Advantech SOM-ETX Design Guide
5.14.5 SMBus
Figure 5-45 I2C Bus Connections
Most SOM-ETX modules provide an SMBus port for communication with external
SMBUS slave devices. This port also is used internally in the SOM-ETX module to
communicate with onboard SMBUS devices such as the SPD EEPROMs on DIMMS,
clock-generator chips, and hardware monitoring devices. The port is externally
available on the SOM-ETX pins SMBDAT and SMBCLK. The addresses for any
external SMBus devices must be chosen so that they do not conflict with the
addresses that are used internally in the SOM-ETX module. If the device offers
externally controllable address options, it is desirable to implement baseboard
resistor straps to allow the device to be set to at least two possible SMBus
addresses.
5.14.6 Power Good / Reset Input
Figure 5-46 Power Good / Reset Input Connections
The SOM-ETX Power Good Input (PWGIN) may be attached to an external power
good circuit if desired, or used as a manual reset input by grounding the pin with a
momentary-contact pushbutton switch. If an external circuit asserts this signal, it
should be driven by an open-drain driver and held low for a minimum of 15mS to
initiate a reset. Use of this input is optional. The SOM-ETX module generates its own
I2C
1
2
3
4
SOM-ETX
Carryboard
SMBDAT
SMBCLK
VCC
+5V or +3.3V
The implementation details and software interface for this port differ between SOM-
ETX modules. Refer to the individual SOM-ETX (rev.2) modules user’s guide for
details. .Special care should be taken in external use of the SMBus because of its
importance to the internal operation of the SOM-ETX module. Consultation with
Advantech technical support is recommended.
PWGIN
74F08
SOM-ETX
Reset Button
0.1uF
Power Good
ATX Power
or Power Circuit
Carryboard
4.7K
Chapter 5 Carrier Board Design Guidelines
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