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Using PS/2 PMOD at 5V?


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Hi,

I have an Arty A7, a PS/2 PMOD and an old PS/2 keyboard. At 3.3V this keyboard doesn't show any sign of life. I suspect it actually needs 5V.
I see there's a jumper on the PMOD that allows you to hook up an external 5V supply. Looking at the schematic however, I don't see how this can work. There's no level shifter on the PMOD. The data and clock lines would get pulled up to 5V and damage the FPGA.
Am I supposed to hook up the PS/2 PMOD to a level shifter PMOD if I want to use it at 5V?
Btw, isn't PS/2 Vcc supposed to be 5V? Is the idea of this PS/2 PMOD to hook up a PS/2 peripheral, which is supposed to run at 5V, to 3V3 instead and hope for the best?

Thanks for any help,

Ruben.

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Hi @epsilon,

If the keyboard itself just needs 5V, you can use the external power connection on JP2 to provide the 5V power and set JP1 to the VE (voltage external) side rather than the Voltage board side. I haven't personally checked, but the keyboard should still be able to read the 3.3V signals coming from the system board. However, the reverse is not true in the sense that the FPGA pins could not handle any 5V signals coming from the keyboard (as per Xilinx's DS181), which is what you intuited.

I looked up the PS/2 voltage and you are correct that those connectors normally run at 5V, so I suppose the plan when it was originally made was that people would hook up the Pmod PS/2 at 3.3V and hope for the best. So if the keyboard is not working at 3.3V, you would probably need a voltage translator of some kind to get it to work properly.

An important note that if you do end up using the external power connection on JP2 is to be very careful about the connection. At one point (this is likely still the case as I doubt the supply of Pmod PS/2 has changed over) it was found that the silkscreen labeling of external power and ground on JP2 is in reverse. This is described in the reference manual here: https://digilent.com/reference/pmod/pmodps2/reference-manual#powering_warning.

Let me know if you have any questions.

Thanks,
JColvin

 

 

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Thank you JColvin,

In the meantime I was able to score another PS/2 keyboard at a garage sale and this one is working fine at 3.3V. I also got a PS/2 mouse, which sadly does not work at 3.3V, but I confirmed it does work at 5V (on a breadboard with a 5V supply and a scope to monitor the signals).

I initially thought that this PMOD could be a solution: https://digilent.com/shop/pmod-lvlshft-logic-level-shifter/

Looking at bit more closely however, it doesn't look like this PMOD will work with one wire interfaces. You have to choose a direction with the jumpers. I guess I'll have to put a bidirectional level shifter on a breadboard and take it from there. That, or I keep buying PS/2 mice at garage sales until I find one that works at 3.3V :-)

Cheers,

Ruben.

 

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