Library that allows for higher resolution and speed than standard mbed PWM library using same syntax (drop-in replacement).

Dependents:   PwmOscillator FastStepDriver TLC5940 CameraTest ... more

FastPWM is a library that unlocks alot more of the potential of the mbed's PWM units than the normal PWM library. It is currently available for the LPC1768, LPC11u24, KLxxZ, K20D50M and most STM32 targets (those lacking are fairly easy to add). (Since I generally forget to update this list, if you want to know if your target is supported see if it compiles).

The two main points it allows for compared to the regular PwmOut library is clock cycle precision PWM and (automated) changing prescalers. It isn't perfect yet, but for now it will have to do ;). For those familiar with version 1, version 2 is almost completely rewritten to add more functions.


FastPWM is largely a drop-in replacement for the normal PwmOut library. All the same functions are available, with some extras.



All prescaler options are disabled for 32-bit PWM units currently, the prescaler is fixed at 1


With this function you can set the value of the prescaler. Aditionally the second argument of the constructor is used for the same to directly set it from the constructor. It returns the actual prescaler which is set. If the requested one isn't available it is always rounded up (unless it is larger than the maximum prescaler).

There are three options for this function. Any value larger than zero will simply be set. (Yes it is signed, so yes you cannot use the full 32-bit prescaler if your device supports it, I cannot imagine why you possibly would want that). If the value is zero dynamic prescaling is disabled and the current prescaler is returned. If the value is -1 dynamic prescaling is enabled and the current prescaler is returned.

So what is dynamic prescaling? This is the default option for FastPWM, don't use any prescaler option and it is enabled. To start with the negative, it adds quite some processing cycles, so changing the period takes longer. Luckily generally the PWM period is constant. The good part is that it automatically adapts the prescaler unit to give as much accuracy as possible: It gives highest accuracy for the duty-cycle, and also allows you to generate a wide range of periods. For example you can now create a LED blinking at 1Hz with FastPWM on the LPC11u24/Nucleo 16-bit PWM units. (On the KL25Z this isn't possible due to limitted value of the prescaler).

As the nice warning message above says, this is currently only implemented for 16-bit PWM units, simply because normally you won't need it for 32-bit PWM units. For those it is automatically disabled, and you cannot enable it. However for example the majority of the PWM units of the LPC11u24 can't be used to make servo signals with PwmOut, with FastPWM they can.

TL;DR, by default it uses dynamic prescaling. Unless period is changed very often just keep it on default and enjoy your larger range of possible periods and higher accuracy.



These two functions allow you to directly write the pwm period and pulsewidth in clock ticks. This is useful if you need to have very little overhead. It is dependent on which device you use, since they have different clock rates. You can get the current clock speed of your device with SystemCoreClock.


PwmOut uses floats for setting the time in seconds, and ints for milliseconds and microseconds. All three of those don't give enough accuracy to fully use the PWM units. Which is why FastPWM uses besides int for milliseconds and microseconds, it uses doubles for seconds and also for microseconds. Generally it is adviced to use these doubles, sometimes you might need to explicitly cast your variables to doubles.

Currently setting pulsewidth in microseconds with an int is a risk with some prescaler values (not on the 32-bit timers). See known-issues.

Adding other microcontrollers

Look at the other device files on how to add other microcontrollers. Functions that need to be implemented:

  • initFastPWM(): Any setups required can be done here. Must set the number of bits of the PWM unit.
  • pulsewidth_ticks( uint32_t ticks ): Set the pulsewidth in ticks
  • period_ticks( uint32_t ticks ): Set the period in ticks
  • getPeriod(): Return the period in ticks
  • setPrescaler(uint32_t reqScale): Set the prescaler. If reqScale is zero, return the current prescaler value only. Otherwise set the requested prescaler, if needed round up to first available prescaler, return the actually set prescaler. If the PWM unit is 32-bit (or for another reason), you can set dynamicPrescaler as false, disabling the dynamic prescaler.

Known Issues

  • Changing the prescaler manually does not adapt periods/pulsewidth
    • Manually re-set the period of each FastPWM object on that PWM unit, this should also set the duty cycle.
  • Changing the period of one FastPWM object does not keep the duty cycle of other PWM objects on that PWM unit constant, but the pulsewidth.
    • Manually re-set the duty cycle of other PWM objects.
  • PwmOut objects run at wrong speed when you use FastPWM
    • Don't use PwmOut objects.
  • On certain prescaler values setting period/pulsewidth in especially microsecond integers, also to lesser extend also millisecond integers, can result in wrong values.
    • The problem is that the number of clock ticks per microsecond/millisecond as integers are pre-calculated for improved speed. However if it isn't an integer number that gives an error.
    • Solution is to preferably use doubles (or ticks). On the 32-bit pwm units this is not an issue, so for them it doesn't matter.
    • I am planning to have a further look into it, but I expect it to stay an issue.

