System Management Bus (SMBus) Specification Version 2.0
SBS Implementers Forum 12
Symbol Parameter Limits Units Comments
Min Max
TLOW: SEXT Cumulative clock low extend time
(slave device)
- 25 ms See note 4
TLOW: MEXT Cumulative clock low extend time
(master device)
- 10 ms See note 5
TF Clock/Data Fall Time - 300 ns See note 6
TR Clock/Data Rise Time - 1000 ns See note 6
TPOR Time in which a device must be
operational after power-on reset
500 ms See section
3.1.4.2
Table 1: SMBus AC specifications
Note 1: The minimum frequency for synchronizing device clocks is defined in section 4.3.3. A master shall
not drive the clock at a frequency below the minimum FSMB. Further, the operating clock frequency shall
not be reduced below the minimum value of F
SMB due to periodic clock extending by slave devices as
defined in section 4.3.3. This limit does not apply to the bus idle condition, and this limit is independent
from the T
LOW: SEXT and TLOW: MEXT limits.
For example, if the SMBCLK is high for T
HIGH,MAX, the clock must not be periodically stretched longer
than 1/F
SMB,MIN – THIGH,MAX. This requirement does not pertain to a device that extends the SMBCLK
low for data processing of a received byte, data buffering and so forth for longer than 100us in a non-
periodic way.
Note 2: Devices participating in a transfer can abort the transfer in progress and release the bus when any
single clock low interval exceeds the value of T
TIMEOUT,MIN. After the master in a transaction detects this
condition, it must generate a stop condition within or after the current data byte in the transfer process.
Devices that have detected this condition must reset their communication and be able to receive a new
START condition no later than T
TIMEOUT,MAX. Typical device examples include the host controller, and
embedded controller and most devices that can master the SMBus. Some simple devices do not contain a
clock low drive circuit; this simple kind of device typically may reset its communications port after a start
or a stop condition.
A timeout condition can only be ensured if the device that is forcing the timeout holds the SMBCLK low
for T
TIMEOUT,MAX or longer.
Note 3: T
HIGH,MAX provides a simple guaranteed method for masters to detect bus idle conditions. A
master can assume that the bus is free if it detects that the clock and data signals have been high for greater
than T
HIGH,MAX.
Note 4: T
LOW:SEXT is the cumulative time a given slave device is allowed to extend the clock cycles in one
message from the initial START to the STOP. It is possible that, another slave device or the master will
also extend the clock causing the combined clock low extend time to be greater than T
LOW:SEXT.
Therefore, this parameter is measured with the slave device as the sole target of a full-speed master.
Note 5: T
LOW:MEXT is the cumulative time a master device is allowed to extend its clock cycles within
each byte of a message as defined from START-to-ACK, ACK-to-ACK, or ACK-to-STOP. It is possible
that a slave device or another master will also extend the clock causing the combined clock low time to be
greater than T
LOW:MEXT on a given byte. Therefore, this parameter is measured with a full speed slave
device as the sole target of the master.
Note 6: Rise and fall time is defined as follows:
T
R = (VILMAX - 0.15) to (VIHMIN + 0.15)
T
F = (VIHMIN + 0.15)to (VILMAX - 0.15)