 Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me. |
Forum Index : Microcontroller and PC projects : PCB for VGAmite?
  Page 4 of 4 |
|||||
| Author | Message | ||||
| matherp Guru  Joined: 11/12/2012 Location: United KingdomPosts: 8575 |
The main CPU clock on STM32 can be perturbed by fast high power I/O. I have included a transistor in my circuit to keep the drive high impedance. Setting up an output to try and drive above the pin current limit of the chip is just asking for issues. Everyone can do as they wish but don't complain if you see poor quality video if overdriving the pin |
||||
Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1593 |
Thanks Peter, enough said. Bill Keep safe. Live long and prosper. |
||||
Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 5712 |
The mono Maximite works fine - the single pin drive capability of the MX170 is 30mA at 2v output (and can get up to about 37mA at 1v). The STM32F411 has a max output of 20mA at 2v. To get close to supply you have to draw less than 6mA. Those figures are at best as far as I can tell. Edited 2021-12-03 22:38 by Mixtel90 Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
||||
| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1593 |
For the record: The mono Maximite uses either the PIC32MX795F512H-80I/PT or the PIC32MX695F512H-80I/PT not the MX170. It can supply +/-25mA from any one I/O pin. Page 61 of my copy of the STM32F411xC STM32F411xE data sheet says the chip can supply +/-25mA from any one I/O pin. I would expect (not assume) that that 25mA would be at a logic '1' level. Bill PS Can someone please tell me the purpose of the 470 ohm base-emitter resistor in Phil's emitter follower circuit? Keep safe. Live long and prosper. |
||||
phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 1779 |
"Can someone please tell me the purpose of the 470 ohm base-emitter resistor in Phil's emitter follower circuit?" Geoff's ASCII terminal can have either VGA or Composite video out. At powerup it tests the VGA pin for a load and switches to Composite if there is none. Vbe is enough to block VGA mode. |
||||
| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1593 |
Thank you (RTFM?). So it should not be needed in Peter's circuit. Bill Keep safe. Live long and prosper. |
||||
| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 5712 |
I stand corrected on the mono Maximite, but in light of what I've written below it might be interesting to have a look at the data sheet for those PICs. PIC32MX795F512H-80I/PT data sheet: Page 355 Absolute max output current +/-25mA Page 366 Param. DO20A - for the high current port C. Other ports can source 14mA. Output high voltage with a 3.3v supply is 1.5v @ 22mA, 2.0v @ 18mA & 3.0v @ 10mA STM32F411xC STM34F4llxE data sheet: Page 61 ... is under "Absolute Maximum Ratings". You must never exceed these or you risk damage. You should never even design close to these unless you like to live life with additional risk. :) It's pretty safe to assume that 25mA will sink the output to zero and heat up the driver. Page 99 Note: "with a relaxed VOL/VOH" - you won't get close to the supply rails. On page 100 the third section of the table (the most optimistic) shows that for 20mA output, you get a guaranteed output of 1.3v while sinking and 1.3v while sourcing with a 3.3v supply. You can design outside this (for a non-steady load), but don't be surprised if it only works on some chips. Full white output will require 28mA into 25R so it's impossible to source this from a single pin on the chip. You will probably still get an output at 28mA, but it won't be at 0.7v into 25R and it might damage the chip. You can drive pulse signals up to quite high current as the heating effect in the driver is based on the *average* current out of the pin. However, the voltage will still sag as the mosfet is a resistance and you can't do anything about that. Edited 2021-12-04 17:55 by Mixtel90 Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
||||
| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1593 |
Thanks Mick, You paint a very pessimistic picture of the chip. The specs you quote do seem contradictory though. Maybe some backside covering? Seems like Peter is lucky his original VGA circuit works as well as it does. Bill Keep safe. Live long and prosper. |
||||
| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 5712 |
The devil is in the detail with data sheets, unfortunately. The bit at the front is advertising blurb and you can't assume it's all possible. You get a similar problem with op-amps - any loading and the output swing really falls away. TBH I hadn't realised that only port C on that PIC is rated for 22mA. There are always surprises! Specifications are the minimum that a chip can handle. It's the design limit. That doesn't mean that you can't push it, all you might do is shorten its anticipated life from 40yrs to 30yrs. Or it might handle additional load anyway - it's just not been tested to that point. The manufacturer has to quote a spec so they quote one that virtually all the devices off the line will meet. :) The original VGA circuit will work (obviously!), as will the others. You'll get video out of a single pin. It's heat that causes damage and that's based on average watts. The sync and video signals are all pulses so the average power dissipated isn't great so nothing is liable to be damaged. The video signal is clamped to 0.7v by the diode so any output above that will give full brightness. You can let the pin voltage sag to 0.7v under load, so 20-odd mA is easily coped with. Also, the problem is only serious with all three video inputs driven. It's less than 10mA per input so any two can be driven within spec. You just might not get full brightness. For the VGAmite I'd be quite happy with a choice of green or amber, which is probably about 15-18mA maximum load (for amber) so it's well in spec. Edited 2021-12-04 19:21 by Mixtel90 Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
||||
| phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 1779 |
@Mixtel90 Thanks for the detailed info. It seems one transistor is a small price to pay to have all colour combinations available. |
||||
| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 5712 |
