LPC1768: PWM

In this tutorial we are going to discuss the PWM module of LPC1768.
First we will see how to configure the PWM registers to generate signals of required PWM, At the end we will see how to use the ExploreEmdedded PWM library.


LPC1768 PWM Module

LPC1768 has 6 PWM output pins which can be used as 6-Single edged or 3-Double edged. There as seven match registers to support these 6 PWM output signals. Below block diagram shows the PWM pins and the associated Match(Duty Cycle) registers.

PWM Channel Port Pin Pin Functions Associated PINSEL Register Corresponding Match Register
PWM_1 P2.0 0-GPIO, 1-PWM1[1], 2-TXD1, 3- 0,1 bits of PINSEL4 MR1
PWM_2 P2.1 0-GPIO, 1-PWM1[2], 2-RXD1, 3- 2,3 bits of PINSEL4 MR2
PWM_3 P2.2 0-GPIO, 1-PWM1[3], 2-CTS1, 3-TRACEDATA[3] 4,5 bits of PINSEL4 MR3
PWM_4 P2.3 0-GPIO, 1-PWM1[4], 2-DCD1, 3-TRACEDATA[2] 6,7 bits of PINSEL4 MR4
PWM_5 P2.4 0-GPIO, 1-PWM1[5], 2-DSR1, 3-TRACEDATA[1] 8,9 bits of PINSEL4 MR5
PWM_6 P2.5 0-GPIO, 1-PWM1[6], 2-DTR1, 3-TRACEDATA[0] 10,11 bits of PINSEL4 MR6




LPC7168 PWM Registers

The below table shows the registers associated with LPC1768 PWM.

Register Description
IR Interrupt Register: The IR can be read to identify which of eight possible interrupt sources are pending. Writing Logic-1 will clear the corresponding interrupt.
TCR Timer Control Register: The TCR is used to control the Timer Counter functions(enable/disable/reset).
TC Timer Counter: The 32-bit TC is incremented every PR+1 cycles of PCLK. The TC is controlled through the TCR.
PR Prescalar Register: This is used to specify the Prescalar value for incrementing the TC.
PC Prescale Counter: The 32-bit PC is a counter which is incremented to the value stored in PR. When the value in PR is reached, the TC is incremented.
MCR Match Control Register: The MCR is used to control the reseting of TC and generating of interrupt whenever a Match occurs.
MR0 Match Register: This register hold the max cycle Time(Ton+Toff).
MR1-MR6 Match Registers: These registers holds the Match value(PWM Duty) for corresponding PWM channels(PWM1-PWM6).
PCR PWM Control Register: PWM Control Register. Enables PWM outputs and selects PWM channel types as either single edge or double edge controlled.
LER Load Enable Register: Enables use of new PWM values once the match occurs.






Register Configuration

The below table shows the registers associated with LPC1768 PWM.

TCR
31:4 3 2 1 0
Reserved PWM Enable Reserved Counter Reset Counter Enable

Bit 0 – Counter Enable
This bit is used to Enable or Disable the PWM Timer and PWM Prescalar Counters
0- Disable the Counters
1- Enable the Counter incrementing.

Bit 1 – Counter reset
This bit is used to clear the PWM Timer and PWM Prescalar Counter values.
0- Do not Clear.
1- The PWM Timer Counter and the PWM Prescale Counter are synchronously reset on the next positive edge of PCLK.

Bit 3 – PWM Enable
Used to Enable or Disable the PWM Block.
0- PWM Disabled
1- PWM Enabled



MCR
31:21 20 19 18 - 5 4 3 2 1 0
Reserved PWMMR6S PWMMR6R PWMMR6I - PWMMR1S PWMMR1R PWMMR1I PWMMR0S PWMMR0R PWMMR0I

PWMMRxI
This bit is used to Enable or Disable the PWM interrupts when the PWMTC matches PWMMRx (x:0-6)
0- Disable the PWM Match interrupt
1- Enable the PWM Match interrupt.

PWMMRxR
This bit is used to Reset PWMTC whenever it Matches PWMRx(x:0-6)
0- Do not Clear.
1- Reset the PWMTC counter value whenever it matches PWMRx.

