Lecture 26: Timer_A recap

10:10 First half of the lecture - refer to slides

10:40 Second half of the lecture - discuss msp432-recorder-demo-2


Architecture of the program:

	- An ISR runs at 8KHz
	- The ISR has two different modes:
			Recording: storing samples from ADC in memory
			Playback: retrieving samples from ADC in memory
	- Playback is based on PWM modulation of 64KHz carrier

	- Timer A0 is used for PWM generation
	- Timer A1 is used for ISR rate generation
	
The program illustrates several interesting features
	- Calibration of samples coming from the ADC
	- Configuration record for upmode config
	- Configuration record for PWM config
	- ISR tied to Timer_A upmode counter
	- PWM generation with varying duty cycle
	
10:43 Calibration of ADC samples

char scaleSample(unsigned vx) {
    // microphone sample is 0x1000 to 0x2FFF midpoint 0x1FFF
    // output is 0x00 to 0xFF midpoint 0x80
    // set gain to 4
    int c = (vx - 0x1FFF) * 0x100 * 4 / 0x2000 + 0x80;
    return (char) c;
}

The program requires 8-bit samples, since we don't want to store
the lower LSBs (to save memory). This means we have to scale samples.

		ADC				CHAR
		
		0x2fff -------- 0xFF
           ||             ||		
		   ||             ||
		0x1fff -------- 0x80
		   ||             ||
		   ||             ||
		0x1000 -------- 0x00
		
		This leads to the following formula:
		
			(vx - 0x1FFF) * 0x100 / 0x2000 + 0x80
			
10:46 PWM configuration of a 64 KHz Carrier

#define CARRIER ((int) (SYSTEMCLOCK / 64000))

Timer_A_PWMConfig pwmConfig = {
        TIMER_A_CLOCKSOURCE_SMCLK,
        TIMER_A_CLOCKSOURCE_DIVIDER_1,       // 3 MHz
        CARRIER,                             // 64 Khz Carrier
        TIMER_A_CAPTURECOMPARE_REGISTER_4,
        TIMER_A_OUTPUTMODE_RESET_SET,
        CARRIER / 2
};

Note the macro CARRIER. If you know the system clock, and the divider is 1,
then you can generate a period value for a give frequency f by computing

				(int) (SYSTEMCLOCK / f)

10:48 Upmode counting with interrupt

#define SAMPLERATE ((int) (SYSTEMCLOCK / 8000))

Timer_A_UpModeConfig upConfig = {
    TIMER_A_CLOCKSOURCE_SMCLK,
    TIMER_A_CLOCKSOURCE_DIVIDER_1,       // 3 MHz
    SAMPLERATE,
    TIMER_A_TAIE_INTERRUPT_DISABLE,
    TIMER_A_CCIE_CCR0_INTERRUPT_ENABLE,
    TIMER_A_SKIP_CLEAR
};

In this case, we enable interrupts on the upmode config counter.
The interrupts will be vectored into:

void TA1_0_IRQHandler() {
   ...
   Timer_A_clearCaptureCompareInterrupt(TIMER_A1_BASE,
                                        TIMER_A_CAPTURECOMPARE_REGISTER_0);

}

10:50 Initialization

Note the initialization routines:

    Timer_A_configureUpMode(TIMER_A1_BASE, &upConfig);
    Interrupt_enableInterrupt(INT_TA1_0);
    Interrupt_enableMaster();
    Timer_A_startCounter(TIMER_A1_BASE, TIMER_A_UP_MODE);

You first have to set Timer_A in upmode, then enable interrupts from
source INT_TA1_0 (which includes CCR0 interrupts from TA1), and
finally start the Timer_A A1 counter.