TLE 4278 G

Semiconductor Group

2

1998-11-01

Two additional features of the TLE 4278 are a load dependent watchdog function as well

as a power on reset and under voltage reset function with an adjustable reset delay time

and adjustable reset switching threshold.

The watchdog function monitors whether the microcontroller is functioning appropriately,

including time base failures. In the case that there is no positive-going edge within a

certain pulse repetition-time the watchdog output is set to LOW. Programming of the

max. repetition time is done by the reset delay capacitor so that no additional external

components are necessary. To prevent the microcontroller from a automatic reset in

case of missing pulses, the watchdog output WO is separated from the reset output RO

for the TLE 4278. The watchdog output can be used as an interrupt signal for the

microcontroller. Pin WO can be externally connected to pin RO.

When the controller is set to sleep mode or low power mode its current consumption

drops and no watchdog pulses are created. In order to prevent the microcontroller from

unnecessary wake ups due to missing pulses at pin WI the watchdog feature can be

disabled as a function of the load. The switch off threshold is set by an external resistor

to pin WADJ. This function can also be used as a timer, which periodically wakes up the

controller. Therefore the pin WADJ has to be connected to the output Q.

The power on reset feature is necessary for a defined start of the microprocessor when

switching on the application. For a certain delay time after the output voltage of the

regulator has surpassed the reset threshold, a reset signal is generated. The delay time

is set by an external delay capacitor. The under voltage reset circuit supervises the

output voltage. In case VQ falls below the reset threshold the reset output is set LOW

after a short reaction time. The reset LOW signal is generated down to an output voltage

VQ of 1 V. In addition the reset switching threshold can be adjusted by an external

voltage divider. This feature is useful with microprocessors which guarantee a safe

operation down to voltages below the internally set reset threshold of 4.65 V typical.