STLinkForm

Initial Hardware Development using STM32 BluePill/BlackPill

This prototype board replaces the STM32 Bluepill proto-board as a result of the chip crisis started on early 20's, which caused a overwhelming cases of STM32F103 clones and counterfeits.

This project is moving to more recent releases of the STM32 as there is barely a chance that the STM32F103 will ever get manufactured again. Or it may happen that very limited production batches will be sold at a premium price, for those searching for original parts.

For the development of the project we need access to the hardware pins and interfaces that realizes our project. So, one of the easiest way to go is the STM32 Blue and Black Pill prototyping board. This board is based on one of the two existing options: The STM32 BluePill and the STM32 BlackPill boards, easily found on eShops.

This prototype board offers the following advantages:

• Dual board support (either Blue- or Blackpill)
• Unbeatable cost/performance combo
• Lots of documentation and community support for both options
• Higher clock rates when compared to most Arduino-like alternatives, with a comparable budget.
• Lots of interface pins
• Possibility to drive JTAG at rates above 4 MHz, supported by the MSP430 JTAG interface
• Integrated USB port to implement a VCP.
• Programmable target voltage (using a PWM pin).
• Voltage level translator, to drive MSP boards with less than 3.3V.
• A Logic analyzer connection.
• Access to most circuit elements for signal monitoring, even for most SMD parts.

Technical Solution Description

The BluePill/BlackPill prototype uses the following special features to implement the Glossy MSP430 device:

• SPI1 pins are used as communication port allowing to use SPI for JTAG transfers. Bit banging is obviously available as a standard GPIO:
  • MISO for TDO (target-side output → MCU input)
  • MOSI for TDI (target-side input → MCU output)
  • SCK for TCK
• TMS is controlled by Timer 1 Channel 3 or bit-banging
• A copy of SCK signal is fed into Timer 1 Channel 1 to allow for TMS automated signal generation.
• Other JTAG signals such as TEST and RST use regular GPIO pins for bit bang.
• The design combines all JTAG interface signals share the same GPIO port, allowing for bit-bang using a single port access, either by software or timer controlled DMA.
• SPI pins are also used for the SBW.
• Timer 1 Channel 2 output is reserved to control the direction of the SBW data pin. This allows the use of SPI bus to implement the SBW signal, with cooperation of the timer.
• A GBD port is provided on USART1, required during the early development phase (see note below).
• TRACESWO is fixed at PB3 and can be connected to a debug emulator that supports it.
• USART2 is connected to the JTAG port, at the moment not used, but reserved for the debug COM port for the target board. This feature will be added when the VCP code is finally developed.
  The pinout of the JTAG connector extends the original TI proposal, like many suggestions for BSL programming found on by many other design references.
• PA0 is an ADC input, used to read the target I/O voltage. The resistor divisor is adjusted to read 50% of VRef input.
• Firmware will sample the PA0 input before setting the output voltage using the Timer 4 Channel 3 PWM output on PB8.
• 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 tasks or, in the worst case, patch the board.

Notes Regarding VCP Emulation

To allow for firmware debug capability on the early stage of development, a GBD port is provided on USART1.
This is currently required, because when a device hosts a VCP, it is required that the firmware is always online to react to USB host packets. USB has a minimal flow of packets required for device management. But, a minimal development framework needs the use of breakpoints. A breakpoint will halt the CPU and cause a drop in the USB link because USB host will stop receiving the minimal USB protocol frames. Therefore, a ordinary USART is required for the step by step debugging.

Currently, the firmware found on the repository uses a stand-alone COM port hardware, so CPU flow may be stopped for debug inspection, while communication link is still online.

Note that the main impact of this combination is performance. Serial port will limit transfers to the serial line speed (bps). In the future, it is planned to implement an own VCP in the firmware, a huge performance gain will happen, because serial port packets on the USB side are essentially virtual and packets arrive directly on the USB payload buffers, no UART serialization is needed anymore. Payloads are handled directly in the firmware and bps won't cause any effect.

