1. Objectives

  • Understanding and using pulse-width modulation (PWM).

2. Parts List

  • LPC1769 LPCXpresso board

  • USB A-Type to Mini-B cable

  • Breadboard

  • RGB-LED or buzzer

  • Seven-segment display

  • Jumper wires

3. Background

3.1. Pulse-Width Modulation (PWM)

A pulse-width modulated signal is a periodic square wave signal. The difference between a PWM signal and a clock signals is the flexibility of its duty cycle.

A periodic square wave is high for some part of its period, and low for the rest of the period. Its duty cycle is the percentage of its high time to the full period duration. A clock waves has a duty cycle of 50%. In a PWM signal, the duty cycle is controllable. The name is derived from this idea, where the width of the high pulse is modulated according to some value.

3.2. PWM applications

PWM has many useful applications in embedded systems. The main two categories are:

  1. When a microcontroller does not have a DAC circuit, PWM can be used to modulate different analog values.

  2. Some devices are built to be used with PWM. The most famous example is servo motors.

    Servo motors usually require a 50 Hz square wave (period of 20 ms). The duration of the high pulse determines the angle. Usually, the full swing of the servo corresponds to a high interval of 1 to 2 ms, where 1.5 ms is the neutral servo position.

3.3. Generating PWM with LPC1769

The LPC1769 features a pulse-width modulator peripheral. The generic steps discussed in Experiment 7 for setting up a peripheral device apply here:

  1. Power: the PWM circuit is powered on by default.

  2. Peripheral Clock (PCLK): recall that the default division factor is 4.

  3. Pin functions: PWM pins must be configured for PWM use. Otherwise, the PWM circuit will act as a standard timer (or counter).

Tip

  1. There is only one PWM circuit, called PWM1. That does not imply that there is a PWM0 or PWM2.

  2. There are six PWM channels, referred to as PWM1.1 to PWM1.6.

  3. You have the option of more than one pin to pin out any of the channels.

Important

You need to disable the pull-up resistor to avoid affecting the PWM voltage. Recall that you can do that using the LPC_PINCON->PINMODEx register.
Exercise

Refer to chapter 8 of the LPC176x manual to determine:

  1. Which pins are you going to use for PWM?

  2. Which PINSELx register should you use?

  3. Which PINSELx bits should you set?

  4. To what value should you set those PINSELx bits?

  5. How to disable the pull-up resistor?

3.3.1. PWM Configuration

To fully specify a PWM signal, you need to specify:

  1. Its period (or frequency)

  2. Its duty cycle

In the simplest configuration, the MR0 register (aka PWM1MR0) determines the period, while MR1 to MR6 determine the duty cycle, as illustrated in the following example.

Example 1. Period and Duty Cycles

If MR0 is set to 80, then:

Register

Value

Duty Cycle

PWM Channel

MR1

40

50%

1 (PWM1.1)

MR2

20

25%

2 (PWM1.2)

MR5

72

90%

5 (PWM1.5)
Note
Single Edge Controlled PWM

In the example above, the periodic signal on all channels will go high at the beginning of the period, and each channel will be reset when matching the number in the corresponding MR1 to MR6 register.

This PWM configuration is called single edge controlled PWM.

To have different PWM channels be set and reset at different times, some PWM channels can be configured as double edge controlled PWM signals.

Note
Double Edge Controlled PWM

In double edge controlled, you can control when to set or reset the pulse within the period, and whether to set or reset first.

The MR0 register still controls the duration of the full period.
Example 2. Double Edge Controlled PWM

PWM channel 2 (PWM1.2) is set by MR1 and reset by MR2.

So, setting MR0 = 100, MR1 = 50, and MR2 = 75 will result in a signal that is low at the beginning of the period, becomes high in the middle of the period, and goes back to low in the middle of the second half of the period.

In contrast, setting MR0 = 100, MR1 = 75, and MR2 = 50 will result a signal that is high at the beginning of the period, becomes low in the middle of the period, and goes back to high in the middle of the second half of the period.

Note

PWM channels can be configured to be single edge controlled or double edge controlled using the PWMSELn bits of the PWM Control Register (PWM1PCR or LPC_PWM1->PCR).

For details, see Table 444 and Table 452 in the LPC176x manual.

3.3.2. The Prescale Register

In most PWM applications, the timing accuracy is crucial. When MR0 is set to 100, every time the PWM Timer Counter register (PWM1TC, or TC for short) matches 100, a new period starts. The question is how long is this 100?

TC is a 32-bit register that is incremented every PR + 1 cycles of PCLK, where PR is the Prescale Register (PWM1PR or LPC_PWM1->PR).

Therefore, TC frequency is given by:

TC frequency in Hz = $\displaystyle\frac{\textrm{System clock}}{\textrm{PCLK divisor} \times (\textrm{PR} + 1)}$

where PCLK divisor is 1, 2, 4, or 8, depending on the setting of the PCLKSELx register (default is 4).

For system clock, you can use the SystemCoreClock variable, which is set by CMSIS to the CPU clock speed.

Example 3. Setting the Prescale Register

To set the prescale register such that TC is incremented every 1 µs (frequency of 1,000,000 Hz):

LPC_PWM1->PR = SystemCoreClock / (4 * 1000000) - 1;

Now you can control the period duration of the PWM signal by setting the MR0 register.

For example:

LPC_PWM1->MR0 = 1000000; // PWM period is 1 second

3.3.3. Other settings

  • LPC_PWM1->LER is used to latch the new MRx values. You must use it every time you change any of the MRx values.

  • LPC_PWM1->PCR is used to enable PWM1 with single or double edge operation. If ignored, PWM will act as a counter.

  • LPC_PWM1->TCR is used to enable, disable, or reset counting in the TC register. You should use it at least once to enable counting.

  • LPC_PWM1->MCR is similar to the timers MCR registers. It is used to generate interrupts or reset TC when matches occur.

4. Tasks

Use the PWM feature of the LPC1769 microcontroller in an experiment of your choosing.

The output PWM signal can be used to control a servo, an RGB LED (color LED), or a buzzer.

5. Resources

[lpc1769-manual]

NXP Semiconductors. UM10360 LPC176x/5x User manual. Rev. 3.1. 2 April 2014.
http://www.nxp.com/documents/user_manual/UM10360.pdf