ECE 447 Fall 2009

Lecture 11: TI MSP430

Timers Compare Modes

ECE447: MSP430 Output: Continuous Mode

• In the continuous mode, the timer repeatedly counts up to 0FFFFh and restarts from zero as shown in the figure.

• The number of timer counts in the period is 0xFFFF + 1

• The capture/compare register TACCR0 works the same way as the other capture/compare registers.

ECE447: MSP430 Output: Up Mode

• The timer repeatedly counts up to the value of compare register TACCR0, which defines the period, as shown in the figure.

• The number of timer counts in the period is TACCR0+1.

• When the timer value equals TACCR0 the timer restarts counting from zero.

ECE447: MSP430 Output: Up/Down Mode

• The timer repeatedly counts up to the value of compare register TACCR0 and back down to zero, as shown in the figure.

• The period is twice the value in TACCR0.

ECE 447: Generating a Periodic Signal

Each time an interval is completed, an interrupt is generated. The next time interval is added to the TACCRx register in the interrupt service routine.

ECE 447: Designing a PWM

pulse width

single pulse periodical signal

period

ECE 447: Code Example

– Review Davies example Listing 8.10 which:• Toggles TA1 with a period of 0.5s, duty cycle 50%,

connected to LED1

• Set/reset TA2 at 100Hz with a 10% duty cycle. Connected to LED2

• Drives a 440Hz wave output to piezo sounder through the Timer_A0 ISR

• Toggles the 440Hz wave on and off every 2s using enabling and disabling of an appropriate interrupt.

Listing 8.10: Illustrating Periodic Outputs from Timer_A

// ----------------------------------------------------------------------#include <io430x11x1.h> // Specific device#include <intrinsics.h> // Intrinsic functions// ----------------------------------------------------------------------// Delays in cycles of timer clock = ACLK = 32KHz = 0x8000Hz#define LED1half 0x2000 // (1/4)s half -period for toggling#define LED2duty 30 // Duration of "on" duty cycle#define LED2period 300 // Total period#define PIEZhalf 37 // Toggle piezo to give f = 440Hz nearly// ----------------------------------------------------------------------void main (void){

WDTCTL = WDTPW | WDTHOLD; // Stop watchdog timer// Channel 0 of timer , interrupts only to drive piezo

TACCR0 = PIEZhalf; // "Compare" value for first interruptTACCTL0 = CCIE; // Enable interrupts only (no output)

// Channel 1 of timer , drive LED1 (P2.3) in toggle mode with interruptsTACCR1 = LED1half; // Compare value for first interruptTACCTL1 = OUTMOD_4 | CCIE; // Toggle mode with interrupts

// Channel 2 of timer , drive LED2 (P2.4) with set , reset and interruptsTACCR2 = LED2duty; // Compare value for first interruptTACCTL2 = OUTMOD_1 | CCIE; // "Set" to turn LED off at next match

// Configure ports 1 and 2; redirect P2.3,4 to Timer_AP1OUT = BIT0 | BIT1; // Output , high for Freq pin and TXDP1DIR = BIT0 | BIT1;P2SEL = BIT3 | BIT4; // Re -route P2.3 to TA1 , P2.4 to TA2P2OUT = BIT0 | BIT3 | BIT4; // LEDs off (active low); prepare piezoP2DIR = BIT0 | BIT3 | BIT4 | BIT5; // Piezo and LED outputs

// Start timer from ACLK , Continuous mode , clear , TAIFG interruptsTACTL = TASSEL_1 | ID_0 | MC_2 | TACLR | TAIE;for (;;) { // Loop forever

__low_power_mode_3 (); // LPM3 , all action in interrupts}

}

Listing 8.10: Cont’d with ISRs// ----------------------------------------------------------------------// Interrupt service routine for TACCR0.CCIFG , called on compare// ----------------------------------------------------------------------#pragma vector = TIMERA0_VECTOR__interrupt void TIMERA0_ISR (void) // Flag cleared automatically {

P2OUT ˆ= (BIT0|BIT5); // Toggle piezo sounderTACCR0 += PIEZhalf; // Calculate next compare value

}// ----------------------------------------------------------------------// Interrupt service routine for TACCRn.CCIFG (n > 0) and TAIFG// ----------------------------------------------------------------------#pragma vector = TIMERA1_VECTOR__interrupt void TIMERA1_ISR (void) // ISR for TACCRn CCIFG and TAIFG{

switch (__even_in_range(TAIV , 10)) { // Acknowledges interruptcase 0: // No interrupt pending

break; // No actioncase TAIV_CCIFG1: // Vector 2: CCIFG1

TACCR1 += LED1half; // Calculate next compare valuebreak; // (LED1 toggled automatically)

case TAIV_CCIFG2: // Vector 4: CCIFG2if (TACCTL2 & OUTMOD_5) { // Was last action Reset (on)?

TACCR2 += LED2duty; // Duration of On part of cycleTACCTL2 = OUTMOD_1 | CCIE; // "Set" turns LED off next

} else { // Last action was Set (off)TACCR2 += (LED2period - LED2duty ); // Duration of Off partTACCTL2 = OUTMOD_5 | CCIE; // "Reset" turns LED on next

}break;

case TAIV_TAIFG: // Vector A: TAIFGTACCTL0_bit.CCIE ˆ= 1; // Toggle CCIFG0 ints (piezo on/off)break;

default: // Should not be possiblefor (;;) {} // Disaster! Loop here for ever

}}

TACTL: Timer_A Control Register

TACCTLx: Capture/Compare Control Register

ECE447: MSP430 Timer_B additional features

Timer_B is identical to Timer_A with the following exceptions:

• The length of Timer_B is programmable to be 8, 10, 12, or 16 bits.

• Timer_B TBCCRx registers are double-buffered and can be grouped.

• All Timer_B outputs can be put into a high-impedance state.

• The SCCI bit function is not implemented in Timer_B.

ECE447: MSP430 Class Exercise

Write a Function that sets up Timer_B in Up Mode to generate a 1000Hz output wave. The duty cycle of the wave will be determined by the value read on PORT1. The value should be interpreted as unsigned and any value over 100 will be treated with a ceiling at 100. As the input on Port1 changes the PWM duty cycle will be adjusted.