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:

Sat Oct 04 13:14:44 2014 +0000
Added NUCLEO F103RB; adapted t 32/16 bit pwm struct for NUCLEO

Who changed what in which revision?

UserRevisionLine numberNew contents of line
Sissors 13:cdefd9d75b64 1 #include "mbed.h"
Sissors 13:cdefd9d75b64 2
Sissors 13:cdefd9d75b64 3 #ifdef TARGET_NUCLEO_F030R8
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
jocis 15:49a7eff133b3 26 #if defined TARGET_NUCLEO_F401RE || defined TARGET_NUCLEO_F411RE
Sissors 13:cdefd9d75b64 27 __IO uint32_t* getChannel(TIM_TypeDef* pwm, PinName pin) {
Sissors 13:cdefd9d75b64 28 switch (pin) {
jocis 16:ec208b5ec0bb 29 // Channels 1 : PWMx/1
Sissors 13:cdefd9d75b64 30 case PA_0: case PA_5: case PA_6: case PA_8: case PA_15: case PB_4: case PB_6: case PC_6: case PA_7: case PB_13:
Sissors 13:cdefd9d75b64 31 return &pwm->CCR1;
Sissors 13:cdefd9d75b64 32
jocis 16:ec208b5ec0bb 33 // Channels 2 : PWMx/2
Sissors 13:cdefd9d75b64 34 case PA_1: case PA_9: case PB_3: case PB_5: case PB_7: case PC_7: case PB_0: case PB_14:
Sissors 13:cdefd9d75b64 35 return &pwm->CCR2;
Sissors 13:cdefd9d75b64 36
jocis 16:ec208b5ec0bb 37 // Channels 3 : PWMx/3
Sissors 13:cdefd9d75b64 38 case PA_2: case PA_10: case PB_8: case PB_10: case PC_8: case PB_1: case PB_15:
Sissors 13:cdefd9d75b64 39 return &pwm->CCR3;
Sissors 13:cdefd9d75b64 40
jocis 16:ec208b5ec0bb 41 // Channels 4 : PWMx/4
Sissors 13:cdefd9d75b64 42 case PA_3: case PA_11: case PB_9: case PC_9:
Sissors 13:cdefd9d75b64 43 return &pwm->CCR4;
Sissors 13:cdefd9d75b64 44 }
Sissors 13:cdefd9d75b64 45 return NULL;
Sissors 13:cdefd9d75b64 46 }
jocis 16:ec208b5ec0bb 47 #endif
jocis 16:ec208b5ec0bb 48
jocis 16:ec208b5ec0bb 49 #if defined TARGET_NUCLEO_F103RB
jocis 17:8378bc456f0d 50 __IO uint16_t* getChannel(TIM_TypeDef* pwm, PinName pin) {
jocis 16:ec208b5ec0bb 51 switch (pin) {
jocis 16:ec208b5ec0bb 52 // Channels 1 : PWMx/1
jocis 16:ec208b5ec0bb 53 case PA_6: case PA_8: case PA_15: case PB_4: case PC_6: case PB_13:
jocis 17:8378bc456f0d 54 return &pwm->CCR1;
jocis 16:ec208b5ec0bb 55
jocis 16:ec208b5ec0bb 56 // Channels 2 : PWMx/2
jocis 16:ec208b5ec0bb 57 case PA_1: case PA_7: case PA_9: case PB_3: case PB_5: case PC_7: case PB_14:
jocis 17:8378bc456f0d 58 return &pwm->CCR2;
jocis 16:ec208b5ec0bb 59
jocis 16:ec208b5ec0bb 60 // Channels 3 : PWMx/3
jocis 16:ec208b5ec0bb 61 case PA_2: case PA_10: case PB_0: case PB_10: case PC_8: case PB_15:
jocis 17:8378bc456f0d 62 return &pwm->CCR3;
jocis 16:ec208b5ec0bb 63
jocis 16:ec208b5ec0bb 64 // Channels 4 : PWMx/4
jocis 16:ec208b5ec0bb 65 case PA_3: case PA_11: case PB_1: case PB_11: case PC_9:
jocis 17:8378bc456f0d 66 return &pwm->CCR4;
jocis 16:ec208b5ec0bb 67 }
jocis 16:ec208b5ec0bb 68 return NULL;
jocis 16:ec208b5ec0bb 69 }
Sissors 13:cdefd9d75b64 70 #endif