ece 371- unit 11 timers and counters (“stop watches and alarm clocks”)
TRANSCRIPT
ECE 371- Unit 11
Timers and Counters
(“Stop Watches and Alarm Clocks”)
Port T – Enhanced Capture Timer
8 Input Capture – Output Compare Pins
“INPUT CAPTURE”
• Used to measure the characteristics of an input signal
- Measure the width of an input pulse
- Determine the period or duty cycle of an input signal.
Pulse width
“INPUT CAPTURE”
• Used to measure the characteristics of an input signal
- Measure the width of an input pulse
- Determine the period or duty cycle of an input signal.
A B
Duty Cycle = A/(A + B) * 100%
“OUTPUT COMPARE”
• Permits generation of output signals to user specification
- Single pulse
- Square wave
- PWM Signal
Single pulse - can control width of pulse and/or delay from trigger signal
T1
Trigger
T2
“OUTPUT COMPARE”
• Permits generation of output signals to user specification
- Single pulse
- Square wave
- PWM Signal
Square wave - can control period.
Period
“OUTPUT COMPARE”
• Permits generation of output signals to user specification
- Single pulse
- Square wave
- PWM Signal
PWM signal- can control duty cycle = T1/ (T1 + T2)
T1T2
Main Components of Timer Module
• Free Running 16-Bit Up-Counter
• Eight Input/Output Lines (Channels)
• Pulse Accumulator System
16-bit Free Running Counter
2 MHz
INPUT CAPTURE (“Stop Watch”/Signal Analysis)
• Captures the Current Count on the Free-Running Counter When a User-Specified Event Occurs on the Channel Pin
• Types of “events”
- rising edge on channel pin
- fall edge on channel pin
- rising or falling edge
INPUT CAPTURE
When the User-Specified Event Occurs:
1. Current value of free-running counter is latched into the Timer Channel Register.
2. Channel Flag is set.
3. Channel will generate an Interrupt (if the Channel’s Interrupt Enable Bit has been set.)
OUTPUT COMPARE
(“Alarm Clock”/Signal Generation)
• Causes a change on the channel output pin when the value in the Free-Running Timer matches the value in the Channel Register.
Actions that can activated at the channel output pin:
• Go high
• Go low
• Toggle
“Output Compare System for One Channel”
Special Channel for Output Compare: Channel 7
• The state of ALL channel output pins can be affected when value of free-running timer matches the target value of Channel 7.
• Two special registers make this possible:
- Mask Register (selects which Channels are to be included in this feature.
- “Value” Register (indicates what value each included channel is to take on)
Software interacts with Counter/Timer using registers:
Main Registers:
• Count Register (contains value of free-running counter)
• Control Registers (used to select option)
• Interrupt Mask Register (used to enable interrupts)
• Flags Registers (used to indicate when an interrupt condition has occurred and when free-running counter rolls over)
• Input Capture/Output Compare Registers (Alarm Clock/Stop Watch value for each timer)
• Input Capture/Output Compare Select Registers (used to specify whether a given channel is to be used as Input Capture or Output Compare.
16-bit Timer Counter Register
Bus ClockPrescaler
PR2,PR1,PR0
16-bitTimer Count Register
TCNTTOF
(free-running counter)
Timer Count Register
• Timer Count Register (TCNT) is constantly being incremented
• TOF is set when TCNT overflows
• Bus Clock Frequency and Prescaler determine the rate of increment
Timer Resolution
• 2 Mhz Bus Clock– Prescale=0b000 => 0.5 usec resolution
• TOF Set every 32758 usec.
– Prescale=0b001 => 1.0 usec resolution• TOF Set every 65536 usec.
