I have a project running on 1.9V, 8 mhz (internal). It accepts the bootloader fine and I can use Nick Gammon's sketch to read all the info from it. However, when I try to upload a sketch via Arduino IDE or Platform.io (over UART) , it gives me a stk500_getsynch() error.
I have tried a FTDI programmer and also a Arduino Uno without a chip. Neither of them seem to work. Because the project is running on 1.9V, I have a TXS0108E inbetween the FTDI-programmer and the Atmega328P-AU. And also a 100nF Ceramic Capacitor between the Reset pin and the DTR pin of the Programmer.
I have tried multiple frequencies: 1 Mhz, 2 Mhz and 8 Mhz. They all give the same result. I have disabled the BOD, since the voltage level is so low. These are the rest of the settings for the Minicore bootloader, the baud rate is set at 9600.
Since my project will run on a battery, has to last quite a while and has to be as small as possible, I want to use the internal oscillator. I found that it is quite innacurate but when testing my idea, I used a Atmega328P-PU with the same settings, and this worked like a charm.
I have tried everything to get this to work and I have run out of ideas. I have searched what feels like the entire internet but I'm also quite new to working with these "custom" chips. Any help is greatly appreciated!