SPI on STM32 won't work without pullup resistors and even then performs poorly
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I've been trying to get the SPI1 on the STM32F103C8 (Blue Pill board) working correctly for some time now. As I'm just starting to learn ARM, I am simply trying to shift data to a 74HC595 shift register and latch it to light up a byte of LEDs. I am not reading back any data, so I only have MOSI, SCK and SS lines. At first I wasn't getting anything out, but reading some online examples I could fix these first problems to get the communication going (I needed to correctly set GPIOA pins and set software SS). The main problem right now is that if I don't include pull-up resistors on all lines (MOSI, SCK, and SS) the microcontroller doesn&#