TC1775 datasheet(54/101 Pages) INFINEON

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TC1775

Data Sheet

V1.2, 2002-05

Preliminary

Watchdog Timer

The Watchdog Timer (WDT) provides a highly reliable and secure way to detect and

recover from software or hardware failure. The WDT helps to abort an accidental

malfunction of the TC1775 in a user-specified time period. When enabled, the WDT will

cause the TC1775 system to be reset if the WDT is not serviced within a user-

programmable time period. The CPU must service the WDT within this time interval to

prevent the WDT from causing a TC1775 system reset. Hence, routine service of the

WDT confirms that the system is functioning properly.

In addition to this standard “Watchdog” function, the WDT incorporates the EndInit

feature and monitors its modifications. A system-wide line is connected to the ENDINIT

bit implemented in a WDT control register, serving as an additional write-protection for

critical registers (besides Supervisor Mode protection).

A further enhancement in the TC1775’s Watchdog Timer is its reset prewarning

operation. Instead of immediately resetting the device on the detection of an error, as

known from standard Watchdogs, the WDT first issues an Non-maskable Interrupt (NMI)

to the CPU before finally resetting the device at a specified time period later. This gives

the CPU a chance to save system state to memory for later examination of the cause of

the malfunction, an important aid in debugging.

Features:

• 16-bit Watchdog counter

• Selectable input frequency:

SYS/256 or fSYS/16384

• 16-bit user-definable reload value for normal Watchdog operation, fixed reload value

for Time-Out and Prewarning Modes

• Incorporation of the ENDINIT bit and monitoring of its modifications

• Sophisticated password access mechanism with fixed and user-definable password

fields

• Proper access always requires two write accesses. The time between the two

accesses is monitored by the WDT and limited.

• Access Error Detection: Invalid password (during first access) or invalid guard bits

(during second access) trigger the Watchdog reset generation.

• Overflow Error Detection: An overflow of the counter triggers the Watchdog reset

generation.

• Watchdog function can be disabled; access protection and ENDINIT monitor function

remain enabled.

• Double Reset Detection: If a Watchdog induced reset occurs twice without a proper

access to its control register in between, a severe system malfunction is assumed and

the TC1775 is held in reset until a power-on reset. This prevents the device from being

periodically reset if, for instance, connection to the external memory has been lost

such that even system initialization could not be performed.

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