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Forum Index : Microcontroller and PC projects : Picomite VGA - mono VGA mode = more I/O?
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| flasherror Senior Member  Joined: 07/01/2019 Location: United StatesPosts: 159 |
I am resuming work on a PCB design for Picomite VGA that I stopped doing a while ago (2022, long time ago, but depression sucks for doing hobbies) Anyway, I am reading PicomiteVGA User Manual v5.08.00 and while it does have info on mono mode, it doesn't say anything about whether mono mode works by driving all 3 R/G/B signal lines or just a single one (green?). I seem to recall that GP21 could have been available for I/O back then for mono mode. I do not know if updated firmware has changed this. I have a use case where VGA mono (single color) is acceptable but I need one more I/O than color VGA. Also, I noticed the VGA wiring diagram does not have clamp diodes from each R/G/B signal pin to GND (to limit to 0.7V). Are these needed to make signals "standard compliant"? I suggest adding to manual: What I/Os mono VGA uses (i.e. only drive green or drives white using all RGB signal lines) If unused VGA signals lines can be used as general purpose I/O Thanks |
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phil99 Guru  Joined: 11/02/2018 Location: AustraliaPosts: 2135 |
All 3 VGA pins (RGB121 so 4 output pins, R, Ghigh, Glow, B) are driven in Mode 1 (640x480), which isn't entirely mono. The editor can be in colour and regular output can be "colorized" with Tiles. In the latest Beta versions the VGA output pins are no longer fixed, however you must still specify 2 for Sync. and 4 for colour whether you want them all or not. Clamping diodes are optional as almost all monitors do that anyway. There may still be some very old (pre 1990) cheap CRT monitors that need them. |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4228 |
hi flasherror, I understand your idea. And I think the very first versions of PicoMite had this option, where you could select mono 640x480, and connect only 1 O/I pin. But the pin could not drive R+G+B directly, so extra circuit was needed. That was even before Geoff issued his PicoMite in silicon chip magazine. But since then, the whole development has gone to support RGB 1:2:1. For 3 years now. Including color coding in the 640x480 editor screen. Quite an achievement. I think Peter will not likely go back to saving (essentially) 3 pins. If you are really short for 3 pins, then omit the SD card (4 pins) or do not use SYSTEM I2C (2 pins). Do not use audio (saves 2 pins). What might be a more sensible question to ask is "can we disable PS2" to save 2 pins. Then you could build a VGA system with 20 free IO pins. And you can always add an IO expander (plenty of examples, I2C and SPI) for a cost near nothing. Regards, Volhout Edited 2024-03-19 18:04 by Volhout PicomiteVGA PETSCII ROBOTS |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6786 |
There is no mono mode as such on the PicoMite VGA and there never has been. You *can* get mono VGA out of it by changing the resistor ladder to suit, but you don't gain any pins, you still have to have 4 video pins and 2 synch pins. The manual is referring to Mode 1 as mono mode and Mode 2 as colour mode. Mode 1 is 640x480 with 2 colours plus a "tile" mode where each tile can have any two colours. Mode 2 is 320x240 with 16 colours 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: 4228 |
Mick, monochrome VGA on GP22 Volhout Edited 2024-03-19 20:42 by Volhout PicomiteVGA PETSCII ROBOTS |
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| flasherror Senior Member Joined: 07/01/2019 Location: United StatesPosts: 159 |
Thanks everyone. Yes, this was a long time ago and the mono mode you describe matches what I remember. This (no mono mode and no pin gain advantage for mono) isn't what I remember, but is likely something that was changed from the early firmware versions. After reading the v5.08.00 manual some more: I think the current description of "mono" is a little misleading given that mono mode is really "hi-resolution with optional limited colors via TILE command"? The Page 6 "Connecting the VGA monitor" text says: "The output is in the standard VGA format with a pixel rate of 25.175MHz and a frame rate of 60Hz. In monochrome mode the resolution is 640 x 480 however when the colour output is enabled (MODE 2) the pixels are duplicated along both the x and y axis giving a 320 x 240 resolution while the monitor still sees a 640 x 480 signal." Maybe adding under the above text: "In monochrome mode, there is the capability to have limited color selection using the TILE command (so that mono can have colors - you can think of "Colored Mono" as Hi-Res with restricted color capability). Note that in mono mode, all VGA color pins are still used so that they are unavailable for general I/O use (even if color TILEs are not used)." |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6786 |
I think you have to be careful with the terminology. Mono VGA always uses R, G, B Hsync and Vsync pins simply because it *is* VGA. It is called "mono" VGA because only two colours are used, one foreground and one background. They could be any two colours as VGA is analogue. Mono combined video, which was an option on the original Maximite, has a single video output with combined Hsync and Vsync. The video is either off or on, but you can feed it into one or more channels of a RGB display to get a colour that you like. :) You can also have mono RGB, which usually carries the sync signals on the green channel so you can get a green and black display from a single pin. but you could also get any colour combination. Sometimes RGB signals are digital, in which case you can only get 8 colours including black. The PicoMite VGA has always been VGA only. Edited 2024-03-19 21:31 by Mixtel90 Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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