how-to-use-USBtoSerialCable-to-upload.txt

串口下载问题
need to pay attention when use USB-to-Serial to program the sipeed-longan-nano board
Step1. 
in "platformio.ini" use below setting
[env:sipeed-longan-nano]
platform = gd32v
framework = gd32vf103-sdk
board = sipeed-longan-nano
monitor_speed = 115200    
upload_protocol = serial  
; change microcontroller
board_build.mcu = GD32VF103CBT6

; change MCU frequency
board_build.f_cpu = 108000000L 

Step 2. 
Connect USB-to-Serial cable (Prolific PL2303 TA chip inside. I bought it $10 on Amazon) pins with pins of sipeed-longan-nano as below match
(RX, TX must be across wired. see below)

USB-to-Serial pins---		sipeed-longan-nano pins
VDD---				VDD
GND---				GND
TX---				RX
RX---				TX

Step3. 
In Win 10.In Device Manager-> Ports (COM & LPT)
select the "USB-to-Serial Comm Port (COMx)-->Properties-->Port Settings. 
set it as below
Bits per second:	115200
Data bits: 		8
Parity:			Even
Stop bits:		1
Flow control:		None

other notes:
1. In the original example code, the longan_led_on()/"GPIO_BOP(LED_GPIO_PORT) = LED_PIN;" is actually turn the LED off. 
Because the RED LED's negative is connected to PIN GPIOC13(PC13) and the LED's positive is connected to VDD.
Please see the schematic of sipeed-longan-nano;
2. sometimes need to "push boot, pusg reset, release boot" twice to make the board to into upload/programming mode.