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README: write documentation

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This firmware template is designed for development boards based around [STM32 F4 series micro-controller](https://www.st.com/en/microcontrollers-microprocessors/stm32f4-series.html).
firmware is for the SWJ finder.
project
=======
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summary
-------
*describe project purpose*
this tools allow to find JTAG and SWD (SWJ) interface on up to 16 channels.
usage
-----
connect the pins or test points of the target device to the SWJ finder channels pins on the connector.
select the first and last channel to probe using the `start <CH>` ans `stop <CH>` commands.
this prevents sending data on the unselected channels.
this will also make the scan faster, else it needs up to 5 seconds.
also connect the target voltage pin to the SWJ finder in order to use the right signal voltage level.
alternatively, the SWJ finder can supply 5V or 3.3V to the target voltage pin using the `voltage 5` or `voltage 3` command.
to revert to using the externally provided target voltage, use the `voltage 0` command.
to measure the target voltage, use the `voltage` command.
to scan for JTAG interfaces, use the `jtag` command.
to scan for SWD interfaces, use the `swd` command.
the scan results and found SWJ pins will be displayed as they are found.
it will also try to identify the target debug core.
the scanning will take up to 5 seconds.
WARNING: data will be sent on the selected channels (see limitations).
you can also reset the target board if you connected to target reset pin to the SWJ finder.
you can select of to drive the reset pin (OD for open-drain, PP for push-pull) and active level (H for high, L for low) using the `reset [ODL|ODH|PPL|PPH]` command.
to assert or release the reset, us the `reset 1` or `reset 0` commands.
alternatively, pressing/releasing the button on the SWJ finder asserts/releases the reset signal
use the `help` command to list all commands.
this will also list the shortcuts for the commands.
technology
----------
*described electronic details*
to scan for JTAG interfaces, it will use all 2-pin combinations as TCK and TMS signals to:
- switch the target to JTAG mode (in case it is in SWD mode)
- put in reset state (TEST-LOGIC-RESET)
- go into SHIFT-DR state to have the target output the IDCODE
any activity on one the other pins could be the IDCODE output on TDO.
this allows to identify the TCk, TMS, and TDO pins.
it will than send a patterns on each of the other pins.
if this pattern is found on the TDO will, the TDI pin is identified.
to scan for SWD interfaces, it will use all 2-pin combinations as SWCLK and SWDIO pins to:
- switch the target to SWD mode (in case it is in JTAG mode)
- put in reset state (using line reset)
- request the DPIDR (e.g. IDCODE)
if the target replies, the SWCLK and SWDIO have been successfully identified.
limitation
----------
the target voltage should not be higher than 5.5V (board I/O-pins limitation).
but I am not sure a SWJ target with signals higher than 3.3V even exist.
it can operate down to 1.5V (BSS138-based level shifter limitation).
all channel pins are pulled to by a 10 kOhm to the target voltage.
this is required to operate the level shifter.
this also means all channel pins are inter-connected by a 20 kOhm resistor when no target voltage is provided.
all channel pins have a 150 Ohm inline resistor protection.
this means the SWJ finder will sink up to 22 mA when transmitting a low level signal (high level is provided by the pull up resistor).
the reset pin has no inline protection resistor and can sink up to 25 mA.
in open drain mode, an external pull-up resistor is required, most often provide by the target device.
in push-pull mode it can only source 3.3V up to 25 mA.
when the IDCODE seems corrupted and the decoded manufacturer looks wrong, it might be due to too low impedance on the clock (SWCLK/TCK) line.
the clock signal needs a very sharp rising edge, but we are limited by the voltage shifter.
try shorter cables, or remove any other device (e.g. logic analyzer) from the test points.
board
=====
The underlying template also supports following board:
- [WeAct MiniF4](https://github.com/WeActTC/MiniF4-STM32F4x1), based on a STM32F401CCU6
**Which board is used is defined in the Makefile**.
This is required to map the user LED and button provided on the board
the underlying hardware uses a [WeAct MiniF4](https://github.com/WeActTC/MiniF4-STM32F4x1) board, based on a STM32F401CCU6.
the bi-directional level shifter are BSS138 n-channel MOSFET based.
switching the target voltage are done using BSS84 p-channel MOSFET.
connections
===========
Connect the peripherals the following way (STM32F4xx signal; STM32F4xx pin; peripheral pin; peripheral signal; comment):
channel pins:
- *list board to peripheral pin connections*
- PB12, high voltage side of level shifter, low voltage side goes to CH0
- PB13, high voltage side of level shifter, low voltage side goes to CH1
- PB14, high voltage side of level shifter, low voltage side goes to CH2
- PB15, high voltage side of level shifter, low voltage side goes to CH3
- PA8, high voltage side of level shifter, low voltage side goes to CH4
- PA9, high voltage side of level shifter, low voltage side goes to CH5
- PA10, high voltage side of level shifter, low voltage side goes to CH6
- PA15, high voltage side of level shifter, low voltage side goes to CH7
- PB3, high voltage side of level shifter, low voltage side goes to CH8
- PB4, high voltage side of level shifter, low voltage side goes to CH9
- PB5, high voltage side of level shifter, low voltage side goes to CH10
- PB6, high voltage side of level shifter, low voltage side goes to CH11
- PB7, high voltage side of level shifter, low voltage side goes to CH12
- PB8, high voltage side of level shifter, low voltage side goes to CH13
- PB9, high voltage side of level shifter, low voltage side goes to CH14
- PB10, high voltage side of level shifter, low voltage side goes to CH15
All pins are configured using `define`s in the corresponding source code.
BSS138 n-channel MOSFET based bi-directional level shifter:
- high voltage supply is 5V
- low voltage supply is connected to drain of BSS84 p-channel MOSFET
- source of BSS84 p-channel MOSFET is target voltage pin
- gate of BSS84 p-channel MOSFET is connected to PA5, pulled up to target voltage using 100 kOhm resistor
BSS84 p-channel MOSFET to supply target voltage with 5V:
- source is connected 5V
- drain is connected to target voltage
- gate is connected to PA7, pulled up to target voltage using 100 kOhm resistor
BSS84 p-channel MOSFET to supply target voltage with 3.3V:
- source is connected 3.3V
- drain is connected to target voltage
- gate is connected to PB0, pulled up to target voltage using 100 kOhm resistor
target voltage measure:
- PA1 is connected to target voltage through 22 kOhm inline resistor
- PA1 is connected to ground through 22 kOhm inline resistor
- this creates a /2 voltage divider, allowing to measure up the 6.6V
other:
- PA6 is connected target reset pin
- reset button is connected to PA0 on one side, ground on the other.
all pins are configured using `define`s in the corresponding source code.
code
====