There seems to be a badgap voltage reference built-in in the STM32F1 microcontrollers (on ADCx_IN17).
Coming from AVRs it seems to me very strange that it's not possible to use this reference as the ADC VREF, at least I think it's impossible. Am I right ? Why is it there than ?
It is possible to use the internal reference, albeit indirectly. You measure it using the Vdd, and then measure the external voltage you are interested in, using the same Vdd as ADC reference. You can then calculate what the supply must be, knowing Vref and the counts it gave. Then you know more accurately what the ADC is using, and can calculate the external channel voltage you want. The assumption here is that Vdd does not move much between the two readings. If this is a concern, measure Vref before and after measuring the channel with your data on, and use an average. For battery operated systems, this gives you the battery voltage as well, which is probably of interest. This is not as accurate as an external precision reference, but is cheaper, uses less power and does not tie up extra pins...
I also find it annoying that I cannot use the internal reference for VREF+. It is extra annoying on smaller pin packages that DO NOT have an external VREF+ input pin. You have to use the MCU's VDD input voltage as your reference. VDD is generally not a very precision voltage so do not expect the best accuracy without a CAL routine. I have also done battery operated systems where VDD comes straight from the battery so that makes using it a real pain.
Microchip PIC's do allow you to use the internal reference for the ADC and some even allow you to divide it by 2 or 4, so it certainly can be done. Hopefully ST will add this feature soon.
from figure 22, page 216
Shows that it isn't connected.
My educated guess here is that while they may have a band-gap they don't have a precision regulated power supply. They are very different things, with the power supply requiring a precision reference (like a band-gap). You have to supply a low impedance, low noise, high PSRR block that burns power. Also band-gaps are lower voltage devices, this power supply is close to the rails. It's safer to put the pins on the outside. And it would probably be lower noise on chip.
What you do, is you provide a stable clean external power on those pins and you use the internal Vref to calibrate your ADC for manufacturing tolerances.
