SPI Chip Select¶
Description¶
SPI master chip select (CSN)
This resource implements Serial Peripheral Interface (SPI) chip select pins (CSN) for the SPI Data Transfer resource. Multiple chip select pins can be used to access multiple SPI slaves, with shared clock and data lines for all slaves. The pins are mapped in the I/O Mapping Panel .
The CSN signal is active low.
The CSN pins can be configured individually as driven high (default) or pulled high while deasserted.
It is assumed that the SPI signals are not shared with another SPI master. The SCLK and MOSI lines are driven while CSN is deasserted to prevent these lines from floating. The MISO line can be configured as no pull, pull-up or pull-down. Note that the MISO input buffer is disabled while CSN is deasserted, and will not cause leakage if the pin is left floating in this state (with no pull).
For information on the SPI_POLx_PHAy constants, see the SPI Data Transfer resource documentation.
Examples¶
Write Multiple SPI Device Configuration Registers¶
// MACRO: Writes an 8-bit value to an accelerometer register with 8-bit address
macro spiWriteAccelReg(addr, value) {
spiBegin(AUXIO_SPI_CSN_ACCEL);
spiTx8bit(ACCEL_SPI_WRITE);
spiTx8bit(addr);
spiTx8bit(value);
spiEnd(AUXIO_SPI_CSN_ACCEL);
}
// Configure the SPI peripheral
spiCfg(SPI_POL0_PHA0, 1);
// Perform soft reset of the accelerometer, and wait for 50 ms
spiWriteAccelReg(ACCEL_REG_SOFT_RESET, ACCEL_SR_KEY);
fwDelayUs(50000);
// Perform one-time configuration of the accelerometer
spiWriteAccelReg(ACCEL_REG_INTMAP1, ACCEL_IM_DATA_READY);
spiWriteAccelReg(ACCEL_REG_POWER_CTL, ACCEL_PC_LOW_NOISE_0 | ACCEL_PC_MEASURE_ON);
Read Array of SPI Device Registers¶
// Read accelerometer data:
// - Write 8-bit register address
// - Read 16-bit value for each axis, most significant byte first (no need to swap endianess)
spiCfg(SPI_POL0_PHA0, 6);
spiBegin(AUXIO_SPI_CSN_ACCEL);
spiTx8bit(0x42);
spiRx16bit(output.x);
spiRx16bit(output.y);
spiRx16bit(output.z);
spiEnd(AUXIO_SPI_CSN_ACCEL);
Read Array of SPI Device Registers with Endianess Conversion¶
// MACRO: Reads a 16-bit value over SPI, and swaps endianess (LSB is received before MSB)
macro spiRx16bitSwapEndianess(value) {
U16 valueLsbFirst;
spiRx16bit(valueLsbFirst);
utilSwapEndianess(valueLsbFirst; value);
}
// Read accelerometer data:
// - Write 8-bit register address
// - Read 16-bit value for each axis, least significant byte first (need to swap endianess)
spiCfg(SPI_POL0_PHA0, 6);
spiBegin(AUXIO_SPI_CSN_ACCEL);
spiTx8bit(0x42);
spiRx16bitSwapEndianess(output.x);
spiRx16bitSwapEndianess(output.y);
spiRx16bitSwapEndianess(output.z);
spiEnd(AUXIO_SPI_CSN_ACCEL);
Procedures Overview¶
| Name | Brief description |
spiBegin() |
Asserts the selected SPI chip select pin (active low). More … |
spiEnd() |
Deasserts the selected SPI chip select pin (active low). More … |
Constants¶
None.
Global Variables¶
None.