ARM Cortex M33 Notes

These are some basic notes on the ARM Cortex M33 device we're using.

Cortex Details

We're using the 48-pin QFN ARM Cortex M33 - the pinout is available on figure 1.6 on page 15 of the datasheet.

Port Reference

• SPI_SSLAx: Slave selection pins (x is 0-3)
• SPI_MOSI_A: Data output from master (Master Out Slave In)
• SPI_MISO_A: Data output from slave (Master In Slave Out)
• SPI_RSPCK_A: Clock pin
• I2C_SDA2: Serial Data line
• I2C_SCL2: Serial Clock line

Our Setup

We have two SPI-connected devices (accelerometer and magnetometer), and 5 I2C-connected devices (accelerometer, pressure sensor, gyroscope, magnetometer and temp sensor).

Since 2 of the 4 SPI chip select ports double as I2C inputs on the MCU, we only have 2 accessible SPI ports.

SPI Notes

Serial Peripheral Interface has four relevant ports: SSLAx (connected to chip selects), MOSI (data from master to slave), MISO (data from slave to master), and RSPCK (the clock).

Since the MCU can only communicate with one SPI device at a time, it must "choose" this device. It does this by activating one of the SSLA (Slave Select Low Active), by settling the SSLA for that device to active low.

Therefore, you need a single SSLA pin per SPI device. The MCU has 4 SSLA pins, so it is able to support up to 4 SPI devices.