– Prescale=0b111 => 64 usec resolution• TOF Set every 4.194304 Seconds
TSCR2 Example Code
/* Bit 4 – TCRE */TSCR2 = TSCR2 & 0xF7; // Set TCRE=0 (prevent special channel 7 // from reseting counter)
/* Bits 2, 1, 0 – Prescale */int prescale; // 3-bit prescale value/* Set Prescale Value */TSCR2 = (TSCR2 & 0xF8) | (0x07 & prescale);/* TSCR2 = xxxx xxxx 0xF8 = 1111 1000TSCR2&0xF8 = xxxx x000 0x07 = 0000 0111 prescale = yyyy yyyyPrescale&0x7= 0000 0yyyTSCR2 = xxxx xyyy */
TSCR2 Example Code
/* Bit 7 – TOI */TSCR2 = TSCR2 | 0x80; //Set TOI=1 to enable Timer Overflow Interrupts/* TSCR2 = xxxx xxxx 0x80 = 1000 0000TSCR2|0x80 = 1xxx xxxx */
TSCR2 = TSCR2 & 0x7F; //Set TOI=0 to disable Timer Overflow Interrupts/* TSCR2 = xxxx xxxx 0x7F = 0111 1111TSCR2&0x7F= 0xxx xxxx */
/* Clear TOF - Timer Overflow Flag */TFLG2 = 0x80;
TSCR1 = TSCR1 | 0x80; // Enable TimerTSCR1 = TSCR1 & 0x7F; //Disable Timer
Timer Overflow Interrupt Example----long unsigned int tnctof = 0; // count timer overflowsvoid tof_isr(void) __attribute__ ((interrupt); // forward reference-----int main(void){// initialize timerSETVECT(0xFFDE,tof_isr); // initialize TOF Interrupt vectorTSCR1 = TSCR1 |0x80; // Enable timer (TEN = 1)TSCR2= 0x80; // Enable interrupt on overflow, prescaler=0 (TOI=1)TFLG2 = 0x80; // Clear Timer Overflow (TOF=1) ENABLE();------}void tof_isr(void){ TFLG2 = 0x80; // Clear Timer Overflow Flag (TOF=1) tnctof++; // count number of overflows}
16-bit Register Per Channel
16-bit Register Per Channel
Input Capture/Output CompareSelection Register
Example – I/O Selection
/* Input Capture/Output Compare Module Selection Timer 7, 6. 3, 1 – Input Capture Timer 5, 4, 2, 0 – Output Compare */
TIOS = 0x35; // 0x35 = 0b00110101
/* Change Timer 6 to Output Compare */ TIOS = TIOS | 0x40; // 0x40 = 0b01000000
/* Change Timer 2 to Input Capture */ TIOS = TIOS & 0xFB; // 0xFB = 0b11111011
Using Channel X for Input Capture
Initialization• Set Timer Channel X to Input Capture in the TOS Register• Select Input Edge Capture Control Mode in Timer Control
Register 3 or 4
Operation • On selected Edge, TCNT is copied into TCx, Flag is Set
Software Action After Flag is Set:• Program reads TCx• Program clears Flag
Timer Control Register
Input Capture Edge Control
Example 1 – Input Capture
/* Input Capture/Output Compare Module Selection Timer 1 – Input Capture Timer 1 - Capture on “0” to “1” */
TIOS = TIOS & 0xFD; // Clear Bit 1 – Set Timer 1 for Input/* Let TIOS = xxxx xxxx 0xFD = 1111 1101TIOS & 0xFD = xxxx xx0x */
TCTL4 = (TCTL4&0xF3) | 0x04; // Use Rising Edge Trigger/* Let TCTL4 = xxxx xxxx 0xF3 = 1111 0011 TCTL4&0xF3 = xxxx 00xx 0x04 = 0000 0100(TCTL4&0xF3)|0x04 = xxxx 01xx */
TFLG1 = 0x02; // clear Timer 1 Interrupt Flag
while((TFLG1&0x02)==0); // Wait for input trigger/* TFLG1 = xxxx xxxx 0x02 = 0000 0010TFLG1&0x02 = 0000 0010 */
unsigned int time1;time1 = TC1; // Read in time of capture
Example 2 – Input CapturePulse Width Measurement
/* Input Capture/Output Compare Module Selection Timer 0 – Alternate between: - Capture on “0” to “1” - Capture on “1” to “0” */TIOS = TIOS & 0xFE; // Clear Bit 0 – Use Timer 0 as Input
/* TIOS = xxxx xxxx 0xFE = 1111 1110TIOS&0xFE = xxxx xxx0 */
TCTL4 = (TCTL4&0xFC) | 0x01; // Trigger on rising edgeTFLG1 = 0x01; // clear Timer 0 Flag
while((TFLG1&0x01)==0); // Wait for input trigger on PT1
Example 2 – Pulse Width
unsigned int time1, time2, pw;time1 = TC0; // Read time of capture
TFLG1 = 0x01; //clear Timer 0 FlagTCTL4 = (TCTL4&0FC) | 0x02; // Change Trigger condition to “falling edge”
while((TFLG1&0x01)==0); // Wait for input triggertime2 = TC0; // read time of capture
/* 2 Cases – tcnt overflow */If(time2 > time1) pw = time2 - time1;else pw = 0x10000 + time2 – time1;
Example 2 – PW Calculation
0 1 2 …… 0xFFFF 0 1 2 …. 0xFFFF 0 1 2 ….. TCNT
0x0100 0xC000 Case 1
Case 2 0xD000 0x2000
PW = 0xC000-0x0100 = 0xBF00
PW = 0x10000 + 0x2000 – 0xD000 = 0x5000
Timer Interrupt Enable Register
Input Capture ExampleUsing Interrupts
/* Use Channel 0 Input Capture to Measure Pulse Width */---unsigned int pw=0;void ic0_isr(void) __attribute__ ((interrupt)); // forward reference---int main(){// init timer, code not shown // init input capture channel 0 TIOS = TIOS & 0xFE; // Set Channel 0 to Input (Clear Bit 0) TCTL4 = (TCTL4&0xFC)| 0x01; // Trigger Channel 0 on rise TFLG1 = 0x01; // Clear Channel 0 Flag TIE = TIE | 0x01; // Enable Channel 0 Interrupt SETVECT(0xFFEE,ic0_isr); // Init Channel 0 Interrupt Vector ENABLE();
