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Forum Index : Microcontroller and PC projects : CMM2 and GPIO connector orientation
| Author | Message | ||||
Atomic_Shrimp Newbie  Joined: 28/07/2020 Location: United KingdomPosts: 3 |
Hi everyone. I'm new here. Just bought a fully assembled CMM2 and I am enjoying reliving and reviving some of the pleasures of boot-to-BASIC computing. I was particularly keen to have a go at various I/O tasks, which I have dipped my toe into with RPi and Arduino with some success, but in those cases, found the dual learning curves of language and hardware made things tricky for me - so the CMM2 is a real boon to me, having grown up in the 80s and learned programming on the ZX81 and Spectrum. Anyway, I rushed out and bought a cheap add-on kit for Raspberry Pi, which includes a breadboard breakout header and cable, and connected it to my Maximite, which I could see from the manual is pin-consistent with the RPi. Only problem is that the rear connector for the Maximite is 180 degrees rotated vs the convention for the RPi - so pin 40 is where pin 1 would be. No problem, I thought - I'll just ignore the labels on the breakout connector and use the pins as they are being passed through, and this seemed to work... BUT... what I did not realise is that on my RPi breakout PCB, all the ground pins are interconnected to a common groundplane - Noob error - which would be fine, except with the rotated connector, this actually means the following pins on my Maximite were shorted together: 2: 5V Power! 7: Analog I/O or Count1 (not 5v tolerant) 11: COM2 RX 20: Ground 21: SPI MISO 27: I2C2 SDA 32: General I/O 35: SPI2 MISO Frankly, I'm a little surprised the magic smoke didn't just come straight out, but oddly, the thing ran OK, with a bit of weirdness like random resets when I presented ground on a regular I/O pin. Having now resolved this problem (by dispensing with the breakout header and just using female to male jumper wires, everything *seems* to be OK - I guess I need to test pin 7, as it's not 5V tolerant, but I wonder if the short also including a ground pin kept it from blowing up. What are the chances I fried something, and what else should I test? Also, feel free to berate me for my misadventurous action. |
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TassyJim Guru  Joined: 07/08/2011 Location: AustraliaPosts: 6283 |
It's unlikely to have done any damage except to your pride. The processor is fairly robust. But don't make a habit of it. Most RPi breakouts are OK to use but there are a few that have really strange pinouts. All I have seen have the GND, 3.3V and 5V lines commoned. Jim VK7JH MMedit |
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KeepIS Guru Joined: 13/10/2014 Location: AustraliaPosts: 1882 |
I filed the raised index tab on the plug down and plugged it in with the correct orientation of pint 1. I think someone else cut a notch in the CMM2 socket when they used one. NANO Inverter: Full download - Only Hex Ver 8.1Ks |
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| Atomic_Shrimp Newbie Joined: 28/07/2020 Location: United KingdomPosts: 3 |
I cut a new notch on the RPI breakout board socket - so I can plug the cable in the right way around now. Gonna test all pins for throughput (and for shorts against all other pins) before I trust this thing again |
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| KeepIS Guru Joined: 13/10/2014 Location: AustraliaPosts: 1882 |
If it's the one I think it is then all pins on mine are correct, but best to be on the safe side for 3v3 pins when +5v is in the cable, I know you were also concerned about this as well. NANO Inverter: Full download - Only Hex Ver 8.1Ks |
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| Atomic_Shrimp Newbie Joined: 28/07/2020 Location: United KingdomPosts: 3 |
Well, the good news is that pin 7 tests just fine. Looks like I got away with being a hasty dumbass just this once more. |
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