Pulse width modulation allows you to create a block wave with a certain duty cycle. It is useful for use cases like dimming LEDs.
<--> pulse-width
___ ____ ____
| | | | | duty-cycle = pulse-width / period
|________| |________| |______
<--period--->
There are 12 PWM channels.
Each channel consists of a free-running 8-bit counter with two configurable comparators. The period comparator resets the counter value back to zero once it reaches a user-configured top value. This sets the period (number of cycles between rising-edges of pulses) of the output waveform. The pulse width comparator changes the channel output from high to low when the counter value surpasses a user-configured pulse width value. This sets the pulse width (number of cycles the output spends high) of the output waveform.
Each channel has a 64-bit section in the address space for configuration.
| Offset | PWM channel |
|---|---|
| 0x00 | Channel 0 |
| 0x08 | Channel 1 |
| 0x10 | Channel 2 |
| 0x18 | Channel 3 |
| 0x20 | Channel 4 |
| 0x28 | Channel 5 |
| 0x30 | Channel 6 |
To see which pins can be connected to which channel please consult the pin multiplexer.
For each channel, there is a 32-bit register defining the pulse width and another 32-bit register above it defining how long the complete wave is (counter top). These registers are write-only, and will return a value of zero if read. The counters are only 8-bit wide, any values written that are larger than 8-bits are invalid.
| Offset | Description | Read/Write |
|---|---|---|
| 0x0 | Pulse width | Write-only |
| 0x4 | Counter top | Write-only |
A channel is enabled by setting an non-zero counter top value.
To generate an always-low signal, set a counter top value of zero. To generate an always-high signal, set a pulse-width value greater than the (non-zero) counter top value.