Input Capture Using Interrupts
pw=0;while(pw==0); // wait for pulse width measurement ----- =pw; // use pulse widthpw=0;-----}
Input Capture Interrupt Example
void ic0_isr(){unsigned int temp; if((TCTL4&0x03)==0x01) {// time was rise temp=TC0; TCTL4=(TCTL4&0xFC)|0x2; // set fall TFLG1=0x01; // clear flag--return from this point leaves processing in “while” loop } else // time was fall {if(temp<TC0) pw=TC0-temp; else pw=0x10000+TC0-temp; TCTL4=(TCTL4&0xFC)|0x01; // set rise TFLG1=0x01; // clear flag }}
Output Compare
• Set Channel X to Output Compare– TIOS = TIOS | (1<<X); // Set Bit X to “1
• Initialize TCx to Time for Compare
• Select Mode
Timer Output Control Register
Timer Output Control Register
Example – Output CompareGenerate Square Wave
/* Initialize Output Compare Channel 2 Complement on Each Compare Timer has been initialized */
TIOS = TIOS | 0x04: // Set Channel 2 to Output Compare – Bit 2TCTL2 = (TCTL2&0xCF) | 0x10; // Toggle Output Line 2TFLG1 = 0x04; // Clear Channel 2 FlagTC2 = TCNT + T/2; // Complement in T/2 Seconds
while(1){ while((TFLG1&0x04)==0); // Wait for Channel 2 flag to go high TC2 = TC2 + T/2; // Complement T/2 from now TFLG1 = 0x04; // Clear Channel 2 Flag }
Square Wave GenerationUsing Interrupts
SETVECT(0xFFEA, oc2_isr); // set interrupt vector--------void oc2_isr(void){TC2 = TC2 + T/2; // Complement T/2 from now TFLG1 = 0x04; // Clear Channel 2 Flag}
#define TIOS _P(0x40) // timer input/output select#define CFORC _P(0x41) // timer compare force#define OC7M _P(0x42) // timer output compare 7 mask#define OC7D _P(0x43) // timer output compare 7 data#define TCNT _LP(0x44) // timer counter register (2 bytes)#define TSCR _P(0x46) // timer system control register#define TTOV _P(0x47) // reserved#define TCTL1 _P(0x48) // timer control register 1#define TCTL2 _P(0x49) // timer control register 2#define TCTL3 _P(0x4A) // timer control register 3#define TCTL4 _P(0x4B) // timer control register 4
Enhanced Timer Port Definitions
#define TMSK1 _P(0x4C) // timer interrupt mask 1 (TIE)#define TMSK2 _P(0x4D) // timer interrupt mask 2 (TSCR2)#define TFLG1 _P(0x4E) // timer flags 1#define TFLG2 _P(0x4F) // timer flags 2#define TC0 _LP(0x50) // timer capture/compare register 0#define TC1 _LP(0x52) // timer capture/compare register 1#define TC2 _LP(0x54) // timer capture/compare register 2#define TC3 _LP(0x56) // timer capture/compare register 3#define TC4 _LP(0x58) // timer capture/compare register 4#define TC5 _LP(0x5A) // timer capture/compare register 5#define TC6 _LP(0x5C) // timer capture/compare register 6#define TC7 _LP(0x5E) // timer capture/compare register 7
Enhanced Timer Port Definitions
#define PACTL _P(0x60) // pulse accumulator controls#define PAFLG _P(0x61) // pulse accumulator flags#define PACN3 _P(0x62) // pulse accumulator counter 3#define PACN2 _P(0x63) // pulse accumulator counter 2#define PACN1 _P(0x64) // pulse accumulator counter 1#define PACN0 _P(0x65) // pulse accumulator counter 0#define MCCTL _P(0x66) // Modulus down conunter control#define MCFLG _P(0x67) // down counter flags#define ICPAR _P(0x68) // Input pulse accumulator control#define DLYCT _P(0x69) // Delay count to down counter#define ICOVW _P(0x6A) // Input control overwrite register#define ICSYS _P(0x6B) // Input control system control
Enhanced Timer Port Definitions
#define TIMTST _P(0x6D) // timer test register
#define PBCTL _P(0x70) // Pulse accumulator B control#define PBFLG _P(0x71) // Pulse accumulator B flags#define PA3H _P(0x72) // Pulse Accumulator holding register 3#define PA2H _P(0x73) // Pulse Accumulator holding register 2#define PA1H _P(0x74) // Pulse Accumulator holding register 1#define PA0H _P(0x75) // Pulse Accumulator holding register 0#define MCCNT _LP(0x76) // Modulus down counter register#define TCOH _P(0x78) // Capture 0 holding register#define TC1H _P(0x7A) // Capture 1 holding register#define TC2H _P(0x7C) // Capture 2 holding register#define TC3H _P(0x7E) // Capture 3 holding register
Enhanced Timer Port Definitions
16-bit Transfers
• A full access for the counter registers or the input capture/output compare registers should take place in one clock cycle.
• Accessing high byte and low byte separately for all of these registers may not yield the same result as accessing them in one word.
16-bit Transfers
• #define _LP(x) *(unsigned int *)(x)
• unsigned int i;
• _LP(0x240) = i;
• i = _LP(0x240);