SPI Clock Speed Calculator

Work out spi clock speed instantly with clear inputs, formula shown and shareable results.

About SPI Clock Speed Calculator

SPI clock is the peripheral clock divided by the prescaler, and transfer time is bits divided by that rate plus per-transfer overhead. Overhead matters: at 6 MHz a 512-byte block takes 683 microseconds so 4 microseconds of setup is negligible, but for single-byte transfers it dominates completely.

Formula

SPI timing

SCK = f / prescaler; transfer time = bytes x 8 / SCK + overhead

Worked example

With the default values below, SPI Clock Speed Calculator returns SPI clock of 6.00. Change any input above and every figure here is recalculated in your browser.

Inputs used

Peripheral clock: 48 MHz
SPI prescaler: 8 x
Bytes per transfer: 512 bytes
Per-transfer overhead: 4 µs

Results

SPI clock: 6.00 MHz
Transfer time: 686.67 µs
Effective throughput: 0.75 MB/s
Bit time: 166.67 ns

Step by step

1Values used
2SPI timing
3SPI clock= 6.00 MHz
4Transfer time= 686.67 µs
5Effective throughput= 0.75 MB/s
6Bit time= 166.67 ns

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

What limits SPI clock speed in practice?

Trace length and capacitance, the slave's maximum clock rating, and round-trip delay on longer buses. Above about 20-30 MHz signal integrity becomes the constraint.

Why use DMA for SPI?

It removes per-byte CPU overhead, which is what actually caps throughput on small transfers and frees the core for other work.