标签 MAX6675 下的文章

1 背景

烘焙咖啡豆的过程中，为了更好地测量和记录温度及其变化，使用ESP32C3制作了一个温度监控模块。

2 需求

如果从烘焙咖啡豆的角度去考虑，参照商用机器的设计，这个温度监测会有很多功能要实现。作为起始的设计，还是先简化需求，逐步实现更多功能。所以第一版先确定以下需求：

  • 计时。这个实现起来简单，记录的温度也需要跟时间关联。
  • 读取测量出的温度值。用于展示、分析、记录等。
  • 展示温度变化。显示当前温度值，和温度与时间的曲线。

3 设计

3.1 元件

  • 主控：ESP32C3-Core，刷上MicroPython固件

    • 此开发板很廉价，最低9.9 RMB包邮。
    • 支持WiFi和蓝牙，数据可以很方便地同步到其它设备。
    • 基于MicroPython开发程序，调试很方便。

  • 温度检测模块：MAX6675，K型热电偶温度传感模块，SPI接口。

    • 廉价。
    • 最低精度为0.25 °C，够用。

  • 温度检测探头：K型铠装热电偶，探头为可弯曲、接壳式、304不锈钢材质。

    • 这个探头比温度检测模块附送的灵敏很多。
    • 一般型号为WRNK191。
    • 参考规格：直径1mm，插深50mm，线长500mm。

  • 显示模块：SSD1306，单色OLED屏，0.96英寸，分辨率128x64，两线I2C接口。

    • 廉价。
    • I2C接口的数据线只有两根，减少GPIO的占用。

3.2 接线

这里忽略电源（VCC 3.3V）和接地（GND）的连接。详细如下：

ESP32C3的接口模块接口
GPIO10MAX6675SO
GPIO02MAX6675SCK
GPIO12MAX6675CS
GPIO05SSD1306SCL
GPIO04SSD1306SDA

接线不是固定的，可以根据实际调整，但是要改main.py的对应配置。

4 程序

相关代码文件：

  • max6675.py：MAX6675的驱动程序。
  • ssd1306.py：SSD1306的驱动程序。
  • series_list.py：温度数据类，方便后面扩展对温度数据的保存。
  • main.py：主程序。

4.1 MAX6675的驱动程序

文件名max6675.py。

# from https://github.com/BetaRavener/micropython-hw-lib/blob/master/MAX6675/max6675.py

import time


class MAX6675:
    MEASUREMENT_PERIOD_MS = 220

    def __init__(self, sck, cs, so):
        """
        Creates new object for controlling MAX6675
        :param sck: SCK (clock) pin, must be configured as Pin.OUT
        :param cs: CS (select) pin, must be configured as Pin.OUT
        :param so: SO (data) pin, must be configured as Pin.IN
        """
        # Thermocouple
        self._sck = sck
        self._sck.off()

        self._cs = cs
        self._cs.on()

        self._so = so
        self._so.off()

        self._last_measurement_start = 0
        self._last_read_temp = 0
        self._error = 0

    def _cycle_sck(self):
        self._sck.on()
        time.sleep_us(1)
        self._sck.off()
        time.sleep_us(1)

    def refresh(self):
        """
        Start a new measurement.
        """
        self._cs.off()
        time.sleep_us(10)
        self._cs.on()
        self._last_measurement_start = time.ticks_ms()

    def ready(self):
        """
        Signals if measurement is finished.
        :return: True if measurement is ready for reading.
        """
        return time.ticks_ms() - self._last_measurement_start > MAX6675.MEASUREMENT_PERIOD_MS

    def error(self):
        """
        Returns error bit of last reading. If this bit is set (=1), there's problem with the
        thermocouple - it can be damaged or loosely connected
        :return: Error bit value
        """
        return self._error

    def read(self):
        """
        Reads last measurement and starts a new one. If new measurement is not ready yet, returns last value.
        Note: The last measurement can be quite old (e.g. since last call to `read`).
        To refresh measurement, call `refresh` and wait for `ready` to become True before reading.
        :return: Measured temperature
        """
        # Check if new reading is available
        if self.ready():
            # Bring CS pin low to start protocol for reading result of
            # the conversion process. Forcing the pin down outputs
            # first (dummy) sign bit 15.
            self._cs.off()
            time.sleep_us(10)

