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ATA6617 View Datasheet(PDF) - Atmel Corporation

Part Name
Description
MFG CO.
'ATA6617' PDF : 308 Pages View PDF
Atmel ATA6616/ATA6617
4.5.3.8
Clock Monitoring
A safe system needs to monitor its clock sources. Two domains need to be monitored:
• Clock sources for peripherals,
• Clocks sources for system clock generation.
In the first domain, the user (code) can easily check the validity of the clock(s) (See “COUT
Command” on page 57.).
In the second domain, the lack of a clock results in the code not running. Thus, the presence
of the system clock needs to be monitored by hardware.
Using the on-chip watchdog allows this monitoring. Normally, the watchdog reloading is per-
formed only if the code reaches some specific software labels, reaching these labels proves
that the system clock is running. Otherwise the watchdog reset is enabled. This behavior can
be considered as a clock monitoring.
If the standard watchdog functionality is not desired, the Atmel® ATtiny87/167 watchdog per-
mits the system clock to be monitored without having to resort to the complexity of a full
software watchdog handler. The solution proposed in the Atmel ATtiny87/167 is to automate
the watchdog reloading with only one command, at the beginning of the session.
So, to monitor the system clock, the user will have two options:
1. Using the standard watchdog features (software reload),
2. Or using the automatic reloading (hardware reload).
The two options are exclusive.
Warning:
These two options make sense ONLY if the clock source at RESET is an INTERNAL
SOURCE. The fuse settings determine this operation.
Figure 4-15. Watchdog Timer with Automatic Reloading
WD
Interrupt
WD
Reset
Register:
WDTCSR
WatchDog
Reload
01
WatchDog Clock
Enable
Checker
Automatic
Reloading
Mode
System CLK
Internal Bus
The ‘Enable Watchdog in Automatic Reload Mode’ command has priority over the standard
watchdog enabling. In this mode, only the reset function of the watchdog is enabled (no more
watchdog interrupt). The WDP3..0 bits of the WDTCSR register always determine the watch-
dog timer prescaling.
As the watchdog will not be active before executing the ‘Enable Watchdog in Automatic
Reload Mode’ command, it is recommended to activate this command before switching to an
external clock source (See following notes).
59
9132D–AUTO–12/10
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