555 watchdog for uC and uP systems - del00013

When the microcontroller hangs due to a spike, EMI or RFI etc. the 7555 will reset the uC, if proper power supply design is done above circuit can also give a clean power on reset, the above circuit you should modify to suit your design.

Digital to Analog Converter with uC Watchdog  This is another way i did the same thing.

Better still use a uC with watchdog built in like some atmel chips, or use the watchdog chips from maxim which can also do RAM battery management.

Another way i have used a 555 - High Resistance Meter

The circuit was developed over a old TI application note, 7555 i think fairchild may be making it, 7555 is CMOS version of 555 timer, advantage of 7555 is that it can go to higher frequency, low power consumption, the disadvantage is its output drive mA is not as good as 555. now why i put it here was that you can see how charge and discharge paths are separated with diodes.

Watchdog Timers can reset a microontroller or microprocessor system when it goes into an endless loop or gets stuck. 555 is used here to keep a tab on the uC activity. It resets when it senses the uC is not pinging it anymore.



See another Application of 555 in a product i once fixed - Microohm Meter with LED Analog Bar

It maybe a good idea to keep the Watchdog timer outside main uC silicon chip. The Watchdog should not be an intelligent circuit. It has to be a generic mixed design so it operates reliably in real-time, independently.

Edit the circuit source file del00013.zip.

555 Pin 3 Inductive Latch up

Inductive Load Latch-up A Negative voltage at pin 3 can cause a latch-up.  The Solution is to add two diodes as shown. Parallel to Load draws away the inductive kickback.



One more method is to have a Freewheeling Diode across the Solenoid or Relay-Coil that the 555 is driving. Driving Lamps and LED is not a problem with 555. Driving coils are best done with an external NPN transistor. You can also use the Astable configuration to pulse drive LEDs without a large Resistor. Use a 30% Duty Cycle and High Clock Rate.

Touch Control Monoshot

Sensible User Interface is as old as 555, see an early application note of a touch switch, useful for embedded systems. Use a 7555 here, cmos for more sensitivity. The Finger Triggers the 555 at pin 2. The Oneshot will go high for RC period. Use a Flip-Flop as latch or memory.

Images above are from the Original Signetics 555 Datasheet

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