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04/20/07 13:53
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 		 #137627 - 75 Ohm it is...
Responding to: ???'s previous message
VGA inputs often have a low input resistance
(75 Ohm if I remember correctly - on some
(very) old monitors you could switch them off)


From memory, I think the *old* CGA/EGA monitors used TTL levels for the color drive rather than 75-ohm 1Vp-p analog.


With 8051's weak internal pullup and your 1 k
external pullup resistor you won't be able to
drive the inputs to appropriate levels.


You are quite correct - this reminds me of the problems I encountered with my 8051 video game console:

http://au.geocities.com/vz300/vgc.htm

To make the 8051 drive into 75-ohms line impeadance. I built a discrete 9-bit color DAC to perform the task. Trouble was, this circuit needed one BC327, 12 resistors and 23 diodes!

On the plus side, it did work and all parts were readily available...

If I were to do it all again, I probably would've used R-2R ladders with some high speed op-amps instead...


Valentin


List of 52 messages in thread
TopicAuthorDate
8051 to VGA            01/01/70 00:00      
   nope..... but i do know            01/01/70 00:00      
      Video            01/01/70 00:00      
         well, if throiughput is a problem            01/01/70 00:00      
         VGA vs. NTSC (OK, PAL)            01/01/70 00:00      
            especially on CRTs?            01/01/70 00:00      
               flicker            01/01/70 00:00      
                  Distortion            01/01/70 00:00      
                     until today...            01/01/70 00:00      
                        Logical Processors            01/01/70 00:00      
                           not likely ...            01/01/70 00:00      
                              sure, but what problem is that            01/01/70 00:00      
               It's the demand for precise timing            01/01/70 00:00      
                  Monochrome?            01/01/70 00:00      
                     He may be a bit misguided            01/01/70 00:00      
                  Ive used an spi interface            01/01/70 00:00      
                     This is the arbitary display overlay jobby            01/01/70 00:00      
      A NICE VIDEO GENERATING AND PROCESSING EXAMPLE!!            01/01/70 00:00      
         Is it VGA?            01/01/70 00:00      
   Video Buffer            01/01/70 00:00      
      and a "video DAC"            01/01/70 00:00      
         Well there are ways and ways ....            01/01/70 00:00      
            but VGA is twice as fast            01/01/70 00:00      
               VGA Pixel rate is faster but ...            01/01/70 00:00      
                  If the MCU can\'t do it all ...            01/01/70 00:00      
                     how about this one            01/01/70 00:00      
                        "stuffer"            01/01/70 00:00      
                           it'd better be a really cheap MCU            01/01/70 00:00      
                              why?            01/01/70 00:00      
                        the guy who pays for it would notice.            01/01/70 00:00      
                           why?            01/01/70 00:00      
                              The extra MCU contributes nothing. Why use it?            01/01/70 00:00      
                                 because            01/01/70 00:00      
                                    TEXT => Pixels = character generator            01/01/70 00:00      
                                       sure, but            01/01/70 00:00      
                                          True, but only sometimes ...            01/01/70 00:00      
                                             HUH?            01/01/70 00:00      
                                                so ... which MCU has those built in?            01/01/70 00:00      
                                                   those?            01/01/70 00:00      
                                                      Here's what "those" are ...            01/01/70 00:00      
                                                         options            01/01/70 00:00      
                                    working examples            01/01/70 00:00      
                     Nice to think folks remember Don's "cheap video"..            01/01/70 00:00      
                        What do you mean, Phillip?            01/01/70 00:00      
                           A notion not to radical or heretical ...            01/01/70 00:00      
                              I still think it's either/or and not both            01/01/70 00:00      
      Like the vhdl that you'll find            01/01/70 00:00      
         Here's an ARM7 Driving a VGA Display direct            01/01/70 00:00      
   75 Ohm?            01/01/70 00:00      
      75 Ohm it is...            01/01/70 00:00      
   dc motor            01/01/70 00:00      
      start a new thead - and be specific            01/01/70 00:00      

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