Friday, February 23, 2018

The Bluepill+

The STM32L443CC 'Bluepill+'

Following up from the work I did on getting Ada going on a Bluepill.

Is a Bluepill a fixed entity?

Looking at the SoC the Bluepill uses we see it is a Cortex-M3 running at 72Mhz with 128k of flash and 20k of ram. There are newer STM32 chips all the way out to Cortex-M7's. I have this tool from ST called the ST MCUFinder. You can screen all their parts by different parameters. I had already hacked Ada onto the STM32L432 for the Make-with-Ada contest last year so the L series was still quite fresh in my mind. So looking at the package the STM32F103C8 on a stock bluepill was there a newer, pin compatible, STM32 cpu across the family that could drop in and work in place of the original? I will spare dear reader the suspense, indeed there are a bunch of parts that have the same footprint.

The candidate CPU for Bluepill+ STM32L443CCT6

That part uses a Cortex-M4F, runs at 80Mhz with 256k of flash and 64k of ram. Plus it has more peripherals than the STM32F103C8. This 443CC has 2 12 bit DACs. I like 2 DACs since you can make oscilloscope vectors with that HW.


  • Core: ARM® 32-bit Cortex®-M4 CPU with FPU, Adaptive real-time accelerator (ART Accelerator™) allowing 0-wait-state execution from Flash memory, frequency up to 80 MHz, MPU, 100DMIPS and DSP instructions
  • Performance benchmark
    • 1.25 DMIPS/MHz (Drystone 2.1)
    • 273.55 CoreMark® (3.42 CoreMark/MHz @ 80 MHz)
  • Energy benchmark
    • 176.7 ULPBench® score
  • Clock Sources
    • 4 to 48 MHz crystal oscillator
    • 32 kHz crystal oscillator for RTC (LSE)
    • Internal 16 MHz factory-trimmed RC (±1%)
    • Internal low-power 32 kHz RC (±5%)
    • Internal multispeed 100 kHz to 48 MHz oscillator, auto-trimmed by LSE (better than ±0.25 % accuracy)
    • Internal 48 MHz with clock recovery
    • 2 PLLs for system clock, USB, audio, ADC
  • Up to 83 fast I/Os, most 5 V-tolerant
  • RTC with HW calendar, alarms and calibration
  • LCD 8× 40 or 4× 44 with step-up converter
  • Up to 21 capacitive sensing channels: support touchkey, linear and rotary touch sensors
  • 11x timers: 1x 16-bit advanced motor-control, 1x 32-bit and 2x 16-bit general purpose, 2x 16-bit basic, 2x low-power 16-bit timers (available in Stop mode), 2x watchdogs, SysTick timer
  • Memories
    • 256 KB single bank Flash, proprietary code readout protection
    • 64 KB of SRAM including 16 KB with hardware parity check
    • Quad SPI memory interface
  • Rich analog peripherals (independent supply)
    • 1× 12-bit ADC 5 Msps, up to 16-bit with hardware oversampling, 200 μA/Msps
    • 2x 12-bit DAC, low-power sample and hold
    • 1x operational amplifier with built-in PGA
    • 2x ultra-low-power comparators
  • AES: 128/256-bit key encryption hardware accelerator
  • 17x communication interfaces
    • USB 2.0 full-speed crystal less solution with LPM and BCD
    • 1x SAI (serial audio interface)
    • 3x I2C FM+(1 Mbit/s), SMBus/PMBus
    • 4x USARTs (ISO 7816, LIN, IrDA, modem)
    • 1x LPUART (Stop2 wake-up)
    • 3x SPIs (4x SPIs with the Quad SPI)
    • CAN (2.0B Active) and SDMMC interface
    • SWPMI single wire protocol master I/F
    • IRTIM (Infrared interface)
  • 14-channel DMA controller
  • True random number generator
  • CRC calculation unit, 96-bit unique ID


If you look on eBay for the price of a Bluepill, its just a tidge over $2. Last I checked, $2.13. Take a look at the 10k quantity for STM32F103C8's btw. Its at that price(!!!). So a Bluepill has: 
1) a pcb
2) STM32F103C8
3) 8Mhz xtal
4) 32khz xtal
5) 5v - 3v reg
6) 2 LEDs
7) reset button
8) USB header

All for the 10k unit quantity of 2) above...

Anyway, the donor CPU an STM32L443CCT6 is about $2.61 in 10,000 unit qty. For me buying 5 or so was about $5 to $6 each so really a Bluepill+ costs about 8$ after surgery. Even at $8 this board is pretty good for what you get.

The donor

A $2 bluepill board was sacrificed to be a donor for Bluepill+ updating. Basically, since the parts are pin compatible you need only use a cheap hot air station (these are sub $100 on eBay) and some tweezers. Get a helper to spread the hot air and gently tweeze the package away from the board as the solder liquefies. I had also tin foiled the rest of the board to avoid stray heat. Once off the board, the donor IC is placed on the footprint (carefully!) and then using an iron, tack down the legs to the liberated pads. The residual solder is enough to make good contact I found even after cleaning the pads up a little bit post removal.

Porting Ada

So, we have a new CPU. Lets get Ada going on it! We will need some low level code changes to get the board initialized to be useful. Things like the PLL has to get locked to the right clock, other L series SoC IP needs to enabled etc. Fortunately, there are ample examples in Ada_Drivers_Library on how other F4 and F7 series cores are handled. The STM32L series is modern relative to the F1 and F4 and shares some commonality with the F7. So we look closely at the F7 when in doubt about init seqs. One easy thing to get going is to grab the SVD files from ST for the L443. These are right on the SoCs ST webpage.  Once you have them, you can run svd2ada to generate the headers, one set for Ada_Drivers_Library and one for the embedded runtimes. I know that this method has be modernized but I have not yet moved over to bb_runtimes at this time. I also wrote a tool in Ruby that sucks in the SVD fie and generates a gdb script. That script is personalized for your SoC, it generates reg dump log files of the entire SoC and saves them to disk. Subsequently, another tool pulls in the svd and the logs and generates a human readable hierarchy dump of each reg. This is documented on the blog. So once the svd files are generated porting begins for the various IP blocks in the design The key ones to port are GPIO, SPI, I2C, TImers, ADC, DMA. This work took some time but I leveraged my work for Make with Ada (an STM32L432). At this point there is parity between my Bluepill port and the Bluepill+ port. All the code with examples is in my STM32 tree:

with libraries:


Also the Ruby SVD reader is here:

One technique I found helpful in my Ada port was that ST via its CubeMX and free SW dev tools has lots of examples. If you get stuck with some IP thats not working in Ada. Build a CubeMX example that initializes or uses the IP you need to port. Once thats up and running on your board, use the Ruby scripts I mentioned to dump the ST example codes reg settings. First you can use that as a reference, and later use logs2dump and diff your port to the example code. Where there are diffs might expain why your port is not working.


Openocd 0.10.0 needed some minor mods to get attached to the bluepill+ Basically, the flash code needed to be told about the STM32L443's flash size based on the unknown part number. Also the target script needed some changes. I will add more to this entry later about this.