Metro Mini USB reset disable?

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mhopeng
 
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Joined: Wed Jan 31, 2018 2:08 am

Metro Mini USB reset disable?

Post by mhopeng »

Hi, I'm interested in modifying the Metro Mini "reset on USB" functionality. When you connect the board to a computer with a USB cable, the processor resets. I would prefer to not have this happen, so I can connect and disconnect while a program is running. You can trigger a reset after connecting by toggling the virtual DTR line. On the schematic, the DTR output from the USB chip is connected through a decoupling cap, C5, to the Atmega328. So, it seems to me that if I remove C5, then the board will not reset when I make a USB connection. Has anyone else done this? Does it make sense? I will try it and see what happens :) but if you have done this or have a better idea I'd be grateful to hear it!
Detail of Metro Mini schematic showing DTR line
Detail of Metro Mini schematic showing DTR line
DTR_to_RESET.png (24.63 KiB) Viewed 89 times

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mhopeng
 
Posts: 9
Joined: Wed Jan 31, 2018 2:08 am

Re: Metro Mini USB reset disable?

Post by mhopeng »

Followup: this worked exactly as I hoped. If you remove C5, the processor does not receive a RESET signal when you connect a USB device. So, for example, you can connect to a Metro Mini that is running, send/receive commands over the serial port, and disconnect the USB cable, and the program will continue to run (assuming there is an external power supply).
One consequence of this, of course, is that if you want to upload a new program (sketch), you will have to press the manual reset button, and start the upload at the same time. The optiboot docs says there is a 1-second timeout after the reset signal, but I felt like I had to be much closer than 1 second between reset and upload to get it to work. But whatever, it did work after a few tries.
For future reference, I've attached a diagram of the Metro Mini pcb layout with the components labeled.
Attachments
Metro Mini Rev B pcb layout.pdf
PCB layout of the Metro Mini with components labeled.
(40.12 KiB) Downloaded 3 times

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