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SNIS128C – AUGUST 2002 – REVISED MARCH 2013
SERIAL INTERFACE RESET
In the event that the SMBus Master is RESET while the LM89 is transmitting on the SMBData line, the LM89
must be returned to a known state in the communication protocol. This may be done in one of two ways:
1. When SMBData is LOW, the LM89 SMBus state machine resets to the SMBus idle state if either SMBData
or SMBCLK are held low for more than 35ms (t TIMEOUT ). Note that according to SMBus specification 2.0 all
devices are to timeout when either the SMBCLK or SMBData lines are held low for 25-35ms. Therefore, to
insure a timeout of all devices on the bus the SMBCLK or SMBData lines must be held low for at least 35ms.
2. When SMBData is HIGH, have the master initiate an SMBus start. The LM89 will respond properly to an
SMBus start condition at any point during the communication. After the start the LM89 will expect an SMBus
address byte.
DIGITAL FILTER
In order to suppress erroneous remote temperature readings due to noise, the LM89 incorporates a user-
configured digital filter. The filter is accessed in the FILTER and ALERT CONFIGURE REGISTER at BFh. The
filter can be set according to the following table.
D2
0
0
1
1
D1
0
1
0
1
Filter
No Filter
Level 1
Level 1
Level 2
Level 2 sets maximum filtering.
Figure 12 depicts the filter output in response to a step input and an impulse input. Figure 13 depicts the digital
filter in use in a Pentium 4 processor system. Note that the two curves, with filter and without, have been
purposely offset so that both responses can be clearly seen. Inserting the filter does not induce an offset as
shown.
Figure 11. Filter Output Response to a Step Input
a) Step Response
Figure 12. Filter Output Response to a Step Input
b) Impulse Response
Copyright ? 2002–2013, Texas Instruments Incorporated
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