Reset pin schematic for ISP and serial programming

Hi all,

I'm working on a very small PCB. I want to use the atmega328 TQFP and so I will need to be able to bootload it using some form of ISP. If possible I would like to continue programming serially.

I noticed that the ISP connects to the rest pin straight away. The serial programming/debugging has a 0.1uF capacitor between the DTR and the reset pin.

I do not fully understand the details for ISP and I have no experience with it. Since my board is very small I was wondering if I can save out a pin and connect the ISP to the same pin as the DTR and thus connect the ISP reset pin via the capacitor.

kind regards,

The capacitor allows to reset the chip by toggling the DTR line, e.g. from the IDE connected to the USB COM port. Since DTR stays active as long as that connection is established, the capacitor releases the reset signal after a short delay. When your programmer provides a true (pulsed) reset signal, it doesn't matter whether you connect it to the RESET pin directly or through the capacitor.

No, you cannot.

ISP reset needs to go direct to reset pin, since reset needs to be held low during ISP programming.

Serial DTR pin needs to be capacitatively coupled to the reset pin, so the transition from high to low is turned into a low pulse. You should also have a diode between the reset pin and Vcc (band towards Vcc), so that when the DTR pin goes high again, there's no chance of the voltage spike putting the chip into HVSP mode.

You can program via ICSP using this adapter. It presses over the chip and connects to the 6 pins - Vcc, Gnd, Reset, SCK, MISO, MOSI.
I have used with Atmel AVR ISP MKii Programmer, just to make a 6-pin to 10-pin adapter (lot of GNDs on the 10-pin end).
MKii only monitors power, so Vcc must come from your circuit.

Great suggestions! Confirms what I was thinking but couldn't find the confirmation.

The custom cable made me think that I could make a custom cable for the serial programming with the capacitor integrated as well.

Thanks a lot guys.

Some thoughts:

You don't need DTR if you have an ICSP header and use the ISP to "File->Upload Using Programmer".

If you have an ICSP header and WANT to use the bootloader you can put the DTR capacitor in the cable and connect through the ICSP Reset pin.

You don't need DTR if you have a way to manually reset the board to initiate an upload.