[STM32F103C8T6] SPI1 does not send any data

[Toy-Jie]

STM32F103C8T6

RCC->APB2ENR |= RCC_APB2ENR_AFIOEN;
AFIO->MAPR |= AFIO_MAPR_SPI1_REMAP;

SPI.begin();
SPI.transfer(0xAA);

SCK -> PB3
MISO -> PB4
MOSI -> PB5

Using an oscilloscope to check, no data is transmitted.