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

  • Jumper wires

  • Servo motor

3. Background

3.1. Pulse-Width Modulation (PWM)

A pulse-width modulated (PWM) 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 the period for which the signal is high. Usually, a clock wave has a duty cycle of 50%. In a PWM signal, the duty cycle is controllable. The name is derived from the idea that 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 motor’s angle. Usually, the full swing of the servo corresponds to a high interval of 1 to 2 ms, whereas a high interval of 1.5 ms corresponds to the neutral servo position [1].

3.3. Generating PWM with LPC1769

The LPC1769 features a pulse-width modulator peripheral. The generic steps discussed in Experiment 5 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: a PWM pin must be configured for PWM use.

Additionally, generating a PWM signal in particular requires:

  1. Setting the period of the PWM signal using the MR0 register.

  2. Specifying the duty cycle using an MRx register, which would control the PWM1.x output.

  3. The PWM circuit should be enabled to generate a PWM signal, otherwise it will act as a standard timer (or counter).

  4. The corresponding PWM1.x output should be enabled.

  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.

If you care about the accuracy of your PWM output voltage levels, you need to disable the pull-up resistor to avoid affecting the PWM voltage. That can be done using the LPC_PINCON→PINMODEx register.

In many applications this is not required.
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. MR0 and MRx

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

  1. Its period (or, equivalently, its frequency)

  2. Its duty cycle

The value of the MR0 register (aka PWM1MR0) determines the period, while any of the MR1 to MR6 registers determine the duty cycle for the corresponding PWM1.1 to PWM1.6 outputs, 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)

MR4

20

75%

4 (PWM1.4)

MR5

72

90%

5 (PWM1.5)

The figure below shows the different PWM outputs for the same MR0.

Different PWM outputs example
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.

In summary:

  1. Control the period duration of the PWM signal by setting the MR0 register.

  2. Use the appropriate MRx register to control the duty cycle of PWM1.x, where x is a number between 0 and 6.

Example 2. A PWM Period of 1 Second
LPC_PWM1->MRx = 1000000; // PWM period is 1 second.

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

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 3. 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 in 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.

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. PWM vs. Timers

From a hardware point of view, PWM is based on the standard timer block, and inherits all of its features [lpc1769-manual].

Let us review the relation between the timer counter, the prescale register, and the prescale counter. 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 in CMSIS).

Recall that you can use the default value of the PR register (0) to simply increment TC every PCLK pulse.

IF PR is set to a non-zero value, TC's frequency would be 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 4. 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;

If MR0 is set to 100, every 100 pulses of the PWM Timer Counter register (PWM1TC, or TC for short), a new PWM period starts. That happens even if TC is not reset. This is an important operational difference between pure timers and a PWM signals. The other crucial difference is the control of the duty cycle, which is at the heart of the the PWM concept.

3.3.3. Summary of Important PWM Control Registers

  • 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 can be used to generate interrupts or reset TC when matches occur if needed.

4. Tasks

  1. Basic operation: Write a program that generates a PWM signal, and use it on an external device.

  2. Control a servo motor: Rotate a servo motor 90 degrees to the right, move it back to the neutral position, then rotate it 90 degrees to the left.

  3. Show different colors on an RGB LED using at least two PWM signals

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

6. Grading Sheet

Task Points

Basic operation

3

Servo Control

7

Bonus: RGB

+2
1. https://circuitdigest.com/article/servo-motor-basics