Hello everyone. I'm a new user of a DSO138 and as much as I am pleased with it, I found the noisy signal traces too much to bear. This led to lots of research around the Internet and a discovery that there were two useful fixes for the noise problem. Reading around this forum led to a reluctance to update the firmware via UART - far too many people reporting problems with problematic USB-UART devices. This made me think about firmware updates via SWD as an alternative.
So, within a week of getting the DSO138 I have fixed the hardware to deal with the incorrect VDDA grounding problem and have changed R11 with an associated update of firmware via SWD (rather than UART) to get the ADC more sensibly.
Other than the silly splash screen about Sainsmart, and the C6 trimmer barely having any effect when calibrating, I am very pleased with the improvement.
But despite the foregoing, my main purpose for my venturing onto the forum is to share my experience of successful firmware updates over SWD rather than UART because I see the UART update process is rather messy and many people have problems with it.
Despite the circuit diagram saying "Use SWD debug mode only", the DSO138 really CAN take a firmware update via SWD and I would argue it is simpler and quicker because no soldering is involved. I am surprised nobody else has mentioned the SWD method.
All you require is a ST-Link device, four jumper wires and the free STM32 ST-LINK Utility.
Better still, if you already happen to have a Nucleo STM32 development board then the upper part of it can be reconfigured to become a standalone ST-Link device. As hints, all you need to do is remove the CN2 links at the top of the board and make use of the top four CN4 header pins as your SWD connections. The exact information you need is all out there in the various user guide documents relating to Nucleo.
So, my recommendation is to stop trying to get those troublesome USB-UART interfaces working and take advantage of the SWD method with an ST-Link device instead. It's simpler, it's easier and everything you need is easily available on eBay.