Revisions Affected

Description

The SDA and SCL outputs are implemented with push-pull 3-state output buffers rather than open-drain output buffers as required by I²C. While possible for the push-pull 3-state output buffers to behave as open-drain outputs, an internal timing skew issue causes the outputs to drive a logic-high for a duration of approximately 1ns–2ns before the outputs are disabled. The unexpected high-level pulse only occurs when the SCL or SDA outputs transition from a driven low state to a high-impedance state.

This short high-level pulse injects energy into the I²C signal traces, which causes the I²C signals to sustain a period of ringing as a result of multiple transmission line reflections. This ringing does not typically cause an issue on the SDA signal because the ringing only occurs at times when SDA is expected to be changing logic levels and the ringing has time to damp before data is latched by the receiving device. The ringing can have enough amplitude to cross the SCL input buffer switching threshold several times during the first few nanoseconds of this ringing period, which can cause clock glitches. This ringing does not typically cause a problem if the amplitude is damped within the first 50ns because I²C devices are required to filter the SCL inputs to remove clock glitches. Therefore, it is important to design the PCB signal traces to limit the duration of the ringing to less than 50ns. One possible way to reduce the ringing is to insert series termination resistors near the SCL and SDA terminals to attenuate transmission line reflections.

This issue can also cause the SDA output to be in contention with the target SDA output for the duration of the unexpected high-level pulse when the target begins an ACK cycle. This occurs because the target can already be driving SDA low before the unexpected high-level pulse occurs. The glitch that occurs on SDA as a result of this short period of contention does not cause any I²C protocol issue but the peak current applies unwanted stress to both I²C devices and potentially increases power supply noise. Therefore, a series termination resistor located near the respective SDA terminal is required to limit the current during the short period of contention.

A similar contention problem can occur on SCL when connected to I²C target devices that support clock stretching. This occurs because the target is driving SCL low before the unexpected high-level pulse occurs. The glitch that occurs on SCL as a result of this short period of contention does not cause any I²C protocol issue because I²C devices are required to apply a glitch filter to the SCL inputs. However, the peak current applies unwanted stress to both I²C devices and potentially increases power supply noise. Therefore, a series termination resistor located near the respective SCL terminal is required to limit the current during the short period of contention.

If another controller is connected, the unexpected high-level pulses on the SCL and SDA outputs can cause contention during clock synchronization and arbitration. The series termination resistors described above also limit the contention current in this use case without creating any I²C protocol issue.

Workaround

Insert series termination resistors on the SCL and SDA signals and locate them near the SCL and SDA terminals along with the SCL and SDA pullup resistors.

The ringing can also be reduced by controlling the output to use minimum drive strength and reduced slew rate. These options are only configurable on pins that support high drive output. Standard drive pins have no configuration options.