I'm using a Nullsom NS-RS232 to convert full-voltage RS232 on a DB9 connector to 5v. http://www.nulsom.com/datasheet/NS-RS232_en.pdf
I want to be able to program an Arduino using a standard USB to FTDI cable.
In one configuration I have an AtMega328P-PU on a breadboard. I connect the NS-RS232's reset pin (in this case, labeled CTS) through a 0.1uF ceramic disc capacitor to the reset pin of the MCU. The reset pin is also tied to 5V through a 10K ohm resistor. This configuration works great.
I used the exact same configuration with an Arduino Nano and get nothing. I tried with and without the 10K ohm resistor. I've tried with and without the capacitor. If I hit the reset button at the right time it will reprogram just fine, but I can't get it to reset automatically.
I replaced the 0.1uF ceramic capacitor with a 1uF electrolytic capacitor and it started working reliably. No need for the resistor because this is done internally. Could too large a capacitor damage the Arduino over time? I didn't have any in-between values.
Next I tried multiple 0.1uF capacitors. Two seemed to work once, but then didn't work. Three does seem to work reliably. My multi-meter measured 0.2uF across the three capacitors (ceramic disc type), each measuring 65-75nF. Do you think my "104" capacitors were produced below spec or do you think my multimeter (cheap Etekcity brand) is inaccurate when measuring capacitors? When I measured the 1uF capacitor it showed like 1.05uF so maybe the ceramic disc capacitors are poor quality?
I connect the NS-RS232's reset pin (in this case, labeled CTS)
Normally, you'd use RTS for reset. RS232 is a bit ambiguous, but make sure that the pin you are using is an OUTPUT...
(No, that doesn't explain why it works on the breadboard circuit. Oh wait - a Nano has a USB/Serial chip wired to RESET. that could be interfering with an external reset circuit. maybe.)