SPI Data Rate Calculator Formula

Understand the math behind the spi data rate calculator. Each variable explained with a worked example.

Use the SPI Data Rate Calculator

Formulas Used

Raw Data Rate

raw_data_rate = spi_clock_mhz * 1e6 / 8 / 1e6

Effective Data Rate

effective_rate = spi_clock_mhz * 1e6 * bits_per_transfer / clocks_per_transfer / 8 / 1e6

Transfers per Second

transfers_per_sec = spi_clock_mhz * 1e6 / clocks_per_transfer

Bus Efficiency

bus_efficiency = bits_per_transfer / clocks_per_transfer * 100

Variables

VariableDescriptionDefault
spi_clock_mhzSPI Clock Speed(MHz)8
bits_per_transferBits per Transfer8
idle_clocksIdle Clocks Between Transfers2
clocks_per_transferDerived value= bits_per_transfer + idle_clockscalculated

How It Works

SPI Data Rate

SPI (Serial Peripheral Interface) transfers one bit per clock cycle. The effective rate depends on clock speed and protocol overhead.

Formulas

Raw Rate = Clock Speed / 8 (bytes per second)

Effective Rate = Clock x Data Bits / (Data Bits + Idle) / 8

Idle clocks account for chip-select toggling and inter-transfer gaps. SPI has no addressing overhead unlike I2C, making it faster for bulk transfers.

Worked Example

8 MHz SPI clock, 8-bit transfers, 2 idle clocks between transfers.

spi_clock_mhz = 8bits_per_transfer = 8idle_clocks = 2
  1. 01Raw rate: 8e6 / 8 = 1.00 MB/s
  2. 02Clocks per transfer: 8 + 2 = 10
  3. 03Effective rate: 8e6 x 8 / 10 / 8 = 0.80 MB/s
  4. 04Bus efficiency: 8 / 10 = 80%

Ready to run the numbers?

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