(My knowledge of analog circuit design is severely limited).
I am trying to generate a reset pulse for an AVR microcontroller. The reset is active LOW, and the maximum threshold voltage is 0.9V, with a pulse of no less than 2.5us.
I am trying to generate the required pulse from the DTS line of an RS232-Level signal. This means that the DTS line voltage is from ~-13V (inactive) to ~+13V (active).
simulate this circuit – Schematic created using CircuitLab
This is the best I could come up with, and it does not work. I have simulated using Spice, and depending on the ramp of the positive edge of the DTR pin, the RESET_N may never go under 2.5v.
The simulation from CircuitLab:
The simulation from NI Multisim:
EDIT: Added Voltage points to the AVR Circuit to clarify that GND is common.
Best Answer
If you need a timed reset pulse off the RISING edge of the DTR signal try this circuit instead. This moves the timing capacitor into the base circuit where it can be managed much more easily.
My simulation shows that this produces a RESET_N pulse of about 30usec. You can adjust by playing with the values of C1 and R3.