Transfer will follow these steps:

• Windows driver queueing requests into USB host hardware (nearly instantaneous);
• Cable transfer at 12 Mb rate (was 115.2 kb in real UART mode)
• STM32 hardware decode and DMA into payload buffers (not measurable yet)
• GDB parsing and JTAG (possibly the single bottleneck on this scenario, but driven by a 72 MHz Cortex 3 kernel, far better than any MSP430 based debug emulator)

Quite promising!

The BluePill Socket

Support for the older Bluepill board is also possible. For this option you should fit the board into the socket, aligned to the left row.

The detailed view can be seen on the following picture:

Consolidated Pinout

MCU: Target Board Voltage Control

MCU: JTAG/SBW Bus Interface

MCU: JTAG/SBW Control Lines

JTAG connector lines are negative logic signals and will have the following scenarios:

MCU: Target Board USART Interface

MCU: USB Interface

MCU: SWD Interface for the Debug Probe Firmware Development

MCU: UART for Development Phase

MCU: Other Functions

MCU: Unassigned Pins

The BlackPill Socket

At the core of this prototype, a BlackPill development board is fitted into the main socket. Note that this socket uses dual row sockets, because it was designed to accommodate either a BlackPil or a BluePill board. The BlackPill should be fitted into the right pin row.

The detailed view can be seen on the picture below:

Please note that firmware development for the BlackPill is suspended because original STM32F411 have also disappeared since the chip crisis.
Worse than that, clones like the AT32F403 implements peripherals of the STM32F103 series, or a combo of both, which is counterproductive.
Newer STM32L4xx or STM32G4xx families are easier to buy and offers a substantial upgrade path.

The TRACESWO Output

The development of a big firmware is almost impossible without a tracing facility. SWO is the standard way to go. So this output has a dedicated access on the board, near to the SWD jumpers of the Blackpill (or Bluepill) board.

The Black Magic Probe provides an input pin for the SWO. Just connect a wire to the pin marked on that device.

Speeds of SWO may be an issue and experimentation proves that values listed on specs are far above the practical limits, when debugging and VCP is also handled simultaneously.

GBD Serial Port

Note that on the final product, GDB connection is provided using a VCP firmware implementation, which couples the tool at the max possible transfer speed.
The advantage of the VCP firmware was covered on a topic above.

Regardless, a simple serial port is used during the core of the firmware development.
Once the firmware is mature, the VCP will be added and this GDB serial port will be deactivated.

This connector can only be found on this prototype board. Final designs found on STLinkForm or StickForm does not incorporate this interface since they are planed when general firmware development gets more mature.

Second LED

Two LEDs are provided to improve usability. One Red LED is already provided on the Blue/Black Pill board and a second, green colored, was added to the board.

Both LEDs share the same pin, like in a regular STLink. Either red or green, the color indicates the state of the JTAG bus and toggles during packet transfers coming from the host.

A red indicates that the JTAG bus is active (low impedance) and the green color indicates standby on a powered device.

On the final design LEDs will se a component with dual LED integration, which is a more consistent solution than the one presented on this prototype board.

The Logic Analyzer Connector

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

This connection is provided at the 3.3V realm, before the voltage translation occurs.

The JTAG Connector

This is the connector used to connect the MSP430 board and follows the standard TI JTAG layout with additional pins for a serial VCP connection like proposed by Elprotronic.

Pins TDO, TDI, TMS, TCK, RST and TEST follows the same convention used by a standard MSP-FET, including both TDO and TCK which are also used for the Spy-bi-Wire connection.

The MSP430 JTAG/BSL connectors from Elprotronic covers the fusion of JTAG and BSL into the same connector. At the end BSL uses a serial link, which is exactly appropriate for the VCP wiring. Note that like usual on a serial connection TXD and RXD interconnection needs to be crossed. So the pin 12 needs to be connected to a RXD pin on the MSP430, while the pin 14 will be tied to TXD pin on the target MCU (i.e. signal crossing occurs on the target board, not in this debug unit).

