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Forum Index : Microcontroller and PC projects : PicoMite VGA - USB to PS2 Keyboard Adapter
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LeoNicolas Guru  Joined: 07/10/2020 Location: CanadaPosts: 479 |
Hello I have a PicoMite VGA with a PS2 keyboard connector. I don't want to buy a new PS2 keyboard. Does anyone know a good PS2 to USB connector adapter? Amazon has a ton of them with mixed reviews. |
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javavi Senior Member  Joined: 01/10/2023 Location: UkrainePosts: 213 |
Hi Leo, For my PicoMite I use a USB keyboard connected via a homemade adapter USB-PS/2 adapter made from RP2040-Zero It is connected directly without unnecessary details :)) It also allows you to simultaneously convert a USB mouse to PS/2 https://github.com/No0ne/ps2x2pico  |
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| LeoNicolas Guru Joined: 07/10/2020 Location: CanadaPosts: 479 |
Thank you I have bought one RP2040-Zero board. I will test it as soon as the board arrive. |
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Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
Using that circuit with no level shifter is adventurous. You will be feeding 5 volt signals into the pico. Not good. The adapters on Amazon and others are meant for keyboards that can automatically switch between USB and PS2, there is no converter inside them little adapter plug. Keyboards that can do that often come with an adapter plug but they seem to be getting harder to find. Bill Keep safe. Live long and prosper. |
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| LeoNicolas Guru Joined: 07/10/2020 Location: CanadaPosts: 479 |
Good to know. I doubt my keyboard has support for PS2. What do I need to do to connect the board to the PS2 connector? |
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| lizby Guru Joined: 17/05/2016 Location: United StatesPosts: 3150 |
I thought the Raspberry Pi Foundation had asserted that the Pico was 5V tolerant (but not much more than 5V). PicoMite, Armmite F4, SensorKits, MMBasic Hardware, Games, etc. on fruitoftheshed |
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| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
Not as I understand it. All designs by matherp and Mixte90 and Silicon Chip use a level shifter. Even if they are 5v tolerant, the keyboard expects 5v logic signals, 3.3v may be a bit marginal for my like. Leo, you need something like this if one didn't come with your keyboard. Bill Keep safe. Live long and prosper. |
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| LeoNicolas Guru Joined: 07/10/2020 Location: CanadaPosts: 479 |
This is probably a dumb question; I'm not a hardware guy. Can I add a small resistor in series to bring down the voltage to save levels? |
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| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
Others may know better but I believe the clock signal is unidirectional, transmitted by the Pico and received by the keyboard. So that should be OK, it may be marginal but if it works then good. The data line is bidirectional so the keyboard is transmitting 5v logic signal to the Pico and the Pico is transmitting 3.3v logic to the keyboard. A 1k resistor in series with the keyboard signal should protect the Pico input. To be safer a 1k8 to 2k2 resistor from the Pico input to GND to would make a voltage divider that will reduce the 5v to something near enough to 3.3v. The data to the keyboard may be marginal but again if it works... A level shifter is not expensive though. The only place I know to get a kit for something like this is Silicon Chip which uses a normal Pico. Bill Keep safe. Live long and prosper. |
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| ebbandflow Newbie  Joined: 31/08/2023 Location: United StatesPosts: 19 |
No, I don't believe so but the github for the ps2x2pico here https://github.com/No0ne/ps2x2pico shows how to solder a little level shifter board directly to a pico. I have built and used the circuit described in the GITHUB and it works well. If you're not using a level shifting circuit on the picomite VGA side, then level shifting is not required and I have had success with the ps2x2pico firmware and this circuit:  Edited 2024-02-18 12:25 by ebbandflow |
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| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
From the rp2040-datasheet:  From that table if you are running the Pico at 3.3v then the maximum voltage on an I/O pin allowed is 3.3v + 0.5v or 3.8v. Bill PS I don't understand how ebbandflow's voltage divider works if the drawing is correct. Keep safe. Live long and prosper. |
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| ebbandflow Newbie Joined: 31/08/2023 Location: United StatesPosts: 19 |
Its not a voltage divider they are pulldowns controlled by the ps2x2pico's gp13. This is the circuit without level shifting. Edited 2024-02-18 13:31 by ebbandflow |
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| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
Thank you, I don't see any reference to that circuit on the Github site but I do see plenty of references to level shifters. I cannot see how it adds anything worthwhile to the circuit. It will not reduce the 5v logic levels to safe limits. The output impedance of the keyboard's data line may be high enough to limit the current to a safe level for the Pico's input protection to handle, but that will depend on the individual keyboard and your luck. Bill Keep safe. Live long and prosper. |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6098 |
Pico to pico is both 3.3V so no level shift needed. It is only when you connect 'real' PS/2 devices or hosts that are expecting real PS/2 levels that the problem is present. Jim VK7JH MMedit MMBasic Help |
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| LeoNicolas Guru Joined: 07/10/2020 Location: CanadaPosts: 479 |
I bought a level shifter board to be at the safe side. How is the connection diagram? |
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| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
Right again Jim. I went off on a tangent again.  Leo, Please ignore my ravings. Bill Keep safe. Live long and prosper. |
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| LeoNicolas Guru Joined: 07/10/2020 Location: CanadaPosts: 479 |
Oh cool, this means it is safe to connect them directly, right? |
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| TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6098 |
Yes, for your purpose, all signals are at 3.3V so no level shifter. VK7JH MMedit MMBasic Help |
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| LeoNicolas Guru Joined: 07/10/2020 Location: CanadaPosts: 479 |
Thank you everyone for helping me  |
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| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
The first circuit plugs into the PS2 connector on the PicoVGA (I Believe) which does use 5v levels but... I should give while I'm only so far behind.  Which circuit do you want to use? The first one shows something which is separate and connects between the keyboard and the PicoVGA. The level shifter in the PicoVGA uses 10k pullup resistors so that should limit any input current to safe levels so a level shifter should not be needed. It could also be fitted internally without the socket and PS2 lead the same as the second one shows. This means surgery on the PicoVGA board. The second one is meant to be fitted internally. This also means surgery on the PicoVGA board. A third option may be to use Peter's "CMM1.5" firmware which means you can plug the keyboard into the Pico's USB socket via a USB A to C adapter. I have not followed it closely enough to know if this is viable. Personally, I would just buy a PS2 keyboard. I bought one from an op-shop for a couple of dollars. Bill Keep safe. Live long and prosper. |
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