User Manual

Application Note
SMBus communication with MLX90614
390119061402 Page 7 of 32 Jan-2008
Rev 004
4.6 Master device timeout definitions and conditions
t
LOW: MEXT
is defined as the cumulative time a master device is allowed to extend its clock cycles
within one byte in a message as measured from:
START to ACK
ACK to ACK
ACK to STOP
A system host may not violate t
LOW:MEXT
except when caused by the combination of its clock
extension with the clock extension from a slave device or another master. A Master is allowed to
abort the transaction in progress to any slave that violates the t
LOW:SEXT
or t
TIMEOUT,MIN
specifications. This can be accomplished by the Master issuing a STOP condition at the
conclusion of the byte transfer in progress.
Note: A Master should take care when evaluating t
LOW:SEXT
violation during arbitration since the clock may be held
low by multiple slave devices simultaneously. The arbitration interval may be extended for several bytes in the case
of devices that respond to commands to the SMBus ARP address. If timeouts are handled at the driver level, the
software may need to allow timeouts to be configured or disabled by applications that use the driver in order to
support older devices that do not fully meet the SMBus timeout specifications. Devices that implement ‘shared’ slave
addresses may also violate this specification due to combined clock stretching by the different devices sharing the
address. T
TIMEOUT,MIN
, however, does not increase due to combined clock stretching. Therefore, this is a safer timeout
parameter for a Master to use when it knows it’s accessing SMBus 2.0 devices.
4.7 Low-power DC specifications
In the table bellow are given low power DC parameters of the SMBus specification.
Table 2
Symbol Parameter Min Max Units Comments
V
IL
Data, Clock Input Low Voltage - 0.8 V
V
IH
Data, Clock Input High Voltage 2.1 V
DD
V
V
OL
Data, Clock Output Low Voltage - 0.4 V
I
LEAK
Input Leakage - ±5 µA Note 1
I
PULLUP
Current trough pull-up resistor or
current source
100 350 µA Note 2
V
DD
Nominal bus voltage 2.7 5.5 V 3V to 5V ±10%
Note 1: Devices must meet this specification whether powered or unpowered. However, a
microcontroller acting as an SMBus host may exceed I
LEAK
by no more than 10 µA.
Note 2: The I
PULLUP,MAX
specification is determined primarily by the need to accommodate a
maximum of 1.1K equivalent series resistor of removable SMBus devices, such as the Smart
Battery, while maintaining the V
OL,MAX
of the bus.
Because of the relatively low pull-up current, the system designer must ensure that the loading
on the bus remains within acceptable limits. Additionally, to prevent bus loading, any devices
that remain connected to the active bus while unpowered (that is, their Vcc lowered to zero),
must also meet the leakage current specification.
4.8 High-power DC specifications
High-power SMBus is specified below. These higher power specifications provide the
robustness necessary, for example, to allow SMBus to cross the PCI connector, thus allowing
SMBus devices on PCI add-in cards to communicate with other devices on both the system
board and other PCI add-in cards in the same system. These higher power electrical