jumper on pins

1-2 SPI flash programmer mode
2-3 TTL serial mode

https://github.com/setarcos/ch341prog

dpavlin@nuc:/nuc/ch341a$ git clone https://github.com/setarcos/ch341prog
Cloning into 'ch341prog'...
remote: Counting objects: 104, done.
remote: Total 104 (delta 0), reused 0 (delta 0), pack-reused 104
Receiving objects: 100% (104/104), 34.79 KiB | 0 bytes/s, done.
Resolving deltas: 100% (61/61), done.
Checking connectivity... done.
dpavlin@nuc:/nuc/ch341a$ cd ch341prog/
dpavlin@nuc:/nuc/ch341a/ch341prog$ make
gcc -std=gnu99 -Wall ch341a.c main.c -o ch341prog  -lusb-1.0
dpavlin@nuc:/nuc/ch341a/ch341prog$ ./ch341prog 

Usage:
 -h, --help             display this message
 -i, --info             read the chip ID info
 -e, --erase            erase the entire chip
 -l, --length <bytes>   manually set length
 -w, --write <filename> write chip with data from filename
 -r, --read <filename>  read chip and save data to filename
 -t, --turbo            increase the i2c bus speed (-tt to use much faster speed)
 -d, --double           double the spi bus speed

I2C

https://sourceforge.net/projects/ch341eepromtool/

dpavlin@nuc:/nuc/ch341a/ch341eepromtool_0.5$ gcc -o ch341eeprom ch341eeprom.c ch341funcs.c -lusb-1.0
dpavlin@nuc:/nuc/ch341a/ch341eepromtool_0.5$ 
dpavlin@nuc:/nuc/ch341a/ch341eepromtool_0.5$ ./ch341eeprom 
ch341eeprom - an i2c EEPROM programming tool for the WCH CH341a IC
Version 0.5 copyright (c) 2011  asbokid <ballymunboy@gmail.com>

This program comes with asbolutely no warranty; This is free software,
and you are welcome to redistribute it under certain conditions:
GNU GPL v3 License: http://www.gnu.org/licenses/gpl.html

Usage:
 -h, --help             display this text
 -v, --verbose          verbose output
 -d, --debug            debug output
 -s, --size             size of EEPROM {24c32|24c64}
 -e, --erase            erase EEPROM (fill with 0xff)
 -w, --write <filename> write EEPROM with image from filename
 -r, --read  <filename> read EEPROM and save image to filename

Example:  ch341eeprom -v -s 24c64 -w bootrom.bin

SPI

git clone https://github.com/urjaman/flashrom/
git checkout -b origin/ch341a origin/ch341a

dpavlin@nuc:/nuc/flashrom$ sudo apt-get install pciutils-dev

linux kernel module

dpavlin@nuc:/nuc$ git clone https://github.com/gschorcht/spi-ch341-usb.git
Cloning into 'spi-ch341-usb'...
remote: Counting objects: 63, done.
remote: Total 63 (delta 0), reused 0 (delta 0), pack-reused 63
Unpacking objects: 100% (63/63), done.
dpavlin@nuc:/nuc$ cd spi-ch341-usb
dpavlin@nuc:/nuc/spi-ch341-usb$ make
make -C /usr/src/linux-headers-4.14.0-3-amd64/ M=/nuc/spi-ch341-usb  modules
make[1]: Entering directory '/usr/src/linux-headers-4.14.0-3-amd64'
  CC [M]  /nuc/spi-ch341-usb/spi-ch341-usb.o
  Building modules, stage 2.
  MODPOST 1 modules
  CC      /nuc/spi-ch341-usb/spi-ch341-usb.mod.o
  LD [M]  /nuc/spi-ch341-usb/spi-ch341-usb.ko
make[1]: Leaving directory '/usr/src/linux-headers-4.14.0-3-amd64'
dpavlin@nuc:/nuc/spi-ch341-usb$ 
dpavlin@nuc:/nuc/spi-ch341-usb$ sudo make install
[sudo] password for dpavlin: 

schematics

• http://onetransistor.blogspot.hr/2017/08/ch341a-mini-programmer-schematic.html