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Forum Index : Microcontroller and PC projects : PicoMite: PicoGAME VGA development
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| Volhout Guru  Joined: 05/03/2018 Location: NetherlandsPosts: 4256 |
Because the protectio diode the rp2040 pin will be driven to maximum 3.3+0.6=3.9V (not 4.4). PicomiteVGA PETSCII ROBOTS |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6814 |
Only if there *is* a protection diode. The schematic in the Pico datasheet states; "ADC GPIO pins have diode to VDDIO (other GPIO do not)", from which I would assume that they aren't there. :) Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1611 |
@Volhout, Sorry, you are right about the base current of the transistor if the Pico's output is low or if the 3.3v is off. But in that situation the current though the 27ohm resistor is still too much for my liking. I would be less unhappy if there was a 1k resistor in series with the base of the transistor but there is still that high CE current. But as you say "it's not for me". Bill Keep safe. Live long and prosper. |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4256 |
For those who are not interested anymore. This is the final circuit I published here in March 2021. Amplifier It does not use the resistor (27 ohm) but a current source for improved linearity. That also gives the advantage that the voltage at the micro pin never exceeds 3.5V. Schematics:  Edited 2022-03-30 00:22 by Volhout PicomiteVGA PETSCII ROBOTS |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6814 |
Once again the voltage on the micro pin is less than 3.6v *if the pin is either high or low*. It's still about 4V7 if the pin is tristate. If you put a 1k8 resistor from the PWM pin to ground the above circuit should be safe as the voltage is about 3V6. 1K5 would be safer still. Alternatively, put a small signal Schottky diode from the PWM pin up to 3V3 as the RP2040 hasn't got one. IMHO this is important. If the pin isn't allocated to something to give it a high or low default then you run the risk of damage. Of course, this isn't an issue if the pin is 5V capable, but unfortunately the Pico hasn't got any of those. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4256 |
So MIck, If I understand you correctly, this would have your blessing:  PicomiteVGA PETSCII ROBOTS |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6814 |
:) Can't see why not - I'd be quite happy to use this. I might do, actually. :) The 1k8 resistor could probably be higher in this circuit for safety, although you can usefully use it to set the bias current. EDIT: I see you might have set the bias. :) It runs nicely in simulation. Edited 2022-03-31 01:38 by Mixtel90 Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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thwill Guru Joined: 16/09/2019 Location: United KingdomPosts: 4047 |
Do you want a couple of these Mick ?  They have been approved by the authorities:  But I'm still waiting on all the parts to populate one, the PCB, not the cat, the cat is fully populated except for one canine and his testicles. Best wishes, Tom Edited 2022-04-07 00:07 by thwill Game*Mite, CMM2 Welcome Tape, Creaky old text adventures |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6814 |
Ooh! yes please! :) I'll need to order some bits too. Lovely puddy tat... Nice to see he's taking an interest in electronics. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| thwill Guru Joined: 16/09/2019 Location: United KingdomPosts: 4047 |
Hopefully I'll drop them in the post by the end of the day. Best wishes, Tom Game*Mite, CMM2 Welcome Tape, Creaky old text adventures |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4256 |
Nice !!! If you have a spare.... I would like to build one. Volhout P.S. red is not my colour, but for this I make an exemption PicomiteVGA PETSCII ROBOTS |
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| thwill Guru Joined: 16/09/2019 Location: United KingdomPosts: 4047 |
Since I owe you a PCB you can have my spare (PM me your address), I just hope I don't cock up one of the other two; I need two to test the "networking". To anyone else who is interested I'll be ordering some more once it is confirmed to be a working board, and within reason I'll be happy to send one to anyone who "promises" to actually build it for the cost of post and packaging - though hopefully one or more of my friends may offer to order boards (at my expense) to service any US and Antipodean shedders. Mick, speak up if I'm overstepping my bounds given that you're the designer, but since you did it as an academic exercise (and to my eccentric controller spec.) it didn't seem likely you were going to "push" these. Best wishes, Tom Edited 2022-04-07 01:34 by thwill Game*Mite, CMM2 Welcome Tape, Creaky old text adventures |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6814 |