Here the TL;DR is: Preferably set the period/prescaler once at the beginning before setting the duty-cycle/pulsewidth. If that isn't possible, take into account duty cyles need to be set again. And preferably use doubles.


Some of the ideas are 'loaned' from Jochen Krapf's fork of the original FastPWM:

Mon Feb 29 19:18:42 2016 +0000
STM32F3, F4, L4, etc now use the channels number from the mbed library. The others still need to use the manual way.

Who changed what in which revision?

UserRevisionLine numberNew contents of line
Sissors 13:cdefd9d75b64 1 #include "mbed.h"
Sissors 13:cdefd9d75b64 2
Sissors 27:7f484dd7323d 3 #if defined (TARGET_NUCLEO_F030R8) || (TARGET_DISCO_F051R8)
Sissors 13:cdefd9d75b64 4 __IO uint32_t* getChannel(TIM_TypeDef* pwm, PinName pin) {
Sissors 13:cdefd9d75b64 5 switch (pin) {
Sissors 13:cdefd9d75b64 6 // Channels 1
Sissors 13:cdefd9d75b64 7 case PA_4: case PA_6: case PB_1: case PB_4: case PB_8: case PB_9: case PB_14: case PC_6: case PB_6: case PB_7:
Sissors 13:cdefd9d75b64 8 return &pwm->CCR1;
Sissors 13:cdefd9d75b64 9
Sissors 13:cdefd9d75b64 10 // Channels 2
Sissors 13:cdefd9d75b64 11 case PA_7: case PB_5: case PC_7:
Sissors 13:cdefd9d75b64 12 return &pwm->CCR2;
Sissors 13:cdefd9d75b64 13
Sissors 13:cdefd9d75b64 14 // Channels 3
Sissors 13:cdefd9d75b64 15 case PB_0: case PC_8:
Sissors 13:cdefd9d75b64 16 return &pwm->CCR3;
Sissors 13:cdefd9d75b64 17
Sissors 13:cdefd9d75b64 18 // Channels 4
Sissors 13:cdefd9d75b64 19 case PC_9:
Sissors 13:cdefd9d75b64 20 return &pwm->CCR4;
Sissors 13:cdefd9d75b64 21 }
Sissors 13:cdefd9d75b64 22 return NULL;
Sissors 13:cdefd9d75b64 23 }
Sissors 13:cdefd9d75b64 24 #endif
Sissors 13:cdefd9d75b64 25
Sissors 27:7f484dd7323d 26 #if defined (TARGET_NUCLEO_F103RB) || (TARGET_DISCO_F100RB)
Sissors 24:1f451660d8c0 27 __IO uint32_t* getChannel(TIM_TypeDef* pwm, PinName pin) {
jocis 16:ec208b5ec0bb 28 switch (pin) {
jocis 16:ec208b5ec0bb 29 // Channels 1 : PWMx/1
jocis 16:ec208b5ec0bb 30 case PA_6: case PA_8: case PA_15: case PB_4: case PC_6: case PB_13:
jocis 17:8378bc456f0d 31 return &pwm->CCR1;
jocis 16:ec208b5ec0bb 32
jocis 16:ec208b5ec0bb 33 // Channels 2 : PWMx/2
jocis 16:ec208b5ec0bb 34 case PA_1: case PA_7: case PA_9: case PB_3: case PB_5: case PC_7: case PB_14:
jocis 17:8378bc456f0d 35 return &pwm->CCR2;
jocis 16:ec208b5ec0bb 36
jocis 16:ec208b5ec0bb 37 // Channels 3 : PWMx/3
jocis 16:ec208b5ec0bb 38 case PA_2: case PA_10: case PB_0: case PB_10: case PC_8: case PB_15:
jocis 17:8378bc456f0d 39 return &pwm->CCR3;
jocis 16:ec208b5ec0bb 40
jocis 16:ec208b5ec0bb 41 // Channels 4 : PWMx/4
jocis 16:ec208b5ec0bb 42 case PA_3: case PA_11: case PB_1: case PB_11: case PC_9:
jocis 17:8378bc456f0d 43 return &pwm->CCR4;
jocis 16:ec208b5ec0bb 44 }
