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The Quecoo t12-958 / KSGER v2.1S is a T12 soldering station using the MM32SPIN27 Arm processor. These are developers' notes. See README.md for users' notes. Switch "Optimize for Debugging" off; this way the code is optimized for size gcc ... -Os

Arduino

The t12-958 soldering station uses an MM32SPIN27 processor. To compile the t12-958 firmware, first install Arduino for MM32SPIN27. In the arduino-ide menu, choose Tools ⇨ Board ⇨ SeekFree ⇨ MM32SPIN27. Switch

Console

RTT Real-Time Terminal is serial port over SWD. RTT is used as console. Using RTT as console frees up the UART pins for other uses, e.g. connecting a temperature sensor.

t12-958
Thursday 2022/9/22 06:36:37
first run
3256 bytes free
use arrow keys
[1]>Iron
[2] Cold end
[3] System
[4] Calibration
[5] Controller
[6] Date and Time
[7] System info
[8] Settings
[9] Overview
[-] Back

The menu on the console and the menu on the oled screen are the same.

RTT

On Arm processors, the debugger can read and write processor memory through SWD or JTAG without stopping the processor. RTT Real-Time Terminal uses this to implement a console: the user program writes console output to a circular buffer, and the debugger reads the circular buffer and prints the console output.

RTT speed depends upon buffer size:

BUFFER_UP RTT speed
bytes char/s
1024 800000
512 450000
256 250000
128 125000

(Speed measured with RTTStream SpeedTest.) To save memory, a buffer size of 256 bytes was used.

If you wish to have system console on the UART, edit t12-common.h, comment out RTTStream.h and uncomment Serial.h.

Menu system

The menu system is ArduinoMenu, with the following tweaks:

  • use interrupt-driven rotary encoder and button as input (file t12-958/encoderIn.h, t12-958/keyIn.h), same as rest of system
  • accept arrow keys as input on the serial console (file ArduinoMenu_library/src/nav.cpp)
  • custom field to print fixed point numbers (class FPx10 in t12-958/menu_screen.cpp )

Graphics

Graphics are U8G2 on a SPI 128x64 OLED. A choice was made to keep the graphics from the STM32 project. The processor has changed from STMicroelectronics STM32F103 to MindMotion MM32SPIN27; the SDK has changed from STM32Cube MX to Arduino; but the display looks the same.

PID controller

The proportional–integral–derivative controller is FastPID, patched to use fixed point Kp, Ki, Kd instead of floating point.

Typical relationship between termocouple temperature and heater PWM: (TC temp in Celsius, PWM from 0 to 32767)

TC temp PWM
200 2300
300 4000
400 6000

NTC Temperature sensor

The pcb contains a voltage divider made with an NTC and a pull-up resistor. The voltage across the NTC resistor is measured with an ADC, and used to calculate temperature.

The Steinhart-Hart equation gives temperature as a function of NTC resistance. The Steinhart-Hart equation uses logarithm and floating point division. The MM32SPIN27 does not have hardware floating-point FPU, so using the Steinhart-Hart equation is slow and impractical for this processor.

Instead, curve fitting has been used to map the adc reading to a temperature. (function NTCVoltageToTemp_x10 in file t12-958/measure.cpp).

The NTC datasheet shows an inflection point at 25C temperature, 10K resistance. It is convenient to use two polynomials, one polynomial for temperatures above 25C, another polynomial for temperatures below 25C.

The polynomial is translated so 25 degrees C is zero on the y axis, and the adc reading at 25 degrees C is zero on the x axis. This keeps numbers small, so everything fits in a 32-bit register. See doc/ntc directory for details how the polynomial was derived.

The resulting polynomial converts an adc reading to temperature with four 32-bit integer multiplications, and four 32-bit integer additions.

For best accuracy, when measuring the NTC voltage, set the ADC to maximum input impedance. In analogSampleTime() set the slowest sample rate.

Open-circuit voltage and input impedance at the NTC pads vary considerably between boards. Perhaps putting a voltage follower opamp between NTC and ADC would help. (MM32SPIN27 has four built-in opamps available.)

Instead of manually calibrating an analogue temperature sensor it is much more convenient to use factory-calibrated digital sensors.

ADC

The analog-to-digital converter ADC is synchronized with the pulse-width modulation PWM. This way the ADC samples when PWM is zero, and all signals have settled.

File adcdma.c handles adc:

  • Soldering iron PWM uses TIMER3, CHANNEL1.
analogWrite(PA6, iron_pwm, PWM_TIM_3);
  • TIMER3, CHANNEL 4 generates a falling pulse on pin PC9, 5ms before the next PWM cycle begins. It does not matter that the package does not have a pin PC9.
analogWrite(PC9, 32000, PWM_TIM_3);
  • the ADC samples when triggered by TIMER3, CHANNEL4:
ADC_InitStructure.ADC_ExternalTrigConv = ADC1_ExternalTrigConv_T3_CC4;
...
ADC_ExternalTrigConvCmd(ADC1, ENABLE);
  • The ADC samples 4 channels:
    • NTC voltage
    • VIN 24V power supply
    • TIP soldering iron thermocouple
    • ATEMP cpu temperature sensor.
  • DMA reads the samples from the ADC and writes the samples to the array adc_value[]
  • when adc_value[] is full, an interrupt is generated and dma1_channel1_irq() is called.

The net result is that every 200ms voltages are measured, and the results written to memory, without the processor having to do anything.

PWM switches a 24V signal, while the termocouple voltage is 1-10mV. To allow voltages to settle, wait 20ms between PWM signal going down and the ADC sampling.

Sampling takes place 20ms after the PWM signal goes down and 5ms before the PWM signal rises. This determines the maximum pwm value allowed. (pwm_max in settings.cpp)

Rotary Encoder

The rotary encoder is decoded in interrupt-driven software.

The MM32SPIN27 timers have an encoder mode, to decode rotary encoders in hardware. Unfortunately, the pcb connects the two lines from the rotary encoder to processor pins that belong to different timers, so this pcb can not be used to decode a rotary encoder in hardware.

Floating point

This firmware does not use floating point. This is a design choice.

There's enough cpu to run PID in software floating point, if you wish. Tested with Arduino-PID-Library

Settings

Settings that are changed only infrequently are stored in MM32SPIN27 flash. Temperature setpoint is stored in DS1302Z battery-backed up ram. This avoids wearing out flash.

Debugger

Various debuggers can be used to download firmware to the MM32SPIN27:

  • Segger JLink does not explicitly support MM32SPIN27 but MM32L072XX works.
  • OpenOCD has an old patch to support the similar MM32L062
  • AN0006 lists officially supported debuggers/flashers (Chinese)
  • H7-Tool supports many "Made in Asia" processors, including MM32. Includes an RTT Viewer. H7-Tool is an open-source commercial product. (Chinese)
  • The MM32 also supports firmware download via serial port, but on the T12-958 pcb the BOOT0 pin has been soldered to ground.

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