Hardware Setup
VDD and VDDIO should be connected to a stable and low-noise 3.3V voltage regulator. Less noise on the supply voltage allows less jitter in the signal generation an clocking of the SAMD20 and therefore a more stable flight behaviour. Using a linear regulator connected directly to the flight battery can cause quite some heat, so bringin the voltage down in multiple steps would be an option. For example a switching regulator from battery to 5V could be used.
| BMF055 PIN | Usage |
|---|---|
| PA22 | motor 0 |
| PA23 | motor 1 |
| PB00 | motor 2 |
| PB01 | motor 3 |
| PA21 | Spektrum receiver TX |
| PA28 | bootpin |
| PA24 | LED |
| PB16 | UART TX |
| PB17 | UART RX |
If a Bootloader is installed bootpin has to be pulled low at startup to enter the bootloader.
Uart is used for bootloader and setting control loop parameters or radio configuration via Multiwii GUI. These settings are stored in the micro controller's flash via EEPROM emulation. To reserve an area in the SAMD20's flash for EEPROM emulation the NVMCTRL_EEPROM_SIZE fuse hat to be set to 0x04.
The SAMD20 allows only limited pin periphrial multiplexing for so the amount of possible configurations for this application is very limited. Here is an overwiew which components are already in use.
| Module | Usage |
|---|---|
| timer/counter 6 | system, milliseconds, microseconds |
| timer/counter 4 | motor 0,1 |
| timer/counter 7 | motor 2,3 |
| Sercom 5 | USART for bootloader and configuration |
| Sercom 3 | USART for Spektrum receiver |
| Sercom 1 | internal sensor communication via SPI |
This image shows which motor is assigned to which pin in software and which rotation direction is expected. The arrow in the middle represtents the forward flight direction.
These pins are blocked and can be re-used for other purposes under some curcumstances. In this software build a double usage of pins is avoided. So these pins are for Sensor internal use only:
| BMF055 PIN | Usage |
|---|---|
| PA19 | sensor SPI MISO |
| PA16 | sensor SPI MOSI |
| PA17 | sensor SPI SCLK |
| PA18 | SPI CS1 |
| PA27 | SPI CS2 |
| PB23 | interrupt 1 |
| PB03 | interrupt 2 |
| PA30 | SWCLK SWD prog |
| PA31 | SWDIO SWD prog |
