In this lesson, you will learn how to squeeze maximum data bandwidth out of your SPI bus by balancing clock frequency, data word sizes, and hardware FIFO utilization.
When interfacing with high-bandwidth sensors or displays, the SPI bus often becomes a bottleneck. To optimize throughput, we must move beyond the simple "send one byte, wait, repeat" approach and focus on three architectural levers.
1. Maximizing Clock Prescalers
The clock prescaler determines how many peripheral clock cycles make up one SPI bit period. While you want the highest frequency possible, you are constrained by the sensor’s maximum rating and signal integrity limits. Always check your datasheet for the "Max SPI Clock Frequency." If your system clock is 80MHz and the sensor supports 10MHz, ensure your prescaler is set to 8 (80/8 = 10) rather than a conservative value like 16 or 32.
2. Tuning Data Word Size
Most SPI peripherals can be configured for 8-bit or 16-bit word sizes. If your sensor sends 16-bit samples, configuring the SPI hardware for 16-bit mode is significantly faster than two 8-bit transfers. This reduces the overhead of the "inter-frame delay"—the mandatory pause the hardware controller often inserts between individual words to reset the shift register logic.
3. Leveraging Hardware FIFOs
Modern microcontrollers include First-In-First-Out (FIFO) buffers for SPI. If you transmit one byte at a time using polling, the CPU spends most of its time waiting for the bus. By filling the hardware FIFO to its capacity (e.g., 8 or 16 levels deep), you allow the SPI hardware to clock out data continuously without waiting for the CPU to provide the next byte.
// Example: Optimizing a bulk transfer using FIFO
void SPI_Send_Burst(uint16_t *data, uint16_t length) {
for (int i = 0; i < length; i++) {
// Wait until FIFO has space
while (!(SPI1->SR & SPI_SR_TXE));
// Write word; hardware handles clocking and FIFO shifting
SPI1->DR = data[i];
}
}
By keeping the FIFO full and using the largest word size supported by both the master and slave, you ensure the SPI clock never "idles" between transmissions, maximizing your effective data rate.
Key Takeaway: Maximize SPI throughput by aligning hardware word sizes with your data packets, setting the highest stable clock frequency, and saturating the hardware FIFO to eliminate idle bus cycles.