jocis 16:ec208b5ec0bb 45 return NULL;
jocis 16:ec208b5ec0bb 46 }
Sissors 19:ba7a5bf634b3 47 #endif
Sissors 19:ba7a5bf634b3 48
altaran 20:3c609bc4ae9c 49 #if defined TARGET_NUCLEO_F072RB
altaran 20:3c609bc4ae9c 50 __IO uint32_t* getChannel(TIM_TypeDef* pwm, PinName pin) {
altaran 20:3c609bc4ae9c 51 switch (pin) {
altaran 20:3c609bc4ae9c 52 // Channels 1 : PWMx/1
altaran 20:3c609bc4ae9c 53 case PA_2: case PA_6: case PA_4: case PA_7: case PA_8: case PB_1: case PB_4: case PB_8: case PB_9: case PB_14: case PC_6:
altaran 21:aa2884be5496 54 // Channels 1N : PWMx/1N
altaran 21:aa2884be5496 55 case PA_1: case PB_6: case PB_7: case PB_13:
altaran 20:3c609bc4ae9c 56 return &pwm->CCR1;
altaran 20:3c609bc4ae9c 57
altaran 20:3c609bc4ae9c 58 // Channels 2 : PWMx/2
altaran 20:3c609bc4ae9c 59 case PA_3: case PA_9: case PB_5: case PC_7: case PB_15:
altaran 20:3c609bc4ae9c 60 return &pwm->CCR2;
altaran 20:3c609bc4ae9c 61
altaran 20:3c609bc4ae9c 62 // Channels 3 : PWMx/3
altaran 20:3c609bc4ae9c 63 case PA_10: case PB_0: case PC_8:
altaran 20:3c609bc4ae9c 64 return &pwm->CCR3;
altaran 20:3c609bc4ae9c 65
altaran 20:3c609bc4ae9c 66 // Channels 4 : PWMx/4
altaran 20:3c609bc4ae9c 67 case PA_11: case PC_9:
altaran 20:3c609bc4ae9c 68 return &pwm->CCR4;
altaran 20:3c609bc4ae9c 69 }
altaran 20:3c609bc4ae9c 70 return NULL;
altaran 20:3c609bc4ae9c 71 }
Sissors 26:0c924507a81f 72 #endif
Sissors 26:0c924507a81f 73
Sissors 28:3c8a0d977bc3 74
Sissors 27:7f484dd7323d 75
Sissors 27:7f484dd7323d 76 #if defined (TARGET_NUCLEO_L152RE)
Sissors 27:7f484dd7323d 77 __IO uint32_t* getChannel(TIM_TypeDef* pwm, PinName pin) {
Sissors 27:7f484dd7323d 78 switch (pin) {
Sissors 27:7f484dd7323d 79 // Channels 1 : PWMx/1
Sissors 27:7f484dd7323d 80 case PA_6: case PB_4: case PB_12: case PB_13: case PC_6:
Sissors 27:7f484dd7323d 81 return &pwm->CCR1;
Sissors 27:7f484dd7323d 82
Sissors 27:7f484dd7323d 83 // Channels 2 : PWMx/2
Sissors 27:7f484dd7323d 84 case PA_1: case PA_7: case PB_3: case PB_5: case PB_14: case PB_7: case PC_7:
Sissors 27:7f484dd7323d 85 return &pwm->CCR2;
Sissors 27:7f484dd7323d 86
Sissors 27:7f484dd7323d 87 // Channels 3 : PWMx/3
Sissors 27:7f484dd7323d 88 case PA_2: case PB_0: case PB_8: case PB_10: case PC_8:
Sissors 27:7f484dd7323d 89 return &pwm->CCR3;
Sissors 27:7f484dd7323d 90
Sissors 27:7f484dd7323d 91 // Channels 4 : PWMx/4
Sissors 27:7f484dd7323d 92 case PA_3: case PB_1:case PB_9: case PB_11: case PC_9:
Sissors 27:7f484dd7323d 93 return &pwm->CCR4;
Sissors 27:7f484dd7323d 94 default:
Sissors 27:7f484dd7323d 95 /* NOP */
Sissors 27:7f484dd7323d 96 break;
Sissors 27:7f484dd7323d 97 }
Sissors 27:7f484dd7323d 98 return NULL;
Sissors 27:7f484dd7323d 99 }
Sissors 27:7f484dd7323d 100 #endif
Sissors 28:3c8a0d977bc3 101
Sissors 28:3c8a0d977bc3 102