I'm using a STM32F103C8T which shall be in circuit programmed by an ESP8266.
For this reason i connected PA9/PA10 to the USART Port of the ESP and the ESP controls Reset and BOOT0 to start the STM32 in Boot or Run mode.
When sending the Sync-Character the STM was not synchronizing and did not respond. After hours of searching i found out that i had to remove external 4K7 pullups attached to PB8 and PB9. Only after that the STM was willing to talk but I did not find any explanation.
45 - PB8 I/O FT PB8 TIM4_CH3(9) I2C1_SCL / CANRX
46 - PB9 I/O FT PB9 TIM4_CH4(9) I2C1_SDA/ CANTX
According to AN2606 the Medium-density STM32F103 only supports USART (V2.2).
CAN is supported for the Connectivity line devices only so I have no clue why high levels on PB8/PB9 are blocking the USART-Bootloader.
What did i forget to read or did something else block my bootloader and I blamed PB8/9 by mistake?
Thanks for your support !