Lab 5 Timer Module PWM ReadMeFirst

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Evelyn Richard
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1 Lab 5 Timer Module PWM ReadMeFirst Lab Folder Content 1) ReadMeFirst 2) Interrupt Vector Table 3) Pin out Summary 4) DriverLib API 5) SineTable Overview In this lab, we are going to use the output hardware of Timer A to implement a useful application: Pulse Width Modulation (PWM). On some microcontrollers and DSPs (Digital Signal Processors), Pulse Width Modulation is already built in as a peripheral in its own, but that is rare. Therefore it is important to know how PWM is produced on a typical MCU using a timer. In this document, we are going to start with an introduction to Pulse Width Modulation, PWM, and Timer A s output mode before we start the experiments. Pulse Width Modulation (PWM) Pulse Width Modulation modulates a signal by producing constant amplitude digital pulses whose pulse width (duty cycle, duration) is proportional to the signal s amplitude at a given sample time. PWM is used in communications and generating control signals among many other applications.

2 Two main parameters are associated with PWM: resolution and sampling period. Resolution is the number of distinct output pulse widths you can have. Each output pulse width corresponds to a signal value/amplitude. Therefore resolution is the number of distinct signal values you can modulate. Sampling time Ts gives the time interval between sampling points. In Fig. 1 sampling time is denoted as Ts. At every Ts, PWM produces a pulse width proportional to the signal (sine wave in the figure) value. Output Modes In the previous experiments we toggled a pin in timer interrupt routines to generate waveforms with various frequencies. In this part we are going to explore the output module of Timer A to generate pulses at fixed intervals and varying widths to produce PWM waves. In Timer_A each Capture/compare Block contains an output unit which can be used to generate output signals such as Pulse Width Modulation (PWM). We include part of the schematics below:

3 As you can see the output mode is handled by hardware. This means that you can generate signals at the output without the need to use ISRs to toggle pins and waste CPU computing cycles when you use this mode. Output pins of Timer_A output module are called TAx.y. x is the Timer number. In the MSP430F5529 we have three Timer_As (Timer_A0, Timer_A1 and Timer_A2) so x can be 0, 1 or 2. y is the Capture Compare Register. For Timer_A0 there are 5 Capture Compare Registers, therefore, y can be 0, 1, 2, 3 or 4. Each output unit has eight operating modes: Output, Set, Toggle/Reset, Set/Reset, Toggle, Reset, Toggle/Set, and Reset/Set. A list of available output modes and their description are summarized in the table below: Notice that there are two types of output modes, one only depends on the Capture Compare Register selected (CCR0, CCR1, CCR2, CCR3, CCR4) like modes 0, 1, 4 and 5. When the timer counts up to the selected CCR register value the corresponding output pin is set, reset or toggled. Name of these modes simply follows the operation: Output, Set, Toggle and Reset. The other type of output mode is related to the selected Capture Compare Register (CCRx where x is not equal to 0) and the special CCR0 register. The output changes when the

4 timer reaches the selected CCRx register and change again when it reaches the CCR0 value. Examples are modes 2, 3, 6 and 7. In the table you can also see the names of these modes: Toggle/Reset, Set/Reset, Toggle/Set and Reset/Set. In these modes we use the output modules of CCRx (x is not equal to 0), it is not useful to use the output mode of CCR0. OUTMOD Illustration in Up Mode The following illustration provide a more straight forward way to understand the operation of different output modes. In this case Timer_A is configured in Up Mode, meaning that the counter counts up to the value of compare register TAxCCR0 and restarts counting from zero when it reaches TAxCCR0. It should be noted that when it is in the Up Mode, Capture Compare Block 0 is used for special purpose (defining the period) and all other CCR blocks can be used normally.

