STM32F103 / STM32F407 boards should use on-chip USB support

[ag88]

hi MKS,
Your boards are quite good, but STM32F103 / STM32F407 boards should use on-chip USB support instead of using a separate USB-UART

There is a problem, as seen in this PR for Marlin
MarlinFirmware/Marlin#22529
Because STM32F103 and STM32F407 UART only has a single byte buffer. When the microcontroller is busy it can skip bytes from the host. So it becomes necessary to increase interrupt priority for the UART, this comes at a cost to interrupts to other peripherals such as HardwareTimer which drives the stepper motors (there is a risk of skipping steps)

Both STM32F103 and STM32F407 has on-chip USB support, please revise your designs to use the on-chip USB rather than using a separate USB-UART bridge.

e.g. your MKS-Nano-V3 is correctly designed
https://github.com/makerbase-mks/MKS-Robin-Nano-V3.X
https://github.com/makerbase-mks/MKS-Robin-Nano-V3.X/blob/main/hardware/MKS%20Robin%20Nano%20V3.0_004/MKS%20Robin%20Nano%20V3.0_004%20SCH.pdf
pins PA11 USB D- and PA12 USB D+ goes direct to USB.
But please update your designs so that the Nano-V1 and Robin E3 etc
https://github.com/makerbase-mks/MKS-Robin-E3-E3D
https://github.com/makerbase-mks/MKS-Robin-E3-E3D/blob/master/hardware/MKS%20Robin%20E3%20V1.1_004/MKS%20Robin%20E3%20V1.1_004%20SCH.pdf
use pins PA11 USB D- and PA12 USB D+ goes direct to USB.

USB is a reliable protocol, if the host (PC) did not get a response, it would poll again and there is a ACK response. So it won't lose bytes. But UART is not a reliable protocol, the receiver needs to keep scanning RX or it would lose bytes.

[mks-viva]

STM32F103 / STM32F407 boards BOOT0 pin should have jumper to VDD 3v3 #63