NXP Semiconductors KL25 Series - KL25 Sub-Family Reference Manual, Rev. 3, September

SDA float high) when it detects any single clock held low longer than T
TIMEOUT,MIN
.
Devices that have detected this condition must reset their communication and be able to
receive a new START condition within the timeframe of T
TIMEOUT,MAX
.
SMBus defines a clock low timeout, T
TIMEOUT
, of 35 ms, specifies T
LOW:SEXT
as the
cumulative clock low extend time for a slave device, and specifies T
LOW:MEXT
as the
cumulative clock low extend time for a master device.
38.4.4.1.1 SCL low timeout
If the SCL line is held low by a slave device on the bus, no further communication is
possible. Furthermore, the master cannot force the SCL line high to correct the error
condition. To solve this problem, the SMBus protocol specifies that devices participating
in a transfer must detect any clock cycle held low longer than a timeout value condition.
Devices that have detected the timeout condition must reset the communication. When
the I2C module is an active master, if it detects that SMBCLK low has exceeded the
value of T
TIMEOUT,MIN
, it must generate a stop condition within or after the current data
byte in the transfer process. When the I2C module is a slave, if it detects the
T
TIMEOUT,MIN
condition, it resets its communication and is then able to receive a new
START condition.
38.4.4.1.2 SCL high timeout
When the I2C module has determined that the SMBCLK and SMBDAT signals have
been high for at least T
HIGH:MAX
, it assumes that the bus is idle.
A HIGH timeout occurs after a START condition appears on the bus but before a STOP
condition appears on the bus. Any master detecting this scenario can assume the bus is
free when either of the following occurs:
• SHTF1 rises.
• The BUSY bit is high and SHTF1 is high.
When the SMBDAT signal is low and the SMBCLK signal is high for a period of time,
another kind of timeout occurs. The time period must be defined in software. SHTF2 is
used as the flag when the time limit is reached. This flag is also an interrupt resource, so
it triggers IICIF.
Functional description
KL25 Sub-Family Reference Manual, Rev. 3, September 2012
710 Freescale Semiconductor, Inc.