The [Audio feature](feature_audio.md) breaks the hardware specifics out into separate, exchangeable driver units, with a common interface to the audio-"core" - which itself handles playing songs and notes while tracking their progress in an internal state, initializing/starting/stopping the driver as needed.
Not all MCUs support every available driver, either the platform-support is not there (yet?) or the MCU simply does not have the required hardware peripheral.
## AVR :id=avr
Boards built around an Atmega32U4 can use two sets of PWM capable pins, each driving a separate speaker.
The possible configurations are:
| | Timer3 | Timer1 |
|--------------|-------------|--------------|
| one speaker | C4,C5 or C6 | |
| one speaker | | B4, B5 or B7 |
| two speakers | C4,C5 or C6 | B4, B5 or B7 |
Currently there is only one/default driver for AVR based boards, which is automatically configured to:
```make
AUDIO_DRIVER = pwm_hardware
```
## ARM :id=arm
For Arm based boards, QMK depends on ChibiOS - hence any MCU supported by the later is likely usable, as long as certain hardware peripherals are available.
Supported wiring configurations, with their ChibiOS/MCU peripheral requirement are listed below;
piezo speakers are marked with :one: for the first/primary and :two: for the secondary.
| `AUDIO_DAC_SAMPLE_MAX` | `4095U` | Highest value allowed. Lower value means lower volume. And 4095U is the upper limit, since this is limited to a 12 bit value. Only effects non-pregenerated samples. |
| `AUDIO_DAC_OFF_VALUE` | `AUDIO_DAC_SAMPLE_MAX / 2` | The value of the DAC when notplaying anything. Some setups may require a high (`AUDIO_DAC_SAMPLE_MAX`) or low (`0`) value here. |
| `AUDIO_MAX_SIMULTANEOUS_TONES` | __see next table__ | The number of tones that can be played simultaneously. A value that is too high may freeze the controller or glitch out when too many tones are being played. |
| `AUDIO_DAC_SAMPLE_RATE` | __see next table__ | Effective bit rate of the DAC (in hertz), higher limits simultaneous tones, and lower sacrifices quality. |
There are a number of predefined quality settings that you can use, with "sane minimum" being the default. You can use custom values by simply defining the sample rate and number of simultaneous tones, instead of using one of the listed presets.
This driver uses the ChibiOS-PWM system to produce a square-wave on specific output pins that are connected to the PWM hardware.
The hardware directly toggles the pin via its alternate function. See your MCU's data-sheet for which pin can be driven by what timer - looking for TIMx_CHy and the corresponding alternate function.
A configuration example for the STM32F103C8 would be:
ChibiOS uses GPIOv1 for the F103, which only knows of one alternate function.
On 'larger' STM32s, GPIOv2 or GPIOv3 are used; with them it is also necessary to configure `AUDIO_PWM_PAL_MODE` to the correct alternate function for the selected pin, timer and timer-channel.
### PWM software :id=pwm-software
This driver uses the PWM callbacks from PWMD1 with TIM1_CH1 to toggle the selected AUDIO_PIN in software.
During the same callback, with AUDIO_PIN_ALT_AS_NEGATIVE set, the AUDIO_PIN_ALT is toggled inversely to AUDIO_PIN. This is useful for setups that drive a piezo from two pins (instead of one and Gnd).
You can also change the timer used for software PWM by defining the driver. For instance:
```c
#define AUDIO_STATE_TIMER GPTD8
```
### Testing Notes :id=testing-notes
While not an exhaustive list, the following table provides the scenarios that have been partially validated:
