I've been attempting to program a few different micro-controllers using nothing but an FTDI 5V TTL cable and self-written software to bitbang the needed pins on the chips. Until now I had success only with my AT89S52 chips but I have a similar problem with the rest. The rest being an ATmega8, ATtiny13 and even a PIC16F84A.

None of these chips seems to respond on the appropriate pins (MISO, SDO, etc) while programming. These pins seems to always be in an high-impedance state and doesn't get pulled either HIGH or LOW.

I've checked this with a simple LED "logic probe", my Arduino as a Logic Analyzer, oscilloscope. There's just no response from these chips. I even tried programming from the Arduino with simple sketches (SPI or bitbang) with exactly the same effect.

Is there something I'm missing or obviously doing wrong? For all micro-controllers I've connected the pins directly to the FTDI cable's wires (or Arduino's pins) and in case HV programming is required I used the following circuit (which seems to work just fine when I'm measuring with my multimeter):


simulate this circuit – Schematic created using CircuitLab


2 Answers 2


One possibility is that the chips you're trying to program are expecting an external oscillator instead of using their internal RC oscillators. I know when I first programmed an AVR I had the fuses set wrong, so to be able to program it again I needed to add an external oscillator.


success only with my AT89S52 chips [...]

That old chip has Reset active high, which is unusual otherwise.

ATmega8, ATtiny13 and even a PIC16F84A

These have Reset active low, and probably need a different reset circuit.

  • \$\begingroup\$ I have different programs for each chip so the reset level is appropriate for the kind of chip. The reset pin is pulled low where it's required by the appropriate wire on the FTDI cable or Arduino pin \$\endgroup\$
    – Sadrok
    Commented Aug 7, 2014 at 14:23

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