I have a tiny embedded system build around a Holtek HT48F06 microcontroller to monitor RFID tags (RDM630). Unfortunately it only detects the arrival, not the removal of a tag. As a workaround, I want to reset the microcontroller periodically (say once per second), since that triggers a new attempt of detection (or does that seem like a bad idea?). Using a 555, I found some suitable values (22µF, 4+100kΩ) for a timer to trigger the reset.
However, in order to delay the power-on-reset, the low-active RESET-pin of the microcontroller is connected to a RC net like the following:
simulate this circuit – Schematic created using CircuitLab
So my question is, what would be a good way to connect a timer to the reset pin, preferably without making any changes to the existing circuit (I am not to keen about fiddling with the tiny smd pcb).
Best Answer
The output of a 555 is normally low. Since the minimum duty cycle for an astable 555 is 50%, you want to use a very high duty cycle (close to 100%) such that the low period is very short which will reset the microcontroller.
So you don't interfere with the existing circuit, you want to make use of an open-drain buffer, like the 74LS07. The 100K resistor R1 will act as a pullup. The circuit will not interfere with the power-up reset since the output of the buffer will be high-impedance when it is a logic 1.