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Co-authored-by: Cem Aksoylar <caksoylar@users.noreply.github.com>
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docs/docs/development/hardware-integration/shift-registers.md
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@ -15,7 +15,7 @@ To understand how shift registers work, we recommend reading through ["How does
## Design Guidelines ## Design Guidelines
The shift register output pins should act as MCU outputs in your design. All MCU inputs should remain connected directly to MCU/board pins. This is to allow the inputs to trigger "interrupts" on the MCU/board, upon which it will begin scanning the keys. Using a shift register for MCU inputs is also possible, but requires you to use a SOPI shift register and will harm your battery life. Note that a [direct kscan](../../config/kscan.md#direct-gpio-driver) keyboard does not have any MCU output pins, but a diodeless keyboard is still possible by making use of a single row pin and using the [matrix kscan](../../config/kscan.md#matrix-driver) driver. The shift register output pins should act as MCU outputs in your design. All MCU inputs should remain connected directly to MCU/board pins. This is to allow the inputs to trigger "interrupts" on the MCU/board, upon which it will begin scanning the keys. Using a shift register for MCU inputs is also possible, but requires you to use a PISO shift register and will harm your battery life. Note that a [direct kscan](../../config/kscan.md#direct-gpio-driver) keyboard does not have any MCU output pins, but a diodeless keyboard is still possible by making use of a single row pin and using the [matrix kscan](../../config/kscan.md#matrix-driver) driver.
:::info :::info
In a diode matrix, MCU output pins are those connected to the [anodes of your diodes](https://learn.sparkfun.com/tutorials/diodes/all#ideal-diodes). You most likely will need to rearrange your matrix to maximize the use of shift register output pins, in order to reduce the total number of GPIO pins connected to your MCU. For example, a 9 column 5 row `col2row` matrix could be rearranged to use 16 columns and 3 rows. In a diode matrix, MCU output pins are those connected to the [anodes of your diodes](https://learn.sparkfun.com/tutorials/diodes/all#ideal-diodes). You most likely will need to rearrange your matrix to maximize the use of shift register output pins, in order to reduce the total number of GPIO pins connected to your MCU. For example, a 9 column 5 row `col2row` matrix could be rearranged to use 16 columns and 3 rows.
@ -23,17 +23,17 @@ In a diode matrix, MCU output pins are those connected to the [anodes of your di
You will want to make sure that the data and clock pins of the shift register are connected to high frequency/SPI-capable pins on your MCU. Make sure that these lie on the same SPI bus for your microcontroller, if applicable -- for instance RP2040 controllers require this, while nRF52840 ones do not. It is generally recommended that you use the pre-defined pins for SPI, if your board comes with them. You will want to make sure that the data and clock pins of the shift register are connected to high frequency/SPI-capable pins on your MCU. Make sure that these lie on the same SPI bus for your microcontroller, if applicable -- for instance RP2040 controllers require this, while nRF52840 ones do not. It is generally recommended that you use the pre-defined pins for SPI, if your board comes with them.
For daisy chaining purposes, ZMK allows you to chain a maximum of four shift registers together. Below is a fragment of a schematic showing two shift registers daisy chained together. ZMK allows you to daisy-chain up to four shift registers. Below is a fragment of a schematic showing two shift registers that are daisy-chained.
![A fragment of a schematic featuring two shift registers daisy chained together.](../../assets/hardware-integration/shift-register-daisy.png) ![A fragment of a schematic featuring two shift registers daisy chained together.](../../assets/hardware-integration/shift-register-daisy.png)
## Configuration ## Configuration
In ZMK, the SPI bus of your MCU is used to communicate with shift registers, using MOSI for SIPO shift registers and MISO for SOPI shift reigsters. Thus you will first need to ensure that your board has SPI configured accordingly. Some boards, such as Seeed Studio's Xiao series, already has particular pins defined and configured for SPI (these can be changed if the MCU allows for alternative selections of SPI pins). Others, such as the nice!nano or any custom board, will need to be configured by you manually. Shift registers can share the SPI bus with other devices with no issues. In ZMK, the SPI bus of your MCU is used to communicate with shift registers, using MOSI for SIPO shift registers and MISO for PISO shift registers. Thus you will first need to ensure that your board has SPI configured accordingly. Some boards, such as Seeed Studio's Xiao series, already has particular pins defined and configured for SPI (these can be changed if the MCU allows for alternative selections of SPI pins). Others, such as the nice!nano or any custom board, will need to be configured by you manually. Shift registers can share the SPI bus with other devices with no issues.
### Configuring the SPI bus ### Configuring the SPI bus
Configuring the pins directly varies depending on your architecture. Presented are methods for overwriting the default SPI bus definitions for boards running the nRF52840 MCU. Alternative MCUs will be similar; refer to the [Zephyr documentation](https://docs.zephyrproject.org/3.5.0/index.html) for these. Also refer to said documentation if you are defining a new bus rather than overwriting an existing one. Configuring the pins directly varies depending on your architecture. Presented are methods for overwriting the default SPI bus definitions for boards running the nRF52840 MCU. Alternative MCUs will be similar; refer to the [Zephyr documentation](https://docs.zephyrproject.org/3.5.0/index.html) for these. Also refer to said documentation if you are defining a new bus rather than reconfiguring an existing one.
:::info :::info
This section can be skipped if you are using the default pins for SPI of a board, e.g. for the Seeed Studio Xiao the pin D8 for the clock signal and D10 for the data signal. However, if you are not making use of the MISO pin and wish to use said pin for alternative purposes, you will need to override the definition. This section can be skipped if you are using the default pins for SPI of a board, e.g. for the Seeed Studio Xiao the pin D8 for the clock signal and D10 for the data signal. However, if you are not making use of the MISO pin and wish to use said pin for alternative purposes, you will need to override the definition.