Generate TTL-level reset pulse from RS-232 level signal

Question

(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.

Answer 1

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.

Answer 2

Well first of all get rid of that capacitor C1. Connect the transistor collector directly to the reset of the AVR processor.

Also make sure that the GND of the AVR subsystem connects through to the GND of the DTR signal setup.

There is no reason to try to limit the reset pulse width with any capacitor here. Just make sure that the DTR goes to its ++V level for as long as you want the AVR reset to be asserted and then return DTR back to GND or the --V level.