doc: Add document for using flashrom with CH341A/B

Document is contributed by Stephan Schneider.

Change-Id: Id6848329d88146fc8a005437a76ec52bec986259
Signed-off-by: Anastasia Klimchuk <aklm@flashrom.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/86019
Reviewed-by: Peter Marheine <pmarheine@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

doc/supported_hw/supported_prog/ch341ab.rst

@@ -0,0 +1,88 @@
+=========
+CH341A/B
+=========
+
+.. meta::
+   :author: Stephan Schneider
+
+The CH341\ **A** is a chip that provides a bridge between USB and serial communication. The
+CH341\ **B** is almost identical, it also has a built-in oscillator. However, it is not
+recommended to use the CH341\ **B** with the built-in oscillator and without a crystal
+as the accuracy of the built-in oscillator is very poor and timing problems can occur
+during data transmission. The serial communication of the CH341A/B can be UART or SPI/I2C,
+here we are only interested in the SPI/I2C mode. The following description applies to both
+chips, it does not matter which one you have. With a crystal, flashrom works with both the
+CH341A and the CH341B without changing the software.
+
+
+There are 2 ready-made boards that are suitable for use with flashrom, one with an IC socket
+(Textool) with a mostly black board and one without an IC socket with a mostly blue board.
+
+Image 1: Black board with IC socket
+
+.. image:: black_board_with_IC_socket.jpg
+
+Image 2: Blue board without IC socket
+
+.. image:: blue_board_without_IC_socket.jpg
+
+Both boards have advantages and disadvantages. The black board with IC socket is more practical
+because the IC socket is already installed. However, it has an error in the circuit diagram that
+must be corrected so as not to jeopardize the EEPROMs. There is a voltage converter on the board to
+convert the 5 volts of the USB to 3.3 volts, the 3.3 volts are then fed to the IC socket, but
+unfortunately the CH341A is still supplied with 5 volts. The CH341A also works with 5 volts, but then
+there is a voltage of just under 5 volts at the outputs of the CH341A that lead to the IC socket,
+which is usually too much for the inputs of the EEPROMs and can cause them to break. The blue board
+without an IC socket is correctly designed, but you also need an IC socket and some connecting wires,
+which makes things complicated and error-prone.
+
+Image 3: Blue board and additional IC socket with wiring
+
+.. image:: blue_board_additional_IC_socket_with_wiring.jpg
+
+The conversion of the black circuit board with IC socket is simple: You need a soldering iron,
+some insulated wire, some insulating tape and a steady hand. First you need to lift pin 28 of the
+CH341A by approx. 1 mm, heat the pin with the soldering iron and insert a thin knife blade between
+the pin and the board. Then stick a small piece of insulating tape between the pin and the circuit
+board to prevent contact. Then you have to connect the bent up pin 28 with 3.3 volts and pin 9 also
+with 3.3 volts, the connections are shown in the next picture.
+
+Image 4: Yellow: insulating tape, pink: new connections
+
+.. image:: yellow_insulating_tape_pink_new_connections.jpg
+
+For programming with flashrom you have to set the SPI/I2C mode of the CH341A, the corresponding jumper
+must be set to the position shown in the pictures (it is the yellow jumper in each case). On the blue
+board without IC socket you have to set the operating voltage with 2 jumpers (here blue), this is
+usually 3.3 volts, there are only a few older EEPROMs that still require 5 volts, please check the data
+sheet of the EEPROM to be on the safe side.
+
+Image 5: Green: 3.3 Volt, orange: 5 Volt
+
+.. image:: green_3_3V_orange_5V.jpg
+
+When using the blue board, you still have to make the correct connections between the EEPROM and the CH341A board:
+
++---------------+--------------+
+| Pin 1 -> CS0  | Pin 8 -> Vcc |
++---------------+--------------+
+| Pin 2 -> MISO | Pin 7 -> Vcc |
++---------------+--------------+
+| Pin 3 -> Vcc  | Pin 6 ➔ SCK  |
++---------------+--------------+
+| Pin 4 -> GND  | Pin 5 ➔ MOSI |
++---------------+--------------+
+
+Pin 1 of the EEPROM is usually marked with a dot or similar, see also the EEPROM data sheet
+
+Image 6: Pinout of an EEPROM
+
+.. image:: pinout_of_EEPROM.jpg
+
+The following commands are some examples how to call flashrom with CH341A::
+
+     flashrom -p ch341a_spi
+     flashrom -p ch341a_spi -r filename.bin
+     flashrom -p ch341a_spi -w filename.bin
+
+That's all, good luck!

doc/supported_hw/supported_prog/index.rst

@@ -16,6 +16,7 @@ Patches to add/update documentation, or migrate docs from `old wiki website <htt
     :maxdepth: 1
 
     buspirate
+    ch341ab
     dummyflasher
     ft2232_spi
     serprog/index