5 PWM Generation The figure below illustrates how to generate PWM using Timer A and Reset/Set output mode for CCR1. In this illustration we set the timer to Up Mode. In Up Mode TAR continues to increment at each Timer_A clock tick until it reaches the value stored in TACCR0 and goes back to 0 again. Here we are using TACCR1 to store the signal value at the sample points. In this figure the output is set to 1 when TAR reaches TACCR0 and the output is reset to 0 when TAR reaches the current value stored in TACCR1. From the table we can see that the corresponding behavior is obtained when the output mode is set to 7, i.e., the Reset/Set mode. Therefore, in this case we should set the output mode to 7 for TA0 Capture Compare Block 1 The corresponding output pin is TA0.1 (on Pin 1.2 according to the Pin out Summary). The signal value is stored in TA0CCR1 and is updated every Ts. The update of TA0CCR1 can be done in either CCR0 or TAR interrupt. Note that it takes CPU clock cycles to update the TA0CCR1 value in the interrupt service routine. Therefore if the signal value is very small then we may miss the signal value while modulating. This problem can be solved by offsetting the signal or using the up down mode.

6 More methods are available based on the signal source and additional hardware attached to them. With an understanding of how PWM signals are generated in the Up Mode we can understand how resolution and sampling period are setup: 1) Resolution value is stored in TACCR0. 2) Sampling period Ts is the Timer A count period = TACCR0*T TACLK (T TACLK is the Timer A Clock) Sampling rate fs (PWM frequency) is 1/Ts. Ts/ T TACLK = TACCR0, f TACLK/fs = TACCR0 As an example, if you are given a design requirement to generate a 12 khz PWM (fs = 1/Ts) of a signal with 4096 (2^12 bits) resolution, you must have a 50 MHz clock (T TACLK ). However the G2553 microcontroller does not have such a high frequency clock, therefore we must compromise and either decrease the sampling frequency 1/Ts to keep the resolution or decrease the resolution to keep the sampling frequency. This example should give you a feel for the constraints required to generate PWM. DriverLib API Select Counting Mode for Timer A Timer_A_initUpMode() Compare Mode Timer_A_initCompareMode() Timer_A_setCompareValue() Output Mode Timer_A_setOutputMode() Timer_A_setOutputForOutputModeOutBitValue() Timer_A_getOutputForOutputModeOutBitValue()

7 Experiments Experiment 1 Compare Mode: Frequency Generation In this experiment, we will start with generating a 50/50 duty cycle square wave with a specific frequency. This would be helpful later in generating a PWM with a specific sampling period. Recall that in our last Lab s Experiment 1, we generated a square wave using Continuous Mode where the pin s output was toggled when an Overflow Interrupt was generated (i.e. the Timer counted to its max value 2^16 1). The operation of Continuous Mode is shown below: Figure. Continuous Mode Up Mode is used if the timer period must be different from 0xFFFF counts. The timer repeatedly counts up to the value in the compare register TAxCCR0, which defines the period as in the following figure. The number of timer counts in the period is TAxCCR0+1. When the timer value equals TAxCCR0, the timer restarts counting from zero. In this mode, the Capture Compare Block 0 works differently from other four Capture Compare Blocks since the value stored in its compare register TAxCCR0 is used to defined the period. Figure. Up Mode Thus we can easily use Up Mode to generate a square wave with any frequency by specifying the max value the timer counts up to. In this experiment, we will generate a frequency of 100Hz in Up Mode using Capture Compare Module 0 (CCR0) in Timer_A0.

8 1) Pin Configuration Configure P3.0 as a digital output pin Configure P2.2 as the output of SMCLK for debugging purposes 2) Clock Configuration Setup SMCLK to run at 1MHz o Measure your actual SMCLK frequency. Take a screenshot 3) Timer Setup Configure Timer_A0 to be sourced from SMCLK and set it in Up Mode Set the compare value of CCR0 o According to your actual SMCLK frequency, theoretically what value should be stored in TA0CCR0 register to generate a 100Hz square wave at P3.0? Show your reasoning in the lab report. Note: If you use structure, please make sure you initialize all members values 4) Interrupt Enable Enable TA0 s Capture Compare Interrupt 0 (CCIFG0) In your main function, enable General Interrupt (GIE) 5) Interrupt Service Routine Toggle P3.0 when an interrupt is generated o What Interrupt Vector should be used? o There is no need to clear the interrupt flag, why? o Use the oscilloscope to measure the output waveform at P3.0. Take a screenshot Experiment 2 DC Pulse Width Modulation In this experiment we are going to test the output module configuration by first generating pulse width modulation of a DC signal on output TA0.1 (on Pin 1.2) which is circled in red in the table below.

