Over voltage protection of TTL GPIO ports

[Ananya.Muddukrishna]

Hi,

What would happen if a 3.3V signal is applied to a TTL GPIO port (EMC2DP) configured to operate at 1.8V?

Something is driving the TTL1-3 GPIO pins high (1.8V) immediately when I turn the board on. The pins stay high as long as the board is powered on. I want to rule out accidental exposure to a higher voltage (3.3V) signal as a cause for the broken behavior I am seeing.

Edit: Changed TTL0-1 to TTL1-3.

[Ananya.Muddukrishna]

There is a typo in the above description.

The pins TTL1 and 3 are always driven high upon power on. I suspect U15 is faulty. Whats U15? Send me an email if you cannot disclose what U15 is here. I assume its some sort of clamper.
TTL0 and 2 seem to be fine.

[Timoteo]

Hi Ananya,
If you power up the board, without programming the FPGA or make the Zynq boot (basically having the TTL pins unconnected from the FPGA's point of view), and you measure the voltage on every pin of JP3, what do you see?
It should be all 0, except VCCIO34 (1.8V).
In case you are seeing 1.8V at TTL1 and TTL3 just when powering up the board without any configuration, there could be a problem, but I have to check why, I know the same as you about U15!
Check what I said above, and if you confirm it, I'll look into it.
Timoteo

[Ananya.Muddukrishna]

Hi Timoteo,

I unplugged the SD card and turned the EMC2DP on. This I assume does not program the FPGA and run software on the Zynq.

I see the same behavior. TTL1 and TTL3 are at 1.8V. TTLVCC is at 1.8V. Other TTL pins pins i.e., TTL0, TTL2, TTLN, TTLP, and TTLGND are at 0V.



- Ananya

[Timoteo]

Hi Timoteo,

I unplugged the SD card and turned the EMC2DP on. This I assume does not program the FPGA and run software on the Zynq.

I see the same behavior. TTL1 and TTL3 are at 1.8V. TTLVCC is at 1.8V. Other TTL pins pins i.e., TTL0, TTL2, TTLN, TTLP, and TTLGND are at 0V.



- Ananya

Hi Ananya.
Do you have a multi meter in hand?
If that's the case, put it in Test mode (diode symbol, or speaker symbol). It should beep if you touch both probes with each other.
Keep the board off, no power.
Put one probe on JP8 or JP7, where 1.8V is (if you have the jumpers for 1.8V position, just touch the metallic area of the jumper), and the other probe in every pin of the header JP3. Only one of the pins should beep, which is the FPGA bank VCC pin.
If any other pins beep, there is a short.
Timoteo.

[Ananya.Muddukrishna]

Hi Timo,

I ran the test you recommended above. No power, check for shorts.

TTL1 pin has a short with the 1.8V track

- Ananya