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ECE/CS 3720: Embedded System Design
(ECE 6960/2 and CS 6968)
Chris J. Myers
Lecture 14: Output Compare
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Basic Principles of Output Capture
(^) can create square waves, generate pulses,
- •^ Each output capture module has:^ •^ Can also use with input capture to measure frequency.implement time delays, and execute periodic interrupts. An external output pin, OCn
A flag bit
A force control bit FOCn
Two control bits, OMn, OLn
An interrupt mask bit (arm)
A 16-bit output compare register
1
Slide 3
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Basic Components of Output Compare
Slide 4
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Basic Principles of Output Compare (cont)
- (^) Output compare event occurs and sets flag when either: • (^) Output compare pin can control an external device.
- The software writes a 1 to the FOC bit.1. The 16-bit TCNT matches the 16-bit OC register
- (^) Two or three actions result from a compare event: • (^) OMn, OLn bits specify effect of event on the output pin.
- An interrupt is requested if the mask is 1.2. The output compare flag is set.1. The OCn output bit changes
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Applications of Output Compare
(^) fixed (^) time delay.
- Calculate TCNT+1. Read the current 16-bit TCNT
fixed
- Set 16-bit output compare register to TCNT+
fixed
- Wait for the output compare flag to be set 4. Clear the output compare flag
- (^) Maximum delay is 65,536 cycles. • (^) Delay of steps 1 to 4 sets the minimum delay.
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Output Compare Interface on 68HC
3
Slide 7
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Control Bits and Flags
OMn
OLn Effect of when TOCn=TCNT
(^0) (^0)
Does not affect OCn
(^0) (^1)
Toggle OCn
(^1) (^0)
Clear OCn=
(^1) (^1)
Set OCn=
Slide 8
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Setting the TFLG1 Register
ldaa #$20^ •^ The following works:^ •^ Care again must be taken when clearing TFLG1.
TFG1 = 0x20;
ldx^ •^ The following does not:staa $ (^) #$
TFLG1 |= 0x20;
bset $23,X,$
4
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Periodic Interrupt Using Output Capture
#define OC5 #define Rate 2000
0x
unsigned int Time;
(^) // Inc every 1ms
void TOC5handler(void){#pragma interrupt_handler TOC5handler() TFLG1=OC5;
// Ack interrupt
Time++; }TOC5=TOC5+Rate; // Executed every 1 ms
asm(" sei");void ritual(void) {
// make atomic
TMSK1|=OC5;
(^) // Arm output compare 5
TFLG1=OC5;Time = 0;
// Initially clear OC5F
TOC5=TCNT+Rate; // First one in 1 ms
asm(" cli"); } & '
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Square-Wave Generation
Component
6811
Process the interrupt (cycles,
μs)
14= μs
Execute the entire handler (cycles,
μs) 34= μs
Minimum period (
μs)
(^24) μs
Interrupt every
Time to process
Freq.
Period (cycles)
(cycles)
Overhead (%)
10 Hz
100 ms 50,
(^48)
100 Hz
10 ms 5,
(^48)
1
1 kHz
1 ms (^500)
(^48)
10
5 kHz
(^200) μs (^200)
(^48)
24
1/P
P (μ s) P
(^48)
4800/P
7
Slide 15
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Initialization for Square-Wave Generation
Period rmb
(^2)
;units sec
Init
sei
;make atomic
oraa #$20ldaa TMSK1 ;Old value
(^) ;TMSK1 OC3I=
anda #$CFldaa TCTL1staa TMSK1 ;Arm OC3F
(^) ;OM3=
oraa #$
(^) ;OL3=
ldaa #$20staa TCTL
(^) ;clear OC3F
lddstaa TFLG (^) TCNT (^) ;current time
stdaddd #2000 ;first in 1 ms (^) TOC
cli
;enable
rts
Slide 16
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ISR for Square-Wave Generation
OC3HAN ldaa #$
;clear OC3F [2]
staa TFLG1 ;Ack
[4]
ldd (^) TOC
[5]
addd Period ;next
[6]
std (^) TOC
[5]
rti
[12]
org (^) $FFE
fdb (^) OC3HAN
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Square-Wave Generation in C
unsigned int Period;
// Period in usec
void TOC3handler(void){#pragma interrupt_handler TOC3handler() TOC3=TOC3+Period;
(^) // calculate Next
TFLG1=0x20;}
// ack,
(^) OC3F=
void ritual(void){ asm(" sei");
// make atomic
TFLG1 = 0x20;
// clear OC3F
TMSK1|= 0x20;
// arm OC
TOC3TCTL1 = (TCTL1&0xCF)|0x10;
= TCNT+50; // first right away
asm(" cli"); }
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Pulse-Width Modulation
Component
Process the interrupt (cycles)
Execute the handler (cycles)
Total time T (cycles)
9
Slide 19
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High rmb
(^) 2 ;number of cycles high
Low (^) rmb (^) 2 ;number of cycles low
RITUAL sei