9 Table. TA0 Signal Connections The timer is set to count in Up Mode, the output mode that we are going to use here is the Reset/Set Mode as shown below. In this mode two Capture Compare Modules are used: the output hardware will set the output signal to 1 when the timer hits the value stored in TAxCCR0 and reset it to 0 when it hits the value stored in TAxCCR1. Remember that each Capture Compare Module comes with its own output hardware and we are using CCR1 s output hardware here. Include the following parts in your program: 1) Pin Configuration Configure P1.2 as a peripheral module function output pin (TA0.1) 2) Clock Configuration Setup SMCLK to run at 1MHz

10 o Measure your actual SMCLK frequency. Take a screenshot 3) Timer Setup Configure Timer_A0 to be sourced from SMCLK with a divider of 8 and set it in Up Mode Set resolution to be 2^10 = 1024 by setting the compare value of CCR0 to 1024 Set the compare value of CCR1 to 50 Configure CCR1 s output mode as Reset/Set mode Note: If you use structure, please make sure you initialize all members values Connect TA0.1 pin out to LED1 on the launchpad using a jumper wire and observe the result via the intensity of the LED. Our eyes will behave like a low pass filter when we see the lighting of LED. Change the value in TA0CCR1 to 900. o What difference did you observe on the LED? o Based on the values stored in CCR0 and CCR1 registers what are the duty cycles of the generated square wave in both cases? Duty cycle = width of a pulse / period of the square wave Bonus Experiment (2pt) The following experiment is NOT required for your lab report. Please ask lab monitors to confirm your results on completion. Experiment 3 Sine Wave PWM Generation In this experiment we are going to generate Pulse Width Modulation from an array of data stored in memory on output TA0.1 (P1.2). We will connect TA0.1 pin to LED1 using a jumper wire and observe the result via the intensity of the LED. For the signal to modulate we will use a table consisting of half a period of a sine wave. As a result we expect to see the brightness of the LED light to vary from dim to bright and then back to dim in the behavior of a half period of a sine wave. The sine wave table will be inserted in the memory of the microcontroller by loading a.dat file into the memory space. The data file (as well as the MATLAB file that generated it) will be provided to you. Based on your code from Experiment 2, follow the instructions below: Add an array[256] of uint16_t Select Timer_A0 s clock source divider of 16 Add an Interrupt Subroutine to update TA0CCR1 value when timer reaches TACCR0 value: use TACCR0 interrupt to update value

Interrupt Enable Download the SineTable zipped file from the website. Unzip the file and save it to your local directory. In debugger, open the memory browser, search for the array name.

Click Finish. As shown in the screenshot below, the array named array starts with the address 0x002400.

11 Interrupt Enable Download the SineTable zipped file from the website. Unzip the file and save it to your local directory. In debugger, open the memory browser, search for the array name. Right click and select Load Memory, browse to the unzipped.dat file and click Next. Set the Start Address as the starting address of the array and set the length to 0x100 (256 in decimal). Click Finish. As shown in the screenshot below, the array named array starts with the address 0x Set the starting address as the starting address of the array: o Observe your output on oscilloscope. o Once a signal is converted to PWM, it can be converted back to the original signal by using an analog filter. What type of filter (low pass, high pass or band pass) would be needed for this conversion, explain. What cutoff frequency should you filter have?

Timer A (0 and 1) and PWM EE3376

Timer A (0 and 1) and PWM EE3376 General Peripheral Programming Model l l l l Each peripheral has a range of addresses in the memory map peripheral has base address (i.e. 0x00A0) each register used in