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Hi, I have connected my controller (ATMEGA2560) input to 5V power supply. At the VCC terminal 5V is shown. The VCC, TX,RX, RESET and GND pins are connected to a FTDI programmer port as shown in figure.

The diode D(SS34A) is connected in series of the VCC line from controller to FTDI programmer port, so that it forms a reverse bias from controller to the FTDI programmer. The 0.1uF capacitor is connected only to supply a stable voltage to the controller.

But, when I connect my FTDI programmer to the FTDI programmer port with other end of the programmer not connected to the USB terminal, I found 5V at the VCC terminal of the FTDI programmer, which should not happen.

So, How the 5V is coming in the VCC terminal of the FTDI port at this condition ?

  • 1
    OP, what voltage do you think should be there? – C_Elegans Dec 5 at 14:55
  • 1
    Ok. You're probably going to have to do some troubleshooting yourself, vs having it done for you. You might start testing things like: is the diode failed short? is the diode in backwards? Has something other than a diode been populated in this position? What is the voltage if you remove the FTDI connector and put a 10k resistor to ground in its place? What measurements have you made other than reading 5V on FTDI? What troubleshooting have you tried on your own? – scorpdaddy Dec 5 at 15:00
  • Simplify your question - Do you have a diode's cathode hooked up to a 5V source and its anode hooked to nothing, and are reading 5V on the anode? – mike65535 Dec 5 at 15:18
  • Maybe you're inadvertently powering the Programmer via the Tx pin on your Controller. It wouldn't be the first time a chip has been powered through an input pin. This can produce some funny results. – Chris Knudsen Dec 5 at 15:24
  • You probably shouldn't have that Vcc connection there. Your diode might be backwards, or you might have a schottky diode with high reverse leakage and no actual load being drawn by the circuit, only a miniscule amount by the meter. Backpowering through the I/Os can be an issue but wouldn't cause you to see 5v on the Vcc, but rather most likely something less. Ultimately you should probably read the FTDI docs on the chip for various power situations and reverse engineer the module. – Chris Stratton Dec 5 at 15:59
up vote 2 down vote accepted

Perhaps you're inadvertently powering the Programmer via the Tx pin on your Controller. It wouldn't be the first time a chip has been powered through an input pin. This can produce some funny results.

schematic

simulate this circuit – Schematic created using CircuitLab

As a good rule, never apply signals to a chip that is not powered.

Many chip inputs have over-voltage protection diodes that connect directly to the chip's supply pins.

If the chip is not powered, and a signal is applied to an input, then the internal input diodes conduct, and the chip becomes somewhat powered by this input signal.

This is a bugger to find, if you're not expecting it (especially while prototyping).

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