Cirrus Logic CS485xx - Manual (Page 80)

8-2 Copyright 2009 Cirrus Logic, Inc. DS734UM7
Watchdog Timer Description
CS485xx Hardware User’s Manual
8.3 Watchdog Timer Description
As mentioned earlier in the overview of the CS485xx, there is an integrated Watchdog Timer that acts as a
“health” monitor for the DSP. The Watchdog Timer must be reset by the DSP before the counter expires, or
the entire chip is reset. This peripheral ensures that the CS485xx will reset itself in the event of a temporary
system failure. In standalone mode (That is, no host MCU is present), the DSP will reboot from external
FLASH. In slave mode (that is, host MCU is present) a GPIO will be used to signal the host that the
Watchdog Timer has expired and the DSP should be rebooted and reconfigured.
The Watchdog Alarm Indicator is a very simple circuit which requires only a GPIO connection between the
CS485xx and the host MCU, and a pull-up resistor a shown in
Figure 8-1 .
Figure 8-1. Typical Connection for Watchdog Timer Alarm
The GPIO pins of the CS485xx are configured as inputs after a hard reset, at which point the line will be
pulled high. Once application code has been loaded and configured on the DSP, the GPIO will be
reconfigured as an output and driven low to indicate that the CS485xx is operating normally. In the event that
something happens to prevent the DSP from servicing the Watchdog Timer, the CS485xx will reset itself
internally and reconfigure the GPIO as an input. At this point, the pull-up (R1) will pull the line high to inform
the MCU that a problem has occurred and the CS485xx should be reconfigured.
A relatively strong resistor is recommended for R1 (10k ohm or smaller) to ensure that the rising edge of the
Watchdog Alarm Indicator remains clean. The CS485xx has integrated pull-up resistors that are engaged
after a reset of the part, but they are very weak (~47k ohm) and should not be used as the only pull-up for
the Watchdog Alarm Indicator.
The GPIO used for the Watchdog Alarm Indicator is selected during the configuration of application code on
the CS485xx. The details of selecting a Watchdog GPIO can be found in AN298.
§§
CS485xx R1
GPIO
+3.3V
Host MCU
Input