Hi,

I have an Arduino MEGA 2560 that is partially not working.

The main MCU functionality is working fine, but the USB connection that allows connection to the PC and the uploading of sketches is not functioning.

I'm trying to learn repair electronics, so would like to understand why it is not working. I've included as much info below as I can.

When I plug the device into my laptop (or any computer) I get this error:



The Arduino is a chinese clone (not genuine), and so uses a chinese USB<>Serial IC called a CH340g (similar to FTDi chip). The datasheet is at https://www.insidegadgets.com/wp-content/uploads/2016/12/ch340g-datasheet.pdf

I probed the CH340G IC pins using a scope (based on the data-sheet) and noticed that the pins for the external crystal were not generating the expected oscillation, just a DC signal of around 5V.



This is what I have attempted so far:

1. I presumed that the crystal must have got damaged, and so replaced it with another (identical - 12MHz), but this did not fix the problem. Still no oscillation.

2. I used the scope to view the oscillator circuit when power is first applied to the board. It seems the oscillator starts, but then the sine wave voltage jumps before it trails off and stops oscillating.



3. From reading online, I understand that the crystal in an oscillator circuit works in conjunction with a pair of capacitors (load capacitors). I have found these SMD capacitors on the board and measured them in circuit using an LCR meter. The capacitors are generic SMD (ceramic?) capacitors, so I have no reference to judge the readings against to determine their health.

Here are the readings:

Capacitor 1 Capacity (Tested @ 100Hz): 4.99uF
Capacitor 2 Capacity (Tested @ 100Hz): 4.95uF

Capacitor 1 Capacity (Tested @ 100KHz): 8.342nF
Capacitor 2 Capacity (Tested @ 100KHz): 6.579nF

Capacitor 1 ESR (Tested @ 100KHz): 1.33kΩ
Capacitor 2 ESR (Tested @ 100KHz): 1.48kΩ

I understand that the pair of capacitors in an oscillator circuit should be matched. The LCR meter indicates a slight difference between the two capacitors, could this indicate an issue with one of the capacitors? Are they a consequence of testing in-circuit? Or are the differences too small to be of importance?

What is more likely to be causing the problem given the above info? Bad capacitor? Bad USB IC chip?

Grateful for any responses. I know it is far easier to replace the arduino board, but I am curious how to fix a problem like this, and I'm trying to improve my troubleshooting skills.

Thanks!

 

Have you loaded the CH340G driver?
http://kig.re/2014/12/31/how-to-use-arduino-nano-mini-pro-with-CH340G-on-mac-osx-yosemite.html

 

Have you loaded the CH340G driver?
http://kig.re/2014/12/31/how-to-use-arduino-nano-mini-pro-with-CH340G-on-mac-osx-yosemite.html

Thanks for the response. Yes, I've previously installed the driver.

However, someone has posted a comment on the driver page you linked to:

Hi, when you get Device Descriptor Request failed you need to restart your computer. Then unplug everything and reconnect. If it still doesn’t work, there may be a power issue – try to use shorter USB cable or connect to other USB port.

The Device Descriptor Request error (43) is what I get in windows. I will try powering the board from external power to see if issue is related to PC not able to supply enough power through USB.

WIll post back later with result.