PWMMRxS
This bit is used to Stop the PWMTC,PWMPC whenever the PWMTC matches PWMMRx(x:0-6).
0- Disable the PWM stop o match feature
1- Enable the PWM Stop feature. This will stop the PWM whenever the PWMTC reaches the Match register value.



PCR
31:15 14-9 8-7 6-2 1-0
Unused PWMENA6-PWMENA1 Unused PWMSEL6-PWMSEL2 Unused

PWMSELx
This bit is used to select the single edged and double edge mode form PWMx (x:2-6)
0- Single Edge mode for PWMx
1- Double Edge Mode for PWMx.

PWMENAx
This bit is used to enable/disable the PWM output for PWMx(x:1-6)
0- PWMx Disable.
1- PWMx Enabled.



LER
31-7 6 5 4 3 2 1 0
Unused LEN6 LEN5 LEN4 LEN3 LEN2 LEN1 LEN0

LENx
This bit is used Enable/Disable the loading of new Match value whenever the PWMTC is reset(x:0-6)
PWMTC will be continously incrementing whenever it reaches the PWMMRO, timer will be reset depeding on PWMTCR configuraion. Once the Timer is reset the New Match values will be loaded from MR0-MR6 depending on bits set in this register.
0- Disable the loading of new Match Values
1- Load the new Match values from MRx when the timer is reset.




PWM Working

After looking into the PWM registers, its time to see how the LPC1768 PWM module works.

The TC is continuously incremented and once it matches the MR1(Duty Cycle) the PWM pin is pulled Low. TC still continues to increment and once it reaches the Cycle time(Ton+Toff) the PWM module does the following things:

  • Reset the TC value.
  • Pull the PWM pin High.
  • Loads the new Match register values.

LPC1768 PWM.gif

Summary of PWM operations for the above image:

  • Slide1: The TC is being incremented as per the Pre-scalar configuration. The PWM output is high as the TC is still less that duty cycle.
  • Slide2: TC is incremented to 40 and still the PWM pin as HIGH.
  • Slide3: TC is incremented to 60 and it matches the Duty Cycle(MR1=60).
  • Slide4: Now the Comparator1(Green) will trigger the R of SR latch and Pulls the PWM output to Zero(Q=0). TC still continues to increment.
  • Slide5: TC is incremented to 80 and PWM pin is low as TC>Duty Cycle.
  • Slide6: Now TC is 100 and it matches the Cycle time(MR0==100).
  • Slide7: Now the Comparator2(Red) will trigger the S of SR latch and pulls the PWM output to ONE(Q==1). It also resets the TC to zero. It updates Shadow buffers with new Match values from MRO,MR1.



Steps to Configure PWM

  1. Configure the GPIO pins for PWM operation in respective PINSEL register.
  2. Configure TCR to enable the Counter for incrementing the TC, and Enable the PWM block.
  3. Set the required pre-scalar value in PR. In our case it will be zero.
  4. Configure MCR to reset the TC whenever it matches MR0.
  5. Update the Cycle time in MR0. In our case it will be 100.
  6. Load the Duty cycles for required PWMx channels in respective match registers MRx(x: 1-6).
  7. Enable the bits in LER register to load and latch the new match values.
  8. Enable the required pwm channels in PCR register.



Example

Program to demonstrates the variable PWM signal generation on PWM_1-PWM_4(P2_0 - P2_3).
Connect the Leds to the pins P2_0 to P2_3 and observe the led brigthness change depending on the dutycycle. 0 LPC1768 PWM.gif


Using ExploreEmbedded Libraries :

In the above tutorial we just discussed how to configure and use the PWM module of LPC1768
Once you know the internals of LPC7168 PWM, you can directly use the ExploreEmbedded libraries to generate the required PWM signals for different duty cycles.
The below sample code shows how to use the ExploreEmbedded PWM library.

Downloads

Download the complete project folder from the below link: https://codeload.github.com/ExploreEmbedded/Explore-Cortex-M3-LPC1768-Stick-DVB-14001/zip/master




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