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Forum Index : Microcontroller and PC projects : MAXIMITE 2 R-2R resistor values

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electricat

Senior Member

Joined: 30/11/2020
Location: Lithuania
Posts: 299

Posted: 07:18am 04 Dec 2020

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Hi all!

This is my first post here and would like to thank Geoff Graham and all kind people for fantastic work done. I have no words describe joy when first time saw it :) It`s like fell in love with computer one more time :D
Perfect toy for boy from 80`s like I am :) I looked for something like Maximite2 for past few years...

So, I ordered parts and hardly can wait arriving them to switch Maximite2 :D
To my surprise CPU board arrived already but discrete components are in stuck somewhere.

So CoreH743I firmware has flashed ok and while awaiting parts I would play with it on prototype board, but found only 220R and 110R resistors in my boxes for that R-2R analogue part of circuit.

Will they be OK instead of 240R and 120R as in schematics leaving 75R as is?
It seems to me should be ok, but not sure.

My MMBasic 'sand box'

Volhout
Guru

Joined: 05/03/2018
Location: Netherlands
Posts: 5091

Posted: 07:49am 04 Dec 2020

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Hi electricat,

You would get a slightly brighter video picture (the picture may get blurring in the brighter sections, depending on your monitor, you may be able to adjust this in the monitors menu).
The STM32 (needs to drive more current on 16 pins) will get slightly warmer, as does the power supply.

This is from simulation (I don't have a CMM2 at hand).

Maximum video level

' 240-120 ohm 220-110 ohm
5bit DAC 761mV 812mV
6bit DAC 774mV 826mV

A brightness increase of about 7% by changing the resistor values to 220-110 ohm.

Volhout
Edited 2020-12-04 18:24 by Volhout

PicomiteVGA PETSCII ROBOTS

electricat

Senior Member

Joined: 30/11/2020
Location: Lithuania
Posts: 299

Posted: 10:18am 04 Dec 2020

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Volhout, thank you for help!

Without exact values am still bit careful about current drawn from chip.
To be sure 100% would be nice to calculate also difference in current drawn per pin.
Now reading some info source about building DAC vor VGAs :)

My MMBasic 'sand box'

matherp
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Joined: 11/12/2012
Location: United Kingdom
Posts: 10315

Posted: 10:23am 04 Dec 2020

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If you have them the safe option is 150/300ohm

electricat

Senior Member

Joined: 30/11/2020
Location: Lithuania
Posts: 299

Posted: 10:42am 04 Dec 2020

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Matherp,

ok. excelent, i have them and soldering station heats on :)

My MMBasic 'sand box'

Volhout
Guru

Joined: 05/03/2018
Location: Netherlands
Posts: 5091

Posted: 04:40pm 04 Dec 2020

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hi electricat,

The values matherp propose are a safe solution for heating and output drive of the STM32.
If you find the brightness levels too low (and cannot correct the brightness on the monitor), you could improve a bit by changing the output resistors on the ladder network from 75 ohm to 100 ohm.

Enjoy your CMM2, it is a very nice product.

Volhout

PicomiteVGA PETSCII ROBOTS

electricat

Senior Member

Joined: 30/11/2020
Location: Lithuania
Posts: 299

Posted: 10:21pm 05 Dec 2020

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So, I`m in :)

(something strange with keyboard, stops functioning randomly. But might be USB shielding or power problems as it realy runs a bit "in air" for now :) Not related to R2R :) anyway )

2020.12.06

All problems solved.

I just can`t express joy about CMM2 in proper words.
The only wish is now await for proper motherboard PCB to put all components politely inside and start play endless games via I2C, SPI, SMBUS etc. Yes, we can do it many different ways and we do. But CMM2 seems to promise some special fun.

I was looking and waiting for such machine for years. Found some nice projects really close, but this one makes me feel Young again! :)
Edited 2020-12-07 06:56 by electricat

My MMBasic 'sand box'

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