Dual RPM-Based PWM Fan Controller

Datasheet

SMSC EMC2302

21

Revision 1.1 (10-12-09)

DATASHEET

4.8

Watchdog Timer

The EMC2302 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.

Writing any other configuration registers will not disable the Watchdog Timer upon power up.

Figure 4.3 Ramp Rate Control

Previous

Setting

Next Desired

Setting

Max

Step

Max

Step

Update

Time

Update

Time

Setting Changed