Maxim Integrated MAXQ7667 - Manual (Page 264)

15.4 External Oscillator Clock/Crystal Failure Switchover
The external oscillator clock/crystal failure can occur during normal operation and is detected when the XTRDY bit switches over to 0,
indicating that the external source has failed. The hardware forces a switch to the internal RC oscillator to keep the MAXQ7667 proces-
sor operational.
The following must be true to detect the failure on the high-frequency input:
• XTRC must be set to logic 1, which enables the external clock source and selects external clock/crystal as the active clock
source.
• XTRDY bit is set to 1, which indicates external clock is ready.
• A falling edge is detected on the XTRDY bit, which signals a crystal/clock source failure.
When the above conditions have been detected, the clock circuitry will:
• Enable the RC oscillator immediately, regardless of the original state of the RCE bit.
• Switch the clock source from external to internal by forcing XTRC to 0.
• Make the system clock ready after a few clock cycles delay.
• Set the high-frequency crystal failure interrupt XTI (AIE.7) flag to 1; an interrupt is generated if the interrupt was enabled, XTIE
= 1 (ASR.7).
This switchover remains until software reconfigures the clock structure.
15.5 Watchdog Timer
Generally, the primary function of the watchdog timer is to provide system control in case a software problem occurs; the watchdog
timer performs a controlled system restart when the selected time interval expires.
The watchdog timer is driven by the internal RC clock, as illustrated in Figure 15-2. If the watchdog is enabled, the RC clock remains
turned on, even after the system clock has been switched over to the external source (XTRC = 1).
_________________________________________________________________________________________________________ 15-12
MAXQ7667 User’s Guide
16MHz INTERNAL RC CLOCK
(MAXIMUM SETTING)
DIVIDE
BY 2
12
256ms 2048ms 16,384ms 131,072ms
DIVIDE
BY 2 3
DIVIDE
BY 2 3
WDCN.3:WDIF
RESET
DELAY
DIVIDE
BY 2 3
TIMER
WD1
TIMEOUT
WATCHDOG
INTERRUPT
WATCHDOG
RESET
WDCN.2:WTRF
WDCN.6:EWDI
WDCN.1:EWT
WD0
RWT
Figure 15-2. Watchdog Timer Function