I just got my Nova with a Sara-R404 working using the uart :-). But I’d love to kill someone at U-Blox! It took umpteen readings of the manual for me to finally realize what these sentences mean: “The module data output line (RXD) […]” and “The module data input line (TXD) […]” Yes, you read that correctly, the module transmits on RXD and receives on TXD. Crazy! I just couldn’t get it to respond for hours, no wonder.
What I did was to cut the 3.8v output of the regulator and the “3.8v discharge” circuit. Using the R404 Nova schematic and bottom layer mask as reference, I cut the (fairly wide) trace between the feedback resistor R13 and the first cap C36. And I desoldered R9 and resoldered it turned 90 degrees so it doesn’t get lost but doesn’t do anything. This makes it relatively easy to put things back in place. I’m feeding power right onto one of the big caps.
In an earlier thread of mine about all this (Using a Nova without USB) the suggestion was made to use a step-up regulator, but that won’t work because as soon as 5v is applied the R4 modem will disable the uart pins and only talk USB. For test purposes I’m powering the thing using a 3.3v regulator, but once I have it working I’m going to hook it up straight to a LiPo since that’s pretty clearly what it’s designed for (see sect 2.2.1.6 of the system integration manual) and where the 3.8v come from.