I got a nice, shiny new Sprint Layout 6.0 to play with the other day. I decided that for the sort of PCBs I do it would probably be more than adequate. The learning curve is proving tobe shallower than Eagle and early results are encouraging. I think I'll be able to produce some Gerbers for the VGA-MITE MINI (as I had to call it on the PCB because the text is a bit small for lower case!) soon (if anyone's interested). It's designed to fit the original case suggested by Peter because it's cheap and very easy to get. I have one on order to test with before I finalize the PCB. Differences from the larger design: * Smaller case (obviously!). * Uses this microSD card adapter. In spite of the ad, it's not level shifting. Even if it is, it works fine on a PicoMite. :) If supplies dry up it's basic enough for someone to make or even put a microSD holder direct on the PCB. Sparkfun do the same holder at a much higher price. * The SDcard enable is selectable by link to either PB1 or PB10. This is handy just in case someone wants to use all the PWM pins (assuming Peter's firmware will allow it). * The 5V supply input is via a 3.5mmx1.3mm barrel jack connector. Leads for USB-A to this are easy to find on ebay and are intended as 5V power supply connections. * GPIO connector is a 2X13 IDE and is not compatible in any way with the CMM2. Pins that are allocated internally (with the exception of the flash enable pin) aren't on the connector. There are 2x 3.3v, 2x GND and 1x 5v pins included. It's possible to power the system via this 5v pin or use it as an output. * A 5-pin female 0.1" SIL connector on the board allows direct plug-in of this USB-serial adapter. Console pins are not on the GPIO connector. A link is provided to disconnect the +5V from the USB source if the device is powered from the console. * I changed my mind about duplicating the onboard LED and put a small sea of holes in that area. The LED pin is, however, on the GPIO together with the RESET pin. * VGA colour option is via a transistor. A bit of stripboard can be used to carry the resistors. It also has a ground pin so that unused VGA inputs can be grounded if desired. Obviously the user can make a few modules and swap them as desired. It has 4 pins at one end and 2 at the other, one of which is spare. * It's got rounded corners (so is liable to a lawsuit from Apple). Edited 2021-12-06 21:37 by Mixtel90 Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
||||
| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 5712 |
This little project isn't dead yet. Just some interest value for anyone considering using these little cases: First, the packing is terrible. A simple plastic bag with the screws and box thrown in and not sealed. That is then put into a second bag which is sealed then into the shipping envelope. The screws *will* be floating about in the packaging when you come to open it and the PCB screws in particular are tiny and easy to lose. Have a resealable bag handy when opening the package. I tried to find working drawings of these cases and managed to find two slightly different ones, one more detailed than the other. Neither match up with the box I got. As there are no manufacturers details and most sales are from box shifters it's unlikely that there are any definitive ones. It's quite possible that there are multiple manufacturers so my comments may not apply to all boxes even if they appear to be identical! Relating to the above, my box is not symmetrical. The top and bottom of the box have different lengths of PCB supports in them (a dimension that was missing from both the drawings). If the PCB is mounted upside down in the top of the case the posts are almost short enough to get the PS/2 connector nicely into the "window" on the end of the case. Incidentally, the "window" isn't symmetrical either - it's about 1mm bigger than the drawings and offset towards the bottom. The "windows" will accept 1.6mm PCB if you wanted to get them cut. For this particular project, getting the VGA and PS/2 connectors side by side is pretty tight. There are moulded bosses inside the case for the recessed fixing screws. These are 8mm dia so you lose a lot of PCB space at that end. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
||||
| Turbo46 Guru Joined: 24/12/2017 Location: AustraliaPosts: 1593 |
Peter, Have you considered the possibility of providing composite video for this board? It would allow the use of some of the small composite video monitors available. Bill Keep safe. Live long and prosper. |
||||
Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9059 |
@ Mixtel90: Congratulations on SL6.  It's my go-to PCB software and I have been using it for years now, so if you ever have any questions about doing something in it, please either start a thread or feel free to send me a PM.Smoke makes things work. When the smoke gets out, it stops! |
||||
| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 5712 |
Thanks, Grogster. :) It's really nice to use. For small boards schematic capture is hardly worth it so I don't miss it. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
||||
| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 5712 |
Final Christmas present for this year. :) Here's the Gerber package for the VGAmite mini (NOT the PicoMite VGA mini). It's unlikely that I'll do anything else with this now. VGAmite mini 001.zip . Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
||||
| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 3507 |
Maybe we should everyone become aware that the VGAmite will not be continued. The future is the VGApicomite. A differet hardware platform with different pinout. People building VGAmite will be on their own.... PicomiteVGA PETSCII ROBOTS |
||||
| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 5712 |
Yep - This is true. MMBasic for the VGAmite is no longer being developed. It has been replaced by the PicoMite VGA edition. However I did say that I'd complete my design for the VGAmite pcb if anyone was interested and Cyber was, so I did just that. :) There is no reason why the same PCB couldn't be used with a Black Pill running, say, Python and you can write your own IO routines or find them in libraries. The same goes for my PicoMite VGA mini design, of course. These things aren't MMBasic specific, they were just designed with that in mind. :) Edited 2021-12-26 06:33 by Mixtel90 Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
||||
| Page 4 of 4 |
|||||
 Print this page |
