STM32F105 Shared Pin PA9 (USART1-TX and VBUS) conflict

[ralf]

Posted on January 05, 2014 at 19:36

Hello,

the STM32F105 Device share USART1-TX and VBUS for USB on Port PA9. I need the USART1 for communication and for the internal Bootloader (only USART1 on my used 64pin Devices possible), if the firmware corrupts. I need the USB-Device possibility for connecting the Device to a HOST-PC sometimes, but its not used for the internal Bootloader. The other STM Devices with only USB-Device function don�t have a VBUS port und use an external resistor for USB recognition. My question is: Can I switch off the VBUS - Port on PA9, still use the STM32F105 as a USB-Device (with external USB Resistor), and use the PA9 for the USART1-TX? Thank you Rheinhold #stm32f105-pa9-usart1-vbus

[chen]

Posted on January 06, 2014 at 10:43

Hi

''Can I switch off the VBUS - Port on PA9, still use the STM32F105 as a USB-Device (with external USB Resistor), and use the PA9 for the USART1-TX?'' Yes you can BUT the VBUS will but toggling while it is being used as USART Tx and USART Tx will be held high/low while being used as USB. You could also electrically cross couple the USB to UART. (I presume you will be using something like a MAX232 for RS232 level shifting) Is this OK behaviour for your application? To overcome this, you could use a IO pin and a buffer (try a 4 port ananlogue buffer) device and route one function through the buffer and disable one of the functions when you do not want it.

[ralf]

Posted on January 10, 2014 at 12:11

Hello,

thank you for your answer. I have designed a switch between USART and VBUS and a Port Pin to switch USART to the alternative Pins after the Firmware is running and switch VBus to the VBUS pin. I hope, it will works. Thank you Reinhold