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%

Frequently Asked Questions

What limits SPI clock speed?

The slave device datasheet specifies the maximum clock. PCB layout, cable length, and signal integrity also limit practical speed.

Is SPI faster than I2C?

Generally yes. SPI runs at 10+ MHz while standard I2C is 100/400 kHz. SPI also has lower overhead per byte.

Can I use SPI with multiple devices?

Yes, using separate chip-select lines for each device. All devices share the clock and data lines.

Ready to run the numbers?

Open SPI Data Rate Calculator