I2C Timing Calculator

What Is an I2C Timing Calculator?

An I2C Timing Calculator evaluates the timing of the SCL clock and associated SDA setup conditions against the requirements of a selected I2C operating mode. It is useful when configuring microcontroller timing registers, validating logic-analyzer measurements or checking whether a generated clock has enough HIGH and LOW time.

Unlike a simple frequency calculator, I2C timing depends on several separate limits. A clock can have the correct average frequency while still violating the minimum HIGH or LOW duration if its duty cycle is poorly chosen.

I2C SCL Period Formula

For a 400 kHz clock:

I2C SCL HIGH and LOW Time

If the clock spends a fraction D of the period LOW:

For a 400 kHz clock with 60% LOW duty:

Both values exceed the Fast-mode minima of 1.3 µs LOW and 0.6 µs HIGH.

I2C Standard, Fast and Fast-mode Plus Timing

These limits are taken from the published I2C timing characteristics for Standard-, Fast- and Fast-mode Plus devices. :contentReference[oaicite:1]{index=1}

Why 50% Duty Cycle Is Not Always Ideal

A mathematically symmetric clock uses equal HIGH and LOW periods. I2C timing requirements are not symmetrical, however.

At 400 kHz, a 50% duty cycle gives:

The HIGH time is comfortably above the Fast-mode minimum, but the LOW time is slightly below the required 1.3 µs. A somewhat longer LOW phase is therefore appropriate when operating close to 400 kHz.

I2C Data Setup Time

Data setup time specifies how long SDA must already be stable before the relevant SCL sampling edge.

The calculator compares the entered setup time directly with the selected mode’s minimum. :contentReference[oaicite:2]{index=2}

I2C Rise and Fall Time

SDA and SCL do not switch instantaneously. Rise time is especially important because the open-drain lines rely on pull-up resistors to charge bus capacitance.

The I2C specification defines maximum edge times that depend on operating mode. The calculator checks the entered rise time against the corresponding mode limit.

How Many Clock Pulses Does an I2C Byte Use?

Each eight-bit data byte is followed by an ACK or NACK clock. Therefore one normal I2C byte consumes nine SCL clock pulses:

I2C Transfer Time Formula

A useful first-order transfer-time estimate is:

START and STOP are bus conditions rather than additional nine-bit characters, so this calculator reports the clocked-data time separately from device processing and clock stretching.

Example: 17 Bytes at 400 kHz

Suppose one address byte plus sixteen data bytes are transferred:

This is the ideal clocked transaction duration before adding clock stretching, software delays or slave processing time.

I2C Clock Stretching

An I2C target can hold SCL LOW to delay the controller when it needs additional time. Clock stretching therefore increases the actual LOW period and total transaction duration.

Because the stretching duration depends on the target and the individual transaction, this calculator does not invent a stretch time. Add measured or datasheet-specific delays separately when estimating worst-case transaction latency.

I2C START and Repeated START Timing

I2C defines minimum setup and hold timing around START and repeated START conditions. For Fast-mode, both repeated-START setup and START hold have a minimum of 0.6 µs; Fast-mode Plus uses 0.26 µs. Standard-mode uses longer limits. :contentReference[oaicite:3]{index=3}

These timing intervals matter especially when configuring an I2C peripheral from low-level timing registers rather than relying on a vendor library.

I2C STOP Timing

The STOP setup time is the interval required before SDA transitions HIGH while SCL is HIGH to terminate the transfer.

The specified minimum is 4.0 µs in Standard-mode, 0.6 µs in Fast-mode and 0.26 µs in Fast-mode Plus. :contentReference[oaicite:4]{index=4}

I2C Timing vs I2C Frequency

A bus can therefore have an acceptable average clock frequency but still fail one of the individual timing requirements.

Common I2C Timing Mistakes

I2C Timing Calculator FAQs