Ground issue with FTDI MM232R or my computers USB ports?

[kodapa85]

Hi everyone!

Recently started playing around with PICs and Atmel chips.

My question is this. Is it possible that my MM232R has a fault, shorting the VCC30 and GND outputs from the FT232RQ or some other fault to raise the GND pin to 3.3V. Or am I simple doing something wrong?

To program a ATMEGA328P-PU I bought a MM232R, wired it up for 5V.

• Jumper connecting pin 14 and 15
• Connected both pins 2 and 3 via my breadboard for the positive rail
• Pin 13 is supposed to be GND and thus connected it to the negative rail

To my surprise the ATMEGA328P didn't seem to do anything. Upon checking the voltage, I found the ATMEGA was only receiving 1.7V. This was because the GND pin was reading 3.3V compared to pin 1, which is connected to the shield.

Needless to say I was able to once I connected the negative rail to shield the ATMEGA works fine and I was able to program it, at last.

Suppose you will want to know the answer to two questions in return :-

• Why didn't I just use a TLL-232R-PCB?
  • Photos made it look like there were only 3 terminals, also requires soldering
  • The MM232R was cheaper (£11.94 vs £14.34) and is more flexible (5V, 3.3V, 2.8V and 1.8V)
• Why haven't I used a voltmeter to test for a short?
  • Good question, just haven't yet. I'm not the most dexterous person I wouldn't want to short it out. Now that it isn't powered up I certainly could test it.