So for example, to establish a UART link to the USART0 of a classic MSP430F149 part, connect JTAG.12 to the P3.5/URXD0 and JTAG.14 to the P3.4/UTXD0 of the MCU.

Now for the Pin 2 TVCC line, this prototype board can supply up to 100 mA which is enough for almost every available daughter-board.
The VCC Sense pin is a reference entry for the cases that a board is self powered. Then this reference voltage is used to adjust the TVCC output voltage so that voltage levels are compatible.

Do not tie pins 2 and 4 together. They are mutually exclusive and official target boards implements a jumper for this selection (this rule is also valid when connecting a MSP430 to the official TI JTAG device).

Voltage Regulator

Note that the Firmware can source a programmable voltage to the TVCC line to supply the voltage translator circuit that can also be used to attach a MSP430 target board using the JTAG connector.
This behavior is also seen on the official TI MSPFET and the older MSPFET430UIF.

The TVCC supply voltage is protected by a 100 mA polyfuse, which should be enough for most practical cases.

Voltage Translator

A voltage translator was added to this test board, walking a step further if compared to the first BluePill prototype board, which allows for the development of the firmware coding that handles programmable supply voltages.

The output signal of this stage have ESD protection, which ensures that connecting cables to that connector is generally safe.

At the time of this writing, firmware still produces a fixed 3.3 V output on the TVCC line, which is used as reference for the target, even if hardware has support for variable voltage.

The circuit is based on a 74xx2T45 and two 74xx125. These circuits are capable of transferring signals at least at a 100 MHz rate, which is far more than the requirements for our application.

Power Supply Test Points

It is very often required to control supply voltages during firmware development, so the board has access to the GND, 3.3V and the TVCC.

Other interesting signals required for the implementation of the TVCC supply voltage are Vref and TVpwm, which are also accessible through the jumper pads around the BluePill/BlackPill.

A Typical Use Case

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

USB cable should be connected to the Black Magic Probe, the BluePill and the Logic Analyzer.

Each element of this picture are detailed next.

Blue/Black Pill Development board

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

In this setup a BluePill is seated at the dual connector. Connection cables are provided for the GDB UART port, the SWD debug port, TRACESWO, the Logic Analyzer and the MSP430 target board.

Debug Unit (Black magic Probe - ARM edition)

At the left you you see a STLink-clone converted to a Black Magic Probe (ARM) according to this article in our wiki.

In this particular conversion, the top connector, originally for the SWIM link, 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. Check the article.

Attached to the 20-pin ARM JTAG connector, an adapter board is used to facilitate the wiring of the SWD+SWO connections. The output of this adapter board has five wires running to the debug port of the BluePill/BlackPill and the SWO jumper on the Development board. Note that connection of the VCC supply is not advised, but here in this image you will see that it has a wire, but although it cannot be verified here, the adapter board has a switch to select the VCC function.
Details to this SWD adapter board can be found on the hardware projects.

The Logic Analyzer

The logic analyzer in this picture is the LA2016 which has 16 inputs, but for this scenario only 6 inputs and the GND wire are required. All other cables are simply left unconnected.

An USB cable needs to be connected between the unit and the PC, so that the bundled software is able to capture signals.

The following image is an example of the 0x28 Shift IR command, while the MSP board returns the 0x91 identification byte:

The SPI channel sends 3 bytes to perform this transmission, while the TMS signal was properly generated using a timer and DMA transactions. Since SPI requires byte aligned transfers we use TMS neutral states so that the some of the clock pulses have no effect on the payload.

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. This uses the standard pinout for MSP430 JTAG emulators, also compatible with the commercial TI MSP-FET.

The particular device used in this case, is the MSP430F2417 and the target board is configured to use the 3.3V power supply provided by the BluePill board.

General Development Environment

A brief documentation of the software development environment is described in here.