I'm having issues with a 100nF capacitor between FT232R DTR and Atmega328p reset line having too much capacitance between toggling the power on/off.

It causes the pc driver not to detect the FT232R (Unknown device).

If I press the reset button while the board is powered down or wait enough time, the capacitor will be discharged and it works then fine.

Schema between the DTR and Atmega reset Line:

        100nF    |      10kOhm
DTR -----||------+------^v^v^----- VCC

Could the problem be that the capacitor is not really 100nf but bigger thus taking more time to discharge after powering down the board?


2 Answers 2


The FT232R and its drivers/firmware are the only important thing for determining the USB device ID, and showing up as something other than Unknown.

It is NOT the reset capacitor from DTR to the ATMEGA328P.

You either have a dead device, faulty USB cable/connections, you've broken the USB device somehow, or some other shenanigans.

The capacitor from DTR to the ATMEGA is there for the auto-reset feature while programming the ATMEGA and has nothing to do with the process of the FT232R enumerating itself as a virtual COM port.

The DTR pin is an output of the FT232R, not related to establishing a connection with the PC. It is software controlled in order to pulse the reset line of the AVR during the programming procedure.

There could be various things wrong as I said before, of why the FT232R is not working properly - I would check for driver issues first. FTDI had in the recent past made a Windows Driver Update which caused counterfeit FTDI chips to become bricked, and you have just encountered this? Either way, try plugging that USB device into other computers which have the FTDI drivers properly installed, to ensure that is not the issue.

  • \$\begingroup\$ The behavior is same on other computers also. The FT232R is legit FTDI. If the FT232R is recognized by the computer TX/RX will work fine and then able to receive and send data through COM <-> ATMEGA. \$\endgroup\$
    – AnttiH
    May 1, 2015 at 23:32
  • \$\begingroup\$ @AnttiH how do you KNOW it's a legit FTDI chip? not even the assembly factory knows sometimes, nor the distributor \$\endgroup\$
    – KyranF
    May 2, 2015 at 2:22

there's a couple of issues here. your RESET/100n cap arrangement shouldn't stop the FT232 from being seen by Windows & enumerating as a valid comm port.

OTOH, if the comm port is showing up in Windows, but your programmer is not seeing a response from the AVR's boot-loader, then:

RESET is an active-low input to the AVR - you pull it low for a short period to reset the AVR. The 10k pull-up to Vcc keeps it high at all times except when either your reset-switch or the FT232 needs to reset the AVR (presumably so that the bootloader will run after reset).

so as far as I can tell, that side of the 100n is OK.

the other side, the FT232 side, should probably have a pull-down resistor, about 1k to 10k, between the 100n cap & Ground.


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