Schematic wanted for Mega2560 (CH340)

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Richard Langner

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May 17, 2021, 12:44:07 PM5/17/21
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Does anyone have a schematic for the Mega2560 which uses a CH340 USB interface. I can find plenty of the later versions but not the CH340 version.

Graham Driver

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May 17, 2021, 1:09:03 PM5/17/21
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Is it likely to be much different? Other than the serial bit? 

From: sheffield-har...@googlegroups.com <sheffield-har...@googlegroups.com> on behalf of Richard Langner <richard...@gmail.com>
Sent: 17 May 2021 16:44
To: Sheffield Hardware Hackers <sheffield-har...@googlegroups.com>
Subject: [SHH:17020] Schematic wanted for Mega2560 (CH340)
 
Does anyone have a schematic for the Mega2560 which uses a CH340 USB interface. I can find plenty of the later versions but not the CH340 version.

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Richard Langner

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May 17, 2021, 1:48:12 PM5/17/21
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The friend who gave me the first faulty Mega, has a second with the same fault.
He followed my video and claims that it's not the same fault.
So I said I'd look at it for him, but I can't find my original schematic.

James Muirhead

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May 17, 2021, 6:18:43 PM5/17/21
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Richard Langner

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May 18, 2021, 5:44:44 AM5/18/21
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Thank you. Although it's not the exact model (component numbers differ) it's plenty good enough for fault-finding.
Richard

On 17/05/2021 23:18, James Muirhead wrote:

Graham Driver

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May 18, 2021, 11:38:16 AM5/18/21
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I like the way input power is selected.

What is that thing next to R13?

Sent: 18 May 2021 09:44
To: James Muirhead <balls...@gmail.com>; sheffield-har...@googlegroups.com <sheffield-har...@googlegroups.com>
Subject: Re: [SHH:17029] Schematic wanted for Mega2560 (CH340)
 

Richard Langner

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May 18, 2021, 12:09:59 PM5/18/21
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I don't know the symbol in parallel with R13, but it may represent a debounce capacitor.
Richard

Graham Driver

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May 18, 2021, 12:14:34 PM5/18/21
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A C seems reasonable.

What is the problem with the board?

Now we all have the circuit perhaps we could all suggest solutions...
Sent: 18 May 2021 16:09
To: sheffield-har...@googlegroups.com <sheffield-har...@googlegroups.com>
Subject: Re: [SHH:17032] Schematic wanted for Mega2560 (CH340)
 

nop head

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May 18, 2021, 1:24:05 PM5/18/21
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The odd symbol next to R13 is also odd because it has no component designator. I can see a device on the board that looks like the symbol and it also has no component designator and it is possibly connected between the 5V rail and the switch.

image.png

Not sure what it is, possibly a diode or a tantalum capacitor.

The reset button is also odd because it has 5 pins. They normally have 4 pins in two pairs of two connected together.


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Adrian Godwin

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May 18, 2021, 2:29:22 PM5/18/21
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It's common to have a diode in this position. It stops the charge on the associated reset capacitor from taking the reset pin above Vcc when Vcc falls. This might avoid current flowing into the reset pin but more importantly, it discharges the capacitor.

I don't know if it's necessary given the arduino reset circuit (the capacitor goes to DTR), and I don't know why they'd show what seems like a picture of a diode rather than a symbol.


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nop head

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May 18, 2021, 3:05:15 PM5/18/21
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Perhaps it was an afterthought added by somebody that didn't really know what they were doing. That would explain a picture instead of a proper schematic and a missing designator. Also if it is a diode it is a power diode when you only need a signal diode to discharge a 100n cap.



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Richard Langner

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May 18, 2021, 5:18:05 PM5/18/21
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Until Nop mentioned it, I hadn't thought to actually find a board like this and take a close look.
I tried several test meters, and one of them showed it was a diode with 0.6v forward voltage.
So I looked further. RTFM is a good motto, and it's all explained here on page 7








On 18/05/2021 20:04, nop head wrote:
Perhaps it was an afterthought added by somebody that didn't really know what they were doing. That would explain a picture instead of a proper schematic and a missing designator. Also if it is a diode it is a power diode when you only need a signal diode to discharge a 100n cap.
On Tue, 18 May 2021 at 19:29, Adrian Godwin <artg...@gmail.com> wrote:
It's common to have a diode in this position. It stops the charge on the associated reset capacitor from taking the reset pin above Vcc when Vcc falls. This might avoid current flowing into the reset pin but more importantly, it discharges the capacitor.

I don't know if it's necessary given the arduino reset circuit (the capacitor goes to DTR), and I don't know why they'd show what seems like a picture of a diode rather than a symbol.
On Tue, May 18, 2021 at 6:24 PM nop head <nop....@gmail.com> wrote:
The odd symbol next to R13 is also odd because it has no component designator. I can see a device on the board that looks like the symbol and it also has no component designator and it is possibly connected between the 5V rail and the switch.

image.png

Not sure what it is, possibly a diode or a tantalum capacitor.

The reset button is also odd because it has 5 pins. They normally have 4 pins in two pairs of two connected together.

Richard Langner

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May 21, 2021, 12:34:20 PM5/21/21
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I've not seen the board as yet, but I am told it gives the same timeout message as in my video.
I must have had a schematic when I fixed the first one by replacing C4 which is on the reset line (in OJ's circuit it's C8) but I can't find it.
It may be a while before the Mega arrives here, then we can see the exact problem.
R.
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