Successfully loaded your register values from Arduino; 6845 is ticking; now I am struggling  with calculation; the 6850 datasheet is very confusing. I assume that for  40x25 characters (8x8) blocks I should have HSYNC 15,750 Hz.  I cannot figure out what  B1 - B10 are on p.19. May try to program registers to reasonable values and tinker with crystals.  Any help or reference will be very appreciated

are you trying to do VGA or connect to a LCD panel?

@vinci18 - i recommend this book: https://www.amazon.com/Cheap-Video-Cookbook-Donald-Lancaster/dp/0672215241

Hello, I am looking at creating a timebase for SHARP MZ-700 computer (need to use 8x8 font and 320x200 screen). I cannot figure out from your schematics what exactly the oscillator frequency should be. Also the content of GAL's will help. Thank you. Alex.

@vinci18 - hey there! i've updated the schematic pdf and uploaded the PLD source files. the board was designed to support a wide range of oscillators because i didn't know which one would work the best. i ended up just using a stock 25.1750MHz oscillator that i purchased on ebay! let me know if you have any more questions! https://www.ebay.com/itm/QTY-20-SG636P-25-1750MHz-SEIKO-EPSON-SMD-CRYSTAL-OSCILLATORS-25-175-MHz/112459687959

Great thanks. Do you have by any chance as an example a  set of 6845 register values for 25.175 oscillator and  any kind of display, say 40x25 so that I figure out everything and can calculate exactly what I need. I will start using Arduino to program 6845, look at the scope and then go further. Thanks

And another question - I see that 25.175 is divided by 16, so 6845 clock is 1.69 MHz. Do you need the high frequency anywhere else? My Z80 runs at 3.54 or 7.37 MHz. Can I clock 6845S with 3.55 MHz?

@vinci18 - i've added a display.asm file that contains the initial register settings i am using with the VGA interface(if i remember correctly!). the 25.175MHz is the standard reference clock for a number of VGA resolutions. this frequency is needed to make sure that the pixel clock is accurate. for my design i am dividing the 25.175MHz in half, which generates 12.5875 which is used for the primary pixel clock (dot clock). the dot clock divided by 2 and divided by 4 are required in order to control the data latching timing. while the dot clock divided by 8 is the character clock needed for the 6845. this page has some info about the VGA timings: http://tinyvga.com/vga-timing

I want  design 8 bit isa vga mono card with AVR ATmega. can you help me?

hello

I have a question.

1. this board have 15 pin vga output?

2. and i use in 8088 pc with vga monitor?

if it is ok , then it is very good project .

3. and it is mono or color?

4. text or graphical?

@M.NOORI there is a separate PCB with the VGA connector and yes this can be used on any motherboard that supports 8-Bit ISA slots. this version of the design is mono and only text.

i've added a PDF schematic for the VGA interface board....

Very cool! I am looking into doing this (and hopefully) more with an FPGA but there is a lot of elegance in this design!

@Ian Hanschen - thanks for the feedback!

Funnily enough I stumbled across a design today that was using a 6545 (so 2.5MHz limited) to generate VGA sort of range video and it's both clever and simple. The row and address lines are wired to two sets of 8bit wide RAM in parallel, and the CPU sees the RAM as addresses from A1 upwards, with A0 selecting between the two 8bit wide RAMs (and gating cs/wr/oe). So the computer thinks it has 8bit wide RAM, the 6845 doesn't care how wide the RAM is and the shift register is now loading 16bits at a time so doubles the pixel clock sustainable.

oh that is clever indeed! any links to that project?

Not online - but it was taken from an app note

http://archive.6502.org/appnotes/synertek_an3_6545_crtc.pdf

"Operating at very fast character clock rates"

@EtchedPixels - dandy thanks!

Thanks for your detailed reply.  I noticed the difference because I am using an Interak computer which displays 80 characters using a 6845.

The characters are 8x10 pixels and the clock is 15MHz but the computer uses a mono CRT monitor.  I don't think it will display on an LCD monitor but it would be better if it did as CRT monitors will not be around forever.

Right, the 15MHz clock is well within the specs of the based MCP6845. most of the older CRT designs used a much lower clock frequency such as those of the IBM Monochrome Display Adapter (MDA) and Color Graphics Adapter (CGA). most likely the resolution you are operating at is 640x200@60Hz (similar to CGA in mono mode). if you do the math of 640x200x60=7680000 pixels per second which could easily be driven by a 15MHz clock source. The issue for driving VGA is the that base clock frequencies start at the 25.175MHz, so you can do some fix ups to make it work. you could easily make a new type of video card for Interak with VGA support using the MCP6845...

do have any links to documentation on the video display unit you are using with your Interak?

Really interesting project, thanks for posting it. 

At the beginning of your description you say that you are using an 8x16 font - I assume this is 8 pixels by 16 pixels. But you also say that you have a resolution of 640 x 480. This would give a text display of 80 characters (8 x 80 = 640) not the display shown of 40 characters.

Can you say why this is?  Thanks, Alan

great question! i probably should have explained that a little better! so.... a normal VGA resolution of 640x480 operates at a clock frequency of 25.175MHz which provides a refresh rate of 60 frames a second or 60Hz. in order for me to use an original MC6845 chip, the maximum frequency i can operate at is 16MHz (2MHz bus times 8 pixels), which is well blow the 25.175MHz needed for the 640x480 resolution at 60Hz. with that in mind, i am actually operating at 1/2 of 25.175MHz ( 12.5875MHz). This effectively causes each pixel in data to be draw twice horizontally, or in other words, it cause the horizontal resolution to be cut in half, giving the final resolution of 320x480 at 60Hz with a character resolution of 40x25 when using the 8x16 font. if 40x25 sounds like a familiar resolution from the late 70's and 80's computer era, this is why! The chips had max clock frequencies so 40x25 was easy to generate!