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Forum Index : Microcontroller and PC projects : PicoMite manual question...

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Grogster

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Posted: 05:44am 22 Jan 2022
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Hi folks.

PicoMite PDF manual as in the RC1 download, manual version 5.07.03, page 149 says:

  Quote  GPIO pins GP21, GP22, and GP26 are reserved to drive the VGA display in monochrome mode. In addition pins GP18, GP19 and GP21 are also not available in MMbasic when in colour mode.


The last bullet point.

GP22 and GP26 - typo?

VGA port uses GP18, GP19, GP20 and GP21 ignoring the Hsync and Vsync lines for now, but it does not use GP22 or GP26 that I can see from the schematics....


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Chopperp

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Posted: 06:00am 22 Jan 2022
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Seems like an error. Mixtel90 picked it up.

See here
ChopperP
 
Grogster

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Posted: 06:15am 22 Jan 2022
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Excellent, thanks.
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Grogster

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Posted: 06:25am 22 Jan 2022
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One more thing:

End of page 4, it states the power requirements as 2.0 to 5.5v @ 10 to 42mA.
Is that the same for the VGA version as the non-VGA version?

I only ask, as the VGA version has the 2nd core and one of the PIO's cranking away there at reasonably high speed, so wondering if the VGA version has a higher current consumption then the non-VGA version.
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robert.rozee
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Posted: 07:19am 22 Jan 2022
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  Grogster said  power requirements as 2.0 to 5.5v @ 10 to 42mA.
Is that the same for the VGA version as the non-VGA version?


another point is that as the supplied voltage drops from 5v down to 2v, so the current consumption into the switchmode converter goes up from 10-42mA to 25-105mA.

the VGA pins, if driving into 75 ohms will each be sourcing 10mA or thereabouts. 10mA x (R+G+G+B+Hsync+Vsync) = up to 60mA @ the 3v3 I/O voltage (assuming continuous drive of each line).

when tethered to a VGA monitor this is not so much of an issue, as the power consumption of said monitor will dwarf the pico's consumption. however, the power dissipation may affect the drive available from the RP2040 chip into other I/O pins.

it may be worthwhile offloading this drive into (as someone else suggested) 6x emitter followers powered from either 3v3 or 5v directly.


cheers,
rob   :-)
Edited 2022-01-22 17:22 by robert.rozee
 
Grogster

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Posted: 07:29am 22 Jan 2022
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Hi Rob.

Assume a constant 5v supply from a 7805 regulator.

Therefore, I will assume a 60mA figure with all VGA + Sync lines being driven, worst-case scenario. (plus some auxiliary stuff also - say 100mA total)

My MAIN concern with the current consumption thing, is that if powered from a 13.8v supply and backup battery, the 7805 regulator will dissipate a certain amount of heat which is hotter the bigger the load vs in-out voltage differential.

You all will know this - it's standard dissipation calculations for linear regulators.

So, assuming the above, the 7805 would dissipate:

(13.8v - 5v) x 0.1A = 0.88 Watts, or 880mW.

I think this is well within the capabilities of the 7805 with even a small heatsink.
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Mixtel90

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Posted: 07:48am 22 Jan 2022
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If you aren't happy with the power dissipated in the 7805 you can fit one of the little switching regs that is pin compatible. Virtually no dissipation - and probably still not as noisy as the onboard SMPS, especially if you add a little more filtering. Alternatively, a series resistor with the 7805 can dissipate a bit of the power elsewhere. You could always shift a switching reg into an external box if you are running from a 13.8V supply, of course. No problems with screening or filtering then. You can have a clean 5V supply.

I tried measuring the supply current to my VGA mini just now, but the little USB power meter isn't reading. I don't think it does anything until you get a bit over 10mA. Actually, I'm not sure it's working because adding the PS/2 keyboard didn't do anything either. lol
Edited 2022-01-22 17:51 by Mixtel90
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robert.rozee
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Posted: 08:10am 22 Jan 2022
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you could also consider using a 3-terminal 3v3 regulator (instead of 5v), and backfeeding this directly into the 3v3out (pin 36) while 3v3en (pin 37) is tied to ground. this will eliminate any noise created by the onboard switchmode, improving the ADC performance.


cheers,
rob   :-)
 
phil99

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Posted: 11:59am 22 Jan 2022
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Re VGA current requirement.
The Sync. pins are high Z so none there and the two greens add up to 9ma, same as R & B. 27ma total to the monitor for a full white screen. If memory serves, the total continuous current for all outputs needs to be less than 50ma. A limitation of the Pico.
 
robert.rozee
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Posted: 12:31pm 22 Jan 2022
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  phil99 said  Re VGA current requirement.
The Sync. pins are high Z so none there and the two greens add up to 9ma, same as R & B. 27ma total to the monitor for a full white screen. If memory serves, the total continuous current for all outputs needs to be less than 50ma. A limitation of the Pico.


fair enough - although even at 27mA, that is still half of your I/O power budget gone. i'd be interested to hear what the actual measured consumption is, ie first driving a real monitor, then with the pico's pins (GP16-GP21) left open-circuit.


cheers,
rob   :-)
 
Grogster

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Posted: 03:46am 23 Jan 2022
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Hi folks.

I need a 5v system voltage to feed to the keyboard and a few other auxiliary things.
Based on the post by phil99 above, 27mA or so for the PicoMite VGA driving white, plus a few other things such as a MAX3232 for an RS232 port, and an RTC module etc, I expect the current to be in the area of 50mA or so total then.  But if I assume 100mA I think I'm covered.

I do have some of those switching regulator modules that are a drop-in replacement for the linear's, so I could always use one of those as mentioned.  Good idea.

The only reason I did not right from the off, is that I like linear regulators, as they are much cleaner on their output - but they are relatively inefficient and dissipate lots of heat.  
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phil99

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Posted: 04:22am 23 Jan 2022
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That 27ma can be saved from 3.3V supply and Pico output budget by using 3 NPN emitter followers with collectors to 5V for the VGA. The green resistor network will need recalculating. Emitter direct to VGA-G pin and a resistor base to emitter as a load for the network . The emitter resistors for R & B = 220R.
Re post when I sort out the values.


Edit
I think this will be adequate (untested). The values can be adjusted for improved balance by trial and error.


Edited 2022-01-23 15:25 by phil99
 
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