Solution
Zyante Chapter 4: Synchronous State Machines (Part A)
A clean timer abstraction and SynchSMs
Like most microcontrollers, the ATmega1284 has built-in timers. Also like most microcontrollers, the ATmega1284's timers have numerous low-level configurable options. However, our disciplined embedded programming approach, as described in PES, uses a clean abstraction of a timer involving just a few simple functions:
void TimerSet(unsigned char M) -- set the timer to tick every M milliseconds
void TimerOn() -- inititialize and start the timer
void TimerOff() -- stop the timer
void TimerISR() -- called automatically when the timer ticks, with contents filled by the user ONLY with an instruction that sets the user-declared global variable TimerFlag=1
The following program contains variable and function declarations that map the ATmega1284's low-level timer constructs to the above clean abstraction. Do NOT copy and paste the code. Type it out by hand to avoid errors.
#include <avr/io.h
#include <avr/interrupt.h
volatile unsigned char TimerFlag = 0; // TimerISR() sets this to 1. C programmer should clear to 0.
Internal variables for mapping AVR's ISR to our cleaner TimerISR model. unsigned long avr_timer_M = 1; // Start count from here, down to 0. Default 1 ms. unsigned long avr_timer_cntcurr = 0; // Current internal count of 1ms ticks
void TimerOn() {
// AVR timer/counter controller register TCCR1
TCCR1B = 0x0B;// bit3 = 0: CTC mode (clear timer on compare)
bit2bit1bit0=011: pre-scaler /64
00001011: 0x0B
SO, 8 MHz clock or 8,000,000 /64 = 125,000 ticks/s
Thus, TCNT1 register will count at 125,000 ticks/s
AVR output compare register OCR1A.
OCR1A = 125; // Timer interrupt will be generated when TCNT1==OCR1A
We want a 1 ms tick. 0.001 s * 125,000 ticks/s = 125
So when TCNT1 register equals 125,
1 ms has passed. Thus, we compare to 125.
AVR timer interrupt mask register
TIMSK1 = 0x02; // bit1: OCIE1A -- enables compare match interrupt
//Initialize avr counter
TCNT1=0;
_avr_timer_cntcurr = avr_timer_M;
// TimerISR will be called every _avr_timer_cntcurr milliseconds
//Enable global interrupts
SREG |= 0x80; // 0x80: 1000000
}
void TimerOff() {
TCCR1B = 0x00; // bit3bit1bit0=000: timer off
}
void TimerISR() {
TimerFlag = 1;
}
In our approach, the C programmer does not touch this ISR, but rather TimerISR() ISR(TIMER1_COMPA_vect) {
CPU automatically calls when TCNT1 == OCR1 (every 1 ms per TimerOn settings) _avr_timer_cntcurr--; // Count down to 0 rather than up to TOP
if (_avr_timer_cntcurr == 0) { // results in a more efficient compare TimerISR(); // Call the ISR that the user uses _avr_timer_cntcurr = avr_timer_M;
}
}
Set TimerISR() to tick every M ms void TimerSet(unsigned long M) {
_avr_timer_M = M;
_avr_timer_cntcurr = avr_timer_M;
}
void main()
{
DDRB = 0xFF; // Set port B to output
PORTB = 0x00; // Init port B to 0s
TimerSet(1000);
TimerOn();
unsigned char tmpB = 0x00;
while(1) {
// User code (i.e. synchSM calls)
tmpB = ~tmpB; // Toggle PORTB; Temporary, bad programming style
PORTB = tmpB;
while (!TimerFlag); // Wait 1 sec
TimerFlag = 0;
Note: For the above a better style would use a synchSM with TickSM()
This example just illustrates the use of the ISR and flag
}
}
The above sample main code toggles port B every 1 second (Note: main's code is used for simple illustration and is itself not good style). Load the project into the chip memory. Any LEDs connected to port B pins should now blink on 1 sec and off 1 sec. (If you're curious, you can try setting the oscillator fuse to a 1 MHz setting, and you'll see slower blinking).
Pre-lab
Draw your synchSM for exercise 1, and write out the C code. Do not use RIBS to auto-generate the C code.
Exercises
Implement the following. For each, create a synchSM, then implement in C, and map to your board. You may optionally use RIBS to help generate code for exercises other than exercise 1.
Create a synchSM to blink three LEDs connected to PB0, PB1, and PB2 in sequence, 1 second each. Implement that synchSM in C using the method defined in class. In addition to demoing your program, you will need to show that your code adheres entirely to the method with no variations.
Video Demonstration: http://youtu.be/ZS1Op26WiBM
(Challenge) Create a simple light game that requires pressing a button on PA0 while the middle of three LEDs on PB0, PB1, and PB2 is lit. The LEDs light for 300 ms each in sequence. When the button is pressed, the currently lit LED stays lit. Pressing the button again restarts the game.
Video Demonstration: http://youtu.be/inmzsXz_HG0
Don’t forget to commit and push your code to Github before shutting down the VM!