;make atomic
oraa #$20ldaa TMSK1 ;Old value
(^) ;TMSK1 OC3I=
oraa #$30ldaa TCTL1staa TMSK1 ;Arm OC3F
(^) ;OM3=1, OL3=
ldaa #$20staa TCTL
(^) ;clear OC3F
lddstaa TFLG (^) TCNT (^) ;current time
addd #
;first in 25s
std (^) TOC
cli
;enable
rts
Slide 20
& ' ISR for Pulse-Width Modulated Square-Wave
OC3HAN ldaa #$
;clear OC3F [2]
staa TFLG1 ;Ack
[4]
bita #$10ldaa TCTL2 ;rise/fall? [4]
[2]
beq (^) zero
[3]
one
ldd (^) TOC
[5]
addd High ;OC3 is 1
[6]
std (^) TOC
[5]
ldaa TCTL
[4]
anda #$BF
[2]$
staa TCLT
[4]
bra (^) done
[3]
10
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Delayed Pulse Generation in C
void Pulse(unsigned int Delay, unsigned int Width){
asm(" sei");
(^) // make atomic
OC1M=0x20;TOC3=TOC1+Width;TOC1=TCNT+Delay;
// connect OC1 to PA5/OC
OC1D=0x20;
// PA5=1 when TOC1=TCNT
TCTL1=(TCTL1&0xCF)|0x20;
// PA5=0 when TOC3=TCNT TFLG1 = 0x20;
// Clear OC3F
TMSK1|= 0x20;
// Arm OC3F
asm(" cli");}
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Delayed Pulse Generation in C (cont)
void TOC3handler(void){ #pragma interrupt_handler TOC3handler() OC1M=0;
// disconnect OC1 from PA
TCTL1&=0xCF;OC1D=0;
(^) // disable OC
TMSK1&=0xDF;} // disarm OC3F
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Slide 27
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Frequency Measurement
- (^) Input Capture handler incrementscapture to create a fixed time interval. • (^) Can use input capture to count pulses, and outputpulses for a fixed amount of time. • (^) Direct measurement of frequency involves counting input
(^) Counter
.
- (^) Output compare handler calculates frequency:
f (^) = fixed timeCounter
- (^) The frequency resolution is:
f (^) =
fixed time
Slide 28
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Frequency Measurement
14
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Frequency Measurement in C
#define IC1F 0x
// connected here
#define Rate 20000
(^) 10 ms
asm(" sei");void ritual(void) {#define OC5F 0x
// make atomic
TOC5=TCNT+Rate;TMSK1|=OC5F+IC1F; // Arm OC5 and IC
// First in 10 ms
TCTL2 = (TCTL2&0xCF)|0x10;
/* IC1F set on rising edges */ Count = 0;
// Set up for first
Done=0;
/* Set on the subsequent measurements */ TFLG1=OC5F+IC1F;
// clear OC5F, IC1F
asm(" cli"); } & '
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Frequency Measurement in C (cont)
void TIC1handler(void){ #pragma interrupt_handler TIC1handler() Count++;
// number of rising edges
TFLG1=IC1F;}
(^) // ack, clear IC1F
void TOC5handler(void){#pragma interrupt_handler TOC5handler() TFLG1= OC5F;
// Acknowledge
Freq = Count;TOC5 = TOC5+Rate; // every 10 ms
// 100 Hz units
Count = 0; }Done = 0xff;
// Setup for next 15
Slide 31
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Conversion Between Frequency and Period
- (^) Could measure frequency from period measurement:
f (^) = p 1
- (^) If range of period measurement is 36
μs to 32ms with
resolution of 500ns, frequency range is 31 to 27,778Hz.
f (^) = p (^) · 1 (^500) ns 1 (^) = 2000000 p
- (^) Resolution relationship is not as obvious: ∆f (^) =
1
(1/f (^) ) (^) − (^) ∆ p (^) − (^) f =
1
(1/f (^) ) (^) − (^500) ns (^) − (^) f
Slide 32
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Relationship Between Frequency and Period
Frequency (Hz)
Period (
μs) ∆f (Hz)
31,
(^32)
500
20,
(^50)
200
10,
100
50
5,
200
13
2,
500
2
1,
1,
500
2,
200
5,
100
10,
(^50)
20,
32,
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Basic Time Involved in Frequency Measurement
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Frequency Measurement in C
#define IC1F 0x
// connected here
#define Rate 50000
(^) 25 ms
asm(" sei");void ritual(void) {#define OC5F 0x
// make atomic
TOC5=TCNT+Rate;TMSK1|=OC5F+IC1F; // Arm OC5 and IC
(^) // First in 25 ms
TCTL2 = (TCTL2&0xCF)|0x10;
/* IC1F set on rising edges */ Count = 0;
// Set up for first
Done=0;
(^) // Set on subsequent meas
TFLG1 = OC5F+IC1F; // Clear OC5F IC1FFourHundred=0;
asm(" cli"); }
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Slide 39
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Frequency Measurement in C (cont)
void TIC1handler(void){ #pragma interrupt_handler TIC1handler() Count++;
// number of rising edges
TFLG1=IC1F;}
(^) // ack, clear IC1F
void TOC5handler(void){#pragma interrupt_handler TOC5handler() TFLG1= OC5F;
// Acknowledge
if (++FourHundred==400){TOC5 = TOC5+Rate; // every 25 ms Freq = Count;
0.1 Hz units
Count = 0; }}Done = 0xff;FourHundred=0;
// Setup for next 20
