spi_master for AVR (#8299)

* Change _delay_ms/us() to wait_ms/us()

* Switch to platform-agnostic GPIO macros

* Add AVR spi_master and migrate Adafruit BLE code

* Set verbose back to false

* Add clock divisor, bit order and SPI mode configuration for init

* Add start and stop functions

* Move configuration of mode, endianness and speed to `spi_start()`

* Some breaks here would be good

* Default Adafruit BLE clock divisor to 4 (2MHz on the Feather 32U4)

* Remove mode and divisor enums

* Add some docs

* No hr at EOF

* Add links in sidebar

1 files changed, 128 insertions, 0 deletions

diff --git a/docs/spi_driver.md b/docs/spi_driver.md
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+# SPI Master Driver
+
+The SPI Master drivers used in QMK have a set of common functions to allow portability between MCUs.
+
+## AVR Configuration
+
+No special setup is required - just connect the `SS`, `SCK`, `MOSI` and `MISO` pins of your SPI devices to the matching pins on the MCU:
+
+|MCU            |`SS`|`SCK`|`MOSI`|`MISO`|
+|---------------|----|-----|------|------|
+|ATMega16/32U2/4|`B0`|`B1` |`B2`  |`B3`  |
+|AT90USB64/128  |`B0`|`B1` |`B2`  |`B3`  |
+|ATmega32A      |`B4`|`B7` |`B5`  |`B6`  |
+|ATmega328P     |`B2`|`B5` |`B3`  |`B4`  |
+
+You may use more than one slave select pin, not just the `SS` pin. This is useful when you have multiple devices connected and need to communicate with them individually.
+`SPI_SS_PIN` can be passed to `spi_start()` to refer to `SS`.
+
+## ARM Configuration
+
+ARM support for this driver is not ready yet. Check back later!
+
+## Functions
+
+### `void spi_init(void)`
+
+Initialize the SPI driver. This function must be called only once, before any of the below functions can be called.
+
+---
+
+### `void spi_start(pin_t slavePin, bool lsbFirst, uint8_t mode, uint8_t divisor)`
+
+Start an SPI transaction.
+
+#### Arguments
+
+ - `pin_t slavePin`  
+   The QMK pin to assert as the slave select pin, eg. `B4`.
+ - `bool lsbFirst`  
+   Determines the endianness of the transmission. If `true`, the least significant bit of each byte is sent first.
+ - `uint8_t mode`  
+   The SPI mode to use:
+
+   |Mode|Clock Polarity      |Clock Phase            |
+   |----|--------------------|-----------------------|
+   |`0` |Leading edge rising |Sample on leading edge |
+   |`1` |Leading edge rising |Sample on trailing edge|
+   |`2` |Leading edge falling|Sample on leading edge |
+   |`3` |Leading edge falling|Sample on trailing edge|
+
+ - `uint8_t divisor`  
+   The SPI clock divisor, will be rounded up to the nearest power of two. This number can be calculated by dividing the MCU's clock speed by the desired SPI clock speed. For example, an MCU running at 8 MHz wanting to talk to an SPI device at 4 MHz would set the divisor to `2`.
+
+---
+
+### `spi_status_t spi_write(uint8_t data, uint16_t timeout)`
+
+Write a byte to the selected SPI device.
+
+#### Arguments
+
+ - `uint8_t data`  
+   The byte to write.
+ - `uint16_t timeout`  
+   The amount of time to wait, in milliseconds, before timing out.
+
+#### Return Value
+
+`SPI_STATUS_TIMEOUT` if the timeout period elapses, or `SPI_STATUS_SUCCESS`.
+
+---
+
+### `spi_status_t spi_read(uint16_t timeout)`
+
+Read a byte from the selected SPI device.
+
+#### Arguments
+
+ - `uint16_t timeout`  
+   The amount of time to wait, in milliseconds, before timing out.
+
+#### Return Value
+
+`SPI_STATUS_TIMEOUT` if the timeout period elapses, or the byte read from the device.
+
+---
+
+### `spi_status_t spi_transmit(const uint8_t *data, uint16_t length, uint16_t timeout)`
+
+Send multiple bytes to the selected SPI device.
+
+#### Arguments
+
+ - `const uint8_t *data`  
+   A pointer to the data to write from.
+ - `uint16_t length`  
+   The number of bytes to write. Take care not to overrun the length of `data`.
+ - `uint16_t timeout`  
+   The amount of time to wait, in milliseconds, before timing out.
+
+#### Return Value
+
+`SPI_STATUS_TIMEOUT` if the timeout period elapses, `SPI_STATUS_SUCCESS` on success, or `SPI_STATUS_ERROR` otherwise.
+
+---
+
+### `spi_status_t spi_receive(uint8_t *data, uint16_t length, uint16_t timeout)`
+
+Receive multiple bytes from the selected SPI device.
+
+#### Arguments
+
+ - `uint8_t *data`  
+   A pointer to the buffer to read into.
+ - `uint16_t length`  
+   The number of bytes to read. Take care not to overrun the length of `data`.
+ - `uint16_t timeout`  
+   The amount of time to wait, in milliseconds, before timing out.
+
+#### Return Value
+
+`SPI_STATUS_TIMEOUT` if the timeout period elapses, `SPI_STATUS_SUCCESS` on success, or `SPI_STATUS_ERROR` otherwise.
+
+---
+
+### `void spi_stop(void)`
+
+End the current SPI transaction. This will deassert the slave select pin and reset the endianness, mode and divisor configured by `spi_start()`.