Multiple RPM-Based PWM Fan Controller for Five Fans
Datasheet
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Figure 4.3 Ramp Rate Control
4.8
Watchdog Timer
The EMC2305 contains an internal Watchdog Timer for all fan drivers. The Watchdog timer monitors
the SMBus traffic for signs of activity and works in two different modes based upon device operation.
These modes are Power Up Operation and Continuous Operation as described below.
For either mode of operation, if four (4) seconds elapse without activity detected by the host, then the
watchdog will be triggered and the following will occur:
1. The WATCH status bit will be set.
2. The fan driver will be set to full scale drive. It will remain at full scale drive until it is disabled.
APPLICATION NOTE: When the Watchdog timer is activated, the Fan Speed Control Algorithm is automatically
disabled. Disabling the Watchdog will not automatically set the fan drive nor re-activate the
Fan Speed Control Algorithm. This must be done manually.
4.8.1
Power Up Operation
The Watchdog Timer only starts immediately after power-up. Once it has been triggered or deactivated,
it will not restart although it can be configured to operate in Continuous operation. While the Watchdog
timer is active, the device will not check for a Stalled Fan condition.
In the Power Up Operation, the Watchdog Timer is disabled by any of the following actions:
1. Writing the Fan Setting Register will disable the Watchdog Timer.
2. Enabling the RPM-based Fan Speed Control Algorithm by setting the EN_ALGO bit will disable the
Watchdog Timer. The fan driver will be set based on the RPM-based Fan Speed Control Algorithm.
Revision 1.1 (10-12-09)
SMSC EMC2305
DATA2S4HEET
SMSC [ SMSC CORPORATION ]