No - I've no problem with it. After all, I wouldn't have designed it if I hadn't wanted to give you yet another distraction. ;) I might "push" my VGA mini 3.0 once it's finally completed (just did a very minor couple of tweaks today), but as a kit with PCB, VGA socket SDcard socket, audio jack, power jack and uncut case. I think that's all the more difficult to source stuff. It's just an idea at the moment. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| al18 Senior Member  Joined: 06/07/2019 Location: United StatesPosts: 205 |
Looking at the photo of the boards, doesn’t look like any of the holes for K4 (PS/2) are plated thru. Looks like the circuits are on the bottom side, so probably works fine electrically. For mechanical strength of connector mounting, plated thru holes are better. |
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| thwill Guru Joined: 16/09/2019 Location: United KingdomPosts: 4047 |
You are correct they are not plated through, there are a couple of non-plated holes on the audio jack too. No big deal, I'm sure that Mick will fix (unless deliberate) them with a Revision 1.1 before I order more boards  .Best wishes, Tom Edited 2022-04-07 02:09 by thwill Game*Mite, CMM2 Welcome Tape, Creaky old text adventures |
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| al18 Senior Member Joined: 06/07/2019 Location: United StatesPosts: 205 |
I think the two nonplated holes for the audio jack were intensional - for mounting posts on the connector. |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6814 |
The PS/2 connector only needs pads on the bottom of the board. There are 3 large solder points there. If it bothers you, just bend the lugs before you solder. You'll probably need a hammer to get it off then. :) The through hole plating adds nothing to the physical strength - it's only microns thick and, even when filled with solder, can be ripped out. Likewise, pads on the top side can be pulled off the FR4. On the other hand, a soldered pad on the reverse side is bigger than the hole and helps a lot. The sources of strength for components mounted on a pcb are the fixing and alignment holes. I couldn't guarantee that the PS/2 connections wouldn't *eventually* short onto the metalwork so all the tracks are underneath. The reason it ended up like this is that originally I used the top groundplane as 3V3 and the bottom as GND. It makes life a lot easier when laying out a tight pcb, but you can't allow any metalwork to short out top and bottom, of course. Hence removing the top groundplane under the PS/2 socket, the reset button and the SDcard holder. EDIT: The audio jack has 2 non-PTH holes for alignment only. Edited 2022-04-07 02:20 by Mixtel90 Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| al18 Senior Member Joined: 06/07/2019 Location: United StatesPosts: 205 |
Mick, Thanks for your design. But I strongly disagree with your statement I worked for decades as an engineer in PCB manufacturing and assembly. Pads on 1 side of the board are not strong - I’ve ripped off pads with too much heat during hand soldering. A plated thru hole with pads on both sides is very strong. While the copper plating in the hole is typically .001 inch thick and surface plating is .0015 to .002 inch thick, the combination acts like a rivet. If I was building your board, I would as some 5 minute epoxy to hold the PS/2 connector to the board on the connector side. |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6814 |
YMMV, of course. :) Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6814 |
PCBs have arrived, thanks very much Tom! :) A lovely bright red. :) Preliminary assembly tests - nothing soldered yet. Audio jack location pins don't line up - just cut them off. I couldn't find a proper drawing of this component and it's *very* hard to see black plastic against black plastic, never mind measure it, so it was half expected. The pins are fine. Alternative source for the VGA and game port connectors is Toby Electronics. VGA socket is their part number HL15S 9-pin connector part number is DMR09P Prices are quite reasonable. The USB-A connector needs the big pins bending very slightly to bring them in line with the front & rear little pins. Once that is done it fits ok. I tried 3mm MF spacers for the joystick module but they don't fit - I made the PCB holes 2.8mm for 2.5mm dia threads. Even so, the one closest to C8 may not work as it's probably too close to the SDcard. A 3mm LED will just fit between the game controller sockets. Holes for the male link headers are a shade too small. Some may fit, most not. You could hard wire or use female headers with plug-in wire links on this version. Something to fix for the next revision!  Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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