EE 308: Microcontrollers

Transcription

1 EE 308: Microcontrollers Timers Aly El-Osery Electrical Engineering Department New Mexico Institute of Mining and Technology Socorro, New Mexico, USA April 2, 2018 Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

2 Timers A counter module provided by most microcontrollers either with 8-bits or 16-bits resolution or both. Timers are used for Counting Timestamp Delays Waveform generation Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

3 Counter For a n-bit resolution counts from 0 to 2 n 1 If using a 16-bit timer on an 8-bit microcontroller, a buffer must be used otherwise the bytes might change during the two read cycles. Can causes an interrupt when an overflow is reached Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

4 Prescaler Provides means to divide the clock to count slower A value is incremented using the system clock and a bit from that value is used to increment the timer Changing the prescaler might cause an offset between data processed at the system clock vs actions done on the timer tick Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

5 Input Capture Captures and timestamps an external event can then be used to frequency duty-cycle other signal features Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

6 Output Compare When a value is reached on the timer, an output is changed The timer value is compared to a value stored in an output compare register Used to change a state or cause an interrupt Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

7 Pulse Width Modulation (PWM) Needs two registers one for the period, and one for the duration during which the pulse is high (or low) Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

8 Registers TCNTn Timer/counter register On reset is set to zero Holds the count value TOVn Timer overflow: when the timer overflows this flag is set TCCRn Timer/control register: sets the mode of operation ICFn Input capture flag OCRn Output compare register: content of TCNTn is compared with the OCRn if equal the output compare flag (OCFn) will be set TIFR Timer/counter interrupt flag register contains: TOVn, OCFn and ICFn Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

9 8-bit Timer BOTTOM: zero value MAX: maximum value, e.g., 255 for 8-bit and for 16-bit TOP: highest assigned value either MAX or OCRn register value Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

10 Timer control unit Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

11 Output compare Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

12 Output match Need to set the data direction of the pin to an output Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

13 Modes of operation Set using the TCCRn register Normal Clear timer on compare match (CTC) Fast PWM Phase correct PWM Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

14 Normal mode 1 counts up 2 resets to zero when it passes max TOP value 3 TOVn flag is set on same clock cycle when TCNTn becomes zero 4 the TOVn flag is cleared by writing a 1 to it (True for all flags) Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

15 Using timers to generate a delay Computing the value to be loaded into the timer to generate a time delay T 1 Compute T clk = 1 F Timer 2 Compute the desired number of clock cycles n = T T clock 3 Load TCNTn register with the hex value of 256 n Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

16 Using timers to generate a delay Computing the value to be loaded into the timer to generate a time delay T 1 Compute T clk = 1 F Timer 2 Compute the desired number of clock cycles n = T T clock 3 Load TCNTn register with the hex value of 256 n Can the TCNTn be computed using negative values? Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

17 Delay subroutine using Timers delay : ldi r20,0 xce out TCNT0, r20 ldi r20,0 x00 out TCCR0A, r20 ldi r20,0 x01 out TCCR0B, r20 again : in r20, TIFR0 sbrs r20, TOV0 rjmp again ldi r20,0 x00 out TCCR0B, r20 ldi r20,(1 < < TOV0 ) out TIFR0, r20 ret Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

18 Delay subroutine using Timers delay : ldi r20,0 xce out TCNT0, r20 ldi r20,0 x00 out TCCR0A, r20 ldi r20,0 x01 out TCCR0B, r20 again : in r20, TIFR0 sbrs r20, TOV0 rjmp again ldi r20,0 x00 out TCCR0B, r20 ldi r20,(1 < < TOV0 ) out TIFR0, r20 ret What is the amount of delay generated excluding the code overhead? Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

19 Using C # define FOSC ( UL) // Clock Speed void delay () void main ( void ) { DDRB = 0 xff ; while (1) { PORTB = PORTB ^ (1<< PB4 ); delay (); } void delay () { TCNT0 = 0 xce ; TCCR0A = 0x00 ; TCCR0B = 0x01 ; } while (( TIFR0 & 0x01 ) == 0); TCCR0B = 0x00 ; TIFR0 = 0 x01 ; Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

20 Counting an external event. nolist. include " m1284def. inc ". list cbi DDRB,0 ldi r20,0 xff out DDRC, r20 ldi r20,0 x00 out TCCR0A, r20 ldi r20,0 x07 out TCCR0B, r20 again : in r20, TCNT0 out PORTC, r20 in r16, TIFR0 sbrs r16, TOV0 rjmp again ldi r16,1<< TOV0 out TIFR0, r16 rjmp again Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

21 Using C # define FOSC ( UL) // Clock Speed # include <avr / io.h> int main () { PORTB = 0 X01 ; DDRC = 0 XFF ; TCCR0A = 0 x00 ; TCCR0B = (1<< CS00 ) (1 < < CS01 ) (1 < < CS02 ); while (1) { do { PORTC = TCNT0 ; } while (( TIFR0 & (1<< TOV0 ))==0); } TIFR0 = (1<< TOV0 ); } return (0); Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20

22 Assembly code. nolist. include " m1284def. inc ". list. section. text. global main. org 0 x00 rjmp main. org OC0Aaddr *2 rjmp oc0_isr main : ldi r16, lo8 ( RAMEND ) out SPL, r16 ldi r16, hi8 ( RAMEND ) out SPH, r16 ; add some port setup code ; configure timer ; initialize timer ; enable output compare interrupt ; start timer sei here : rjmp here oc0_isr : ; add your interrupt code reti Aly El-Osery (NMT) EE 308: Microcontrollers April 2, / 20