Lab #3: PWM and Low Power Modes

Part 1: Different Clocks, Low Power Mode, and PWM

For Lab 2, you learned how to switch the timer interrupt from the DCO to the VLO by, in part, changing its source from the SMCLK to the ACLK. As discussed in class, the primary advantage of the VLO is much lower power consumption than the DCO. It also is lower frequency, which can be convenient if you want infrequent timer interrupts.

1. (Review, not scored) Using the DCO, what is the minimum frequency for a timer interrupt? What is the typical default frequency of the VLO? How do you turn on the VLO, and how do you change code from last week to run off the VLO instead of the DCO?

LPM4 consumes very little power. I noticed, playing around with my Lab 2 pendant, that if I removed the battery while in LPM4, the capacitory between power and ground held enough charge to keep the device functioning (in LPM4) for at least a few seconds. To see this yourself, remember that on first power up, the pendant will always start with LED1 illluminated. So start it going, and then go into LPM4 when the next LED will not be LED1. Remove the battery, wait a while, then put the battery back and push the button again. You’ll see that it will continue on where it left off. If you push the button while it’s asleep, you’ll rapidly discharge the capacitor and when you put the battery back in, it will start with LED1 again.

2. According to the datasheet, power consumption in LPM4 is ~100 nA @ 2.2V. Lets assume we can model the device in LPM4 as a resistor. What is the resistance? Assume that right before you take the battery out, the capacitor is charged to 3V (i.e., battery voltage). Then, we can define the system as a capacitor discharging into a resistor. The brownout circuit will trigger a device reset when the power level drops too low. Let’s assume that that value is 1.35V. Finally, let’s assume that the capacitor between power and ground which you soldered on is (actually) 10 µF. How long will it take for the voltage to drop from 3V to 1.35V? Experiment with your Lab 2 PCB. Do you find that the state is maintained as long as you predicted?

In Lab 2, you used the timer interrupt to wake up the CPU periodically to control which LED was lit on your PCB. For Labs 3 – 5, we want to generate PWM signals, which will end up potentially using both of the TimerA modules on the MSP430G2553. There is a third, easily forgogtten, timer module on the MSP430G2553, though. Before you look in the skeleton code for Lab 3, see if you can remember what it is.

Now take a look at the skeleton code. This program follows the same code pattern as in Lab 2. Succinclty, this pattern involves a main loop which contains the program logic. At the end of each main loop cycle, the device goes into LPM3, and then the timer interrupt wakes it back up.

3. What is the function of the function call __bic_SR_register_on_exit(LPM3_bits);? What would be the result to the program if we replaced this with __bic_SR_register(LPM3_bits);?
4. (not graded) What clock sources can drive the ACLK? In which low power modes is only ACLK active? If we only want to enable the VLO when not in an interrupt, what low power mode should we put the MSP430 into?
5. (not graded) What is the difference in supply current between LPM1 (when SMCLK is set to 1 MHz from the DCO) and LPM3 (when ACLK is set to use the VLO)? Note that when the DCO is on, LPM1 power will be dominated by the DCO/DC generator and performance will be more like LPM0.
6. In VLO-driven mode for the ACLK, what frequencies are available for the WDT+ interrupt? What code in the skeleton that sets the interrupt nominally at ~1.5 Hz? Why do we describe the WDT+ interrupt rates as “nominal”? (One answer has to do with the fact that 12 kHz divided by whatever is not exactly 1.5 Hz. That’s not the answer I’m hoping for!)

PWM

To change the brightness of an LED (or most other analog devices), we use PWM. This essentially changes the brightness by altering the duty cycle of the output signal. We could do this manually – i.e., write code using the timer interrupts that makes the appropriate output bit high or low for the apropriate amount of time. However, many microcontrollers include special hardware that allows for this to be done automatically. In the MSP430G2553, the two TimerA modules can drive up to 3 pins each with PWM signals, with the limitation that the fundamental frequency for the 3 pins must be the same – the module’s selected clock frequency. Because PWM is a useful function for controlling intensity, the Timer modules are designed so that this constraint will still let a variety of pulse patterns to be generated. Hence the various modes (continuous, up, up/down).

7. The timer counter register (TAR) is 16-bits. Assuming we use the VLO to drive the Timer module at 12 kHz, and run the counter in up/overflow mode, where it increments to 0xFFFF then overflows to 0x0, at what frequency will the TAR overflow? If we want our PWM modulation to not appear to flicker, the minimum modulation frequency is about 100-200 Hz. Is this achievable if the TAR is configured to overflow?

The TimerA module allows us to set various pins to be on for a fraction of the timer counter period, creating a PWM signal when it is pulsed fast enough. These pins are depicted in the device datasheet.

8. Say we want to use Timer A1 for our PWM signal. Which pins is register 1 of Timer A1 capable of sending a PWM signal to? What should PxSEL, PxSEL2, and PxDIR be for these pins?
9. (not graded) In up mode, the timer will continuously count up to the value in TA1CCR0, resetting to 0 every time it reaches it. If we want our output (using TA1CCR1) to be initially on for some fraction of each cycle and then turn off when TA1CCR1 is reached, what output mode should we put the capture compare block in? How do we actually set it to this mode?
10. To drive an RGB LED, you in general need 3 output pins. In general, what is the maximum number of PWM signals that can be generated by the Timer A1 module? What if the module is configured in up mode?
11. If you had to generate 4 different PWM signals (i.e., using both Timer A1 and Timer A0) what is a valid set of 4 output pins?

Write some code to toss all this together. Source Timer A1 off the VLO clock, leaving it at its default frequency. Configure your registers such that the duty cycle of the LED input signal is 50%, and make sure to enable the LPM found in b). Since the PWM signals are automatically generated, you should not need an ISR.

11. If you were to set CCR0 to be 500, why does the LED flicker and not dim?

Lab 3 programming task

Now, combine the PWM code you just wrote with the skeleton code. Your goal is for the LED to change duty cycle from 0% to 100% in 5% steps every time the WDT interrupts. You should be able to accomplish this by changing the value of the CCR1 register. Demo this functionality and turn in your code.

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