README.md

Initial Hardware Development using STM32 BluePill

THIS DEVELOPMENT BOARD WAS DEPRECATED IN AUG/2022

For the development of the project we need access to the hardware pins and components. So, one of the easiest way to go is the STM32F103 Blue Pill.

This is a really cheap and powerful solution. The chip offers following advantages if compared to other 8 or 16-bit cheap modules:

• Unbeatable price/performance
• Lots of documentation and community support
• Higher clock rates when compared to MSP430 alternatives
• Lots of interface pins
• Possibility to bit-bang pin at a rate near 10 MHz, which is the upper limit of the MSP430 JTAG interface
• A SPI peripheral, also able to run on those high clock speeds, which is a real alternative to the bit bang option
• Possibility to DMA timer transitions and emulate the TMS signal at high speeds and take advantage of the SPI interface at 9MHz bit rate.
• Possibility to timer a DMA to bit bang GPIO port at these high rates (clock generation for the Flash erase command).
• Integrated USB port to implement a VCP.

Note that using the Blue Pill you are at a fixed supply rate of 3.3V, which may not be compatible to all MSP430 designs out there.

Feature Description

The Bluepill prototype has the following features:

• SPI2 pins is used as communication port allowing to use SPI for JTAG transfers. Bit banging is obviously available as a standard GPIO:
  • MISO for TDI
  • MOSI for TDO
  • SCK for TCK
• TMS is controlled by Timer 4 Channel 1 or bit-banging
• A copy of SCK signal is fed into Timer 4 Channel 2 to allow for TMS automated signal generation.
• Other JTAG signals such as TEST and RST use regular GPIO pins for bit bang. In the design all signals share the same GPIO port, allowing for bit-bang using a single port access, either by software or timer controlled DMA.
• A GBD port is provided on USART1, as USB COM emulation would limit SWD debug capabilities during the early stage of development. Currently firmware uses an independent COM port hardware, so CPU flow may be stopped by breakpoints and communication resources are keep online.
• TRACESWO is fixed at PB3 and can be connected to a debug emulator that supports it, or optionally a FTDI device, which allows very high COM port speeds.
• USART3 is connected to the JTAG port, at the moment not used, but reserved for the debug COM port for the target board. This feature comes when the USB firmware part is finally developed.
  This pinout is suggested for BSL programming by many other references on the internet.
• PA0 is configured exactly as in the ST-Link schematics and is tied to the ADC input to read I/O voltage, which in this case will always be 50% of 3.3V. On other schematics this wiring will be kept for this function as they have variable I/O.
• A dedicated jumper set is provided to connect a Logic Analyzer, which is a very useful tool to check the pulse streams and shapes. For bit-banging output, widths are a critical part, as the STM32 sometimes generates too fast widths, which causes failures. Pulses cannot be shorter than 50 ns.
• Other jumpers are available for 3.3V and GND power lines and also access to other STM32 pins which could help other development tests.

This is the board without the Blue-Pill and the FTDI proto-boards:

Board Layout

Top layout:

Bottom layout:

The Blue-pill Socket

At the core of these prototype, a Blue-pill development board is fitted. It is a very common development resource and most hobbyists will have one laying around.

The PC development is done using Visual Studio 2019/2022 using the VIsualGDB plugin. A Standard ARM debugger is connected to the 4 debug jumpers of the Blue-pill, which allows us to download and debug the firmware.

At the moment ths debugger is a Black Magic Probe (the normal ARM Cortex version), but one can use a J-Link or STLink.

The chosen I/O pins to communicate with the MSP430 target board are 5V tolerant. This is not a must, but could help handle 3.6V voltages, which is a valid range for MSP430. Not a feature that we plan to use, but a possible consideration if requirements change in the future.

The TRACESWO Output

The development of a big firmware is almost not possible without a tracing facility. SWO is the standard way to go. So this output has a clear access on the board.

Speeds of SWO may be an issue and experimentation proves that values listed on specs are far above the practical limits.

Regardless, a Black Magic Probe provides an input pin for the SWO. Just connect a wire to the pin marked on the board.

Alternatively, it is possible to install a standard FTDI serial proto-board in the provided socket. In this case you can read the message tracing using a serial port.

GBD Serial Port

Note that on a standard Black Magic Probe, GDB connection is provided using a VCP, which couples the tool at the max possible transfer speed. The advantage of the VCP is that regardless of the BAUD rate of the VCP, there is actually no serial line where this BAUD rate is applied. The packets of the VCP driver are handled internally and directly causes actions on the firmware, producing top speed.

On the other side, a VCP firmware needs to be online so that communication is not shutdown by the USB host.
This means that on the when analyzing JTAG protocol and signals, a breakpoint would harm the VCP link and lots of communication issues would happen.

To avoid this effect, a simple serial port is used during the development. When the JTAG communication is matured the VCP will be added and the existing GDB serial port will be deactivated.

Second LED

Two LEDs are provided to improve usability. One LED is already provided on the Blue Pill and a second was added to the board.

One LED will indicate when a JTAG link was established, so that no one disconnects the JTAG link while this is active. THe second will blink at every command received by the host.

The LogicAna Connector

For a complex bus like the JTAG it is very handy to control JTAG signals using a Logic Analyzer. So the board offers a dedicated connector for this purpose.

The JTAG Connector

This is the connector used to connect the MSP430 board. Since the Blue Pill comes with a decent 3.3V regulator you are allowed to connect simple boards without an additional power source, specially because a MSP430 is a ultra low power MCU.

Please note that the Vcc pin supplies a fixed voltage of 3.3V. Not a real problem, since almost every MSP430 devices supports this operating voltage.

A Typical Use Case

The image below shows a typical use case of a firmware debug session:

Each element of this picture are detailed next.

Blue Pill Development board

At the center you see the development board described on this topic.

In this setup a Blue-Pill is seated at the provided connector. Connections cables are provided for the SWD debug port, the GDB UART port, TRACESWO, the Logic Analyzer, the MSP430 target board and a USB cable, currently used as power supply.

Debug Unit (Black magic Probe - ARM edition)

At the left bottom you see a STLink-clone converted to a Black Magic Probe (ARM).

In this particular conversion, the top connector, originally a SWIM connector was converted to a 3.3V UART port. An internal hardware modification was required for this functionality and this port is used as the GDB debug port.

USB cable connects the unit to the PC, so that the VisualGDB software can perform the firmware download as development occurs. As this provides the control of the debug session.

Attached to the 20-pin ARM JTAG connector, an adapter board is used to facilitate the wiring of the SWD connection. The output of this adapter board has four wires running to the debug port of the Blue-pill. Note that the VCC is not required, even if it was wired, but the adapter board has a jumper to select the VCC function.

At the back of the adapter board I made a MOD to add
an additional jumper binding two wires to the TRACESWO connection points, on the space reserved for the optional FTDI-232 board, which is not mounted here, since BMP already supports this functionality.

The Logic Analyzer

The logic analyzer LA2016 has 16 inputs, but we need just 6 inputs and a GND wire. All other cables are simply left unconnected.

The USB cable connects the unit to the PC so the bundled software is able to capture the JTAG pulses.

The MSP430 Target Board

On this repository you will find schematics and PCB for some MSP430 devices.

In this picture a MSP Proto Board is connected using a standard MSP430 14-pin flat cable, and the pinout is compatible with other existing JTAG emulators, such as the TI MSP-FET.

The particular device used in this case, is the MSP430F2417 and the target board uses the 3.3V power supply provided by the Blue-Pill board.