Communication Delay
Time elapsed between the initiation of a data request by the master controller and the receipt of the final bit from the slave device defines the speed of the interface. This spi latency includes the setup time for the chip select signal, the clocking of the actual data bits and the processing time required by the sensor to prepare the response. High speed control loops require this delay to be minimized to ensure that the sensor information is as fresh as possible when the algorithm executes.
Because the serial peripheral interface is synchronous, the clock frequency sets the hard floor for this duration.
Clock Frequency
Throughput of the bus depends heavily on the maximum frequency supported by the slowest device on the line. Long traces on the circuit board introduce capacitance that rounds off the square waves.
Interrupt Overhead
Software execution on the main processor adds a measured delay to the hardware communication cycle. When the SPI peripheral finishes a transfer, the processor must respond to an interrupt to move the data from the buffer to the main memory. If the processor is busy with higher priority tasks, the data sits in the buffer, increasing the effective spi latency of the system.
Direct memory access controllers bypass the CPU to move data directly, which dramatically reduces this software induced delay.
Propagation Boundary
Physical distance between the devices limits the maximum speed due to the flight time of the signal along the copper. At very high clock rates, the delay for the signal to travel from the master to the slave and back can exceed the timing window for a valid bit capture. This effect becomes critical in systems where the sensor is located on a separate board connected by a ribbon cable.
Most designs keep these traces short to ensure the spi latency remains dominated by the device logic instead of the cable.
