3.4
USB bridge
The communication between the
and the PC is managed by the latter as a standard serial
peripheral; the IC U3 converts the USB connection into a virtual COM port (refer to the electrical schematic).
By default, the virtual COM port operates at 921600 bps.
A yellow LED near the mini-B USB connector turns on when the CP2102 has been recognized (enumerated) by
the host operating system.
The VCP RX and TX signals are then isolated thanks to the opto-couplers U1 and U2 and connected to the
STM32F3 (U9) microcontroller USART1.
Important:
The USB port and the remaining part of the board are isolated from the mains.
The microcontroller performs:
•
Conversion between the host UART and I²C protocols
–
The I²C speed can rise up to 1 MHz (maximum speed allowed by the
–
The STM32F3 allows bidirectional communication between the PC and the target device through the
UART to I²C conversion.
•
Conversion between the host UART and the STNRG011 UART
.
This is mainly baud rate matching: STNRG011 operates at 19200 bps, whereas the host UART operates at
921600 bps.
Note:
The microcontroller also manages the muxing of the UART and I²C protocols on the same interface.
3.5
VCC monitoring
The MCU also monitors the
STNRG011 VCC is sampled periodically by the MCU via a simple resistive bridge divider plus a low-pass filter
using R20, R21 and C19. The divider ratio is 10/78=1/7.8.
The divided voltage is then sent to STM32F3 PA0 pin on a regular 12-bit ADC.
For instance, this allows preventing the use of the on-board VCC when the STNRG011 is already operating.
Note:
This feature accuracy is ±100 mV.
UM2342
USB bridge
UM2342
-
Rev 1
page 7/57
