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Forum Index : Microcontroller and PC projects : Shall I call this one Gemini?

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Volhout
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Joined: 05/03/2018
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Posted: 07:29am 22 Aug 2022
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AFAIK the gemini board does not use diodes in the VGA circuit.

Please elaborate what diodes you have changed.

Volhout
PicomiteVGA PETSCII ROBOTS
 
Mixtel90

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Posted: 08:48am 22 Aug 2022
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It's a PicoMite Pear, original version. That uses the 1N4148 diode clamps on RGB.
Mick

Zilog Inside! nascom.info for Nascom & Gemini
Preliminary MMBasic docs & my PCB designs
 
Volhout
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Posted: 09:56am 22 Aug 2022
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- remove the BAT85 diodes entirely
- then the video should be a bit bright
- to tune: 220 ohm -> 270 ohm
          330 ohm -> 390 ohm
          680 ohm -> 820 ohm

This should solve the problem.
BAT85 is not a good replacement for 1N4148.
PicomiteVGA PETSCII ROBOTS
 
lizby
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Posted: 11:43am 22 Aug 2022
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  Volhout said  - remove the BAT85 diodes entirely

...

BAT85 is not a good replacement for 1N4148.

For the purpose of education, can you explain why the BAT85 is not a good replacement here?
PicoMite, Armmite F4, SensorKits, MMBasic Hardware, Games, etc. on fruitoftheshed
 
Volhout
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Posted: 12:06pm 22 Aug 2022
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Hi Lizby,

Generic:
A diode is a semiconductor. It conducts (with some losses) current in one direction (anode to cathode) and blocks current in the other direction (cathode to anode).

For a 1N4148 silicon diode to conduct it needs a minimum voltage between anode and cathode. This voltage is around 0.6V. Below 0.6V it will nor conduct (or really minimal leakage), and above 0.6V is will conduct current.
In case of the VGA circuit the picomite IO pin delivers 3.3V to a 220 ohm resistor to the silicon diode. This is sufficient high to make the diode conduct (when RGB is bright 3.3V) and not conduct when the screen is dark (0 volt). The conducting diode works like a limiter at 0.6..0.7V. This is in line with the 0.7V that VGA is specified at. 0.7V for 100% brightness.

The BAT85 is a Schottky diode. A schottky diode is also a diode, but it does not need the minimum voltage of 0.6V to start conducting. The BAT85 starts conducting at 0.1V or even less. If it where used for the same VGA application it will limit the VGA voltage to 0.1 or 0.2V, much lower that 0.6-0.7V for the silicon diode. And low voltages show as low brightness in the VGA display (near black).

You can simply see this difference if you look at the "forward voltage" in the datasheet of the diodes.

So the short answer is: despite that the 1N4148 and BAT85 look similar from the outside, inside they are different technology. Like a petrol car and a diesel car.

Volhout

P.S. the BAT85 is often used as a protection diode for IO pins that cannot exceed 3.3V. Connect it from the pin (anode) to the 3.3V supply (cathode). The 1N4148 will not protect the IO pin sufficiently when used in that application.
PicomiteVGA PETSCII ROBOTS
 
JohnS
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Posted: 02:08pm 22 Aug 2022
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  Volhout said  P.S. the BAT85 is often used as a protection diode for IO pins that cannot exceed 3.3V. Connect it from the pin (anode) to the 3.3V supply (cathode). The 1N4148 will not protect the IO pin sufficiently when used in that application.

If it has a forward voltage drop of about 0.1V doesn't that mean the pin can get to 3.4V?

If not, what have I got wrong?

Otherwise, how is that OK?

John
 
Volhout
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Posted: 02:34pm 22 Aug 2022
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Correct. And the chip specification says that when powered from 3.3V the IO voltage shoul be limitted to 3.6V.
So 3.4 is safe.
PicomiteVGA PETSCII ROBOTS
 
lizby
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Posted: 02:49pm 22 Aug 2022
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Thanks. I knew of the use of the Schottky diode to limit the forward voltage drop, but hadn't considered that the non-Schottky might be specified here because of the greater forward drop. Analog mysteries to a digital person.

I snipped off all three diodes (D2, D3, D4), and now what I get is a slightly tan-ish white, regardless of use of RGB(red), RGB(blue) or RGB(green). Any further suggestions?
PicoMite, Armmite F4, SensorKits, MMBasic Hardware, Games, etc. on fruitoftheshed
 
Volhout
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Posted: 02:51pm 22 Aug 2022
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Type "mode 2"
Try again. The picomite is monochrome in mode 1
PicomiteVGA PETSCII ROBOTS
 
matherp
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Posted: 03:06pm 22 Aug 2022
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  Quote  I snipped off all three diodes (D2, D3, D4), and now what I get is a slightly tan-ish white, regardless of use of RGB(red), RGB(blue) or RGB(green). Any further suggestions?


There are two circuits known to give good colour balance

The original with 220 ohm resistors, two diodes and a trimmer. The second with the resistor values given above by Volhout
         220 ohm -> 270 ohm
         330 ohm -> 390 ohm
         680 ohm -> 820 ohm
and no diodes

Your circuit may give good balance but in its case the 1n4148 diodes are essential
 
lizby
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Posted: 03:44pm 22 Aug 2022
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  Volhout said  Type "mode 2"
Try again. The picomite is monochrome in mode 1

Thank you. That did the trick, though white is on the yellowish side (not that it matters to me--yet).



With this code:

 mode 2
 dim integer i,j,k,c,x,y=1
 dim integer xx=mm.hres-1,yi=mm.vres/8
 for i=0 to 1
   for j=0 to 1:
     for k=0 to 1
       c=rgb(i*255,j*255,k*255)
       box x,y,xx,yi,,c,c
       y=y+yi
     next k
   next j
 next i

PicoMite, Armmite F4, SensorKits, MMBasic Hardware, Games, etc. on fruitoftheshed
 
Volhout
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Posted: 07:38pm 22 Aug 2022
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Hi Lizby,

The white balance should be far better that what your monitor shows.
Maybe you have swapped 2 resistors. Check:

GP21 connects to a 270 ohm resistor
GP20 connects to a 390 ohm resistor
GP19 connects to a 820 ohm resistor
GP18 connects to a 270 ohm resistor

Do not put diodes in

Volhout
PicomiteVGA PETSCII ROBOTS
 
lizby
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Posted: 10:53pm 22 Aug 2022
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No diodes (at D2, D3, D4), but I didn't swap the resistors from what the board having the resistors showed. If I get back to it, I'll swap the resistors for the values you suggest. DMV showed that the resistors are as specified on the board (but correct diodes were supposed, so that may explain the colors I see).
PicoMite, Armmite F4, SensorKits, MMBasic Hardware, Games, etc. on fruitoftheshed
 
JohnS
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Joined: 18/11/2011
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Posts: 3641
Posted: 06:58am 23 Aug 2022
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  Volhout said  Correct. And the chip specification says that when powered from 3.3V the IO voltage shoul be limitted to 3.6V.
So 3.4 is safe.

Ah! Thank you!

John
 
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