            # Read temperature bits 14-3 from MAX6675.
            value = 0
            for i in range(12):
                # SCK should resemble clock signal and new SO value
                # is presented at falling edge
                self._cycle_sck()
                value += self._so.value() << (11 - i)

            # Read the TC Input pin to check if the input is open
            self._cycle_sck()
            self._error = self._so.value()

            # Read the last two bits to complete protocol
            for i in range(2):
                self._cycle_sck()

            # Finish protocol and start new measurement
            self._cs.on()
            self._last_measurement_start = time.ticks_ms()

            self._last_read_temp = value * 0.25

        return self._last_read_temp

4.2 SSD1306的驱动程序

文件名ssd1306.py。

这个驱动继承了FrameBuffer类，显示文字或绘图的方法，参考FrameBuffer类的文档即可（代码有说明）。

# MicroPython SSD1306 OLED driver, I2C and SPI interfaces
# from https://github.com/micropython/micropython-lib/blob/master/micropython/drivers/display/ssd1306/ssd1306.py

from micropython import const
import framebuf


# register definitions
SET_CONTRAST = const(0x81)
SET_ENTIRE_ON = const(0xA4)
SET_NORM_INV = const(0xA6)
SET_DISP = const(0xAE)
SET_MEM_ADDR = const(0x20)
SET_COL_ADDR = const(0x21)
SET_PAGE_ADDR = const(0x22)
SET_DISP_START_LINE = const(0x40)
SET_SEG_REMAP = const(0xA0)
SET_MUX_RATIO = const(0xA8)
SET_IREF_SELECT = const(0xAD)
SET_COM_OUT_DIR = const(0xC0)
SET_DISP_OFFSET = const(0xD3)
SET_COM_PIN_CFG = const(0xDA)
SET_DISP_CLK_DIV = const(0xD5)
SET_PRECHARGE = const(0xD9)
SET_VCOM_DESEL = const(0xDB)
SET_CHARGE_PUMP = const(0x8D)

# Subclassing FrameBuffer provides support for graphics primitives
# http://docs.micropython.org/en/latest/pyboard/library/framebuf.html
class SSD1306(framebuf.FrameBuffer):
    def __init__(self, width, height, external_vcc):
        self.width = width
        self.height = height
        self.external_vcc = external_vcc
        self.pages = self.height // 8
        self.buffer = bytearray(self.pages * self.width)
        super().__init__(self.buffer, self.width, self.height, framebuf.MONO_VLSB)
        self.init_display()

    def init_display(self):
        for cmd in (
            SET_DISP,  # display off
            # address setting
            SET_MEM_ADDR,
            0x00,  # horizontal
            # resolution and layout
            SET_DISP_START_LINE,  # start at line 0
            SET_SEG_REMAP | 0x01,  # column addr 127 mapped to SEG0
            SET_MUX_RATIO,
            self.height - 1,
            SET_COM_OUT_DIR | 0x08,  # scan from COM[N] to COM0
            SET_DISP_OFFSET,
            0x00,
            SET_COM_PIN_CFG,
            0x02 if self.width > 2 * self.height else 0x12,
            # timing and driving scheme
            SET_DISP_CLK_DIV,
            0x80,
            SET_PRECHARGE,
            0x22 if self.external_vcc else 0xF1,
            SET_VCOM_DESEL,
            0x30,  # 0.83*Vcc
            # display
            SET_CONTRAST,
            0xFF,  # maximum
            SET_ENTIRE_ON,  # output follows RAM contents
            SET_NORM_INV,  # not inverted
            SET_IREF_SELECT,
            0x30,  # enable internal IREF during display on
            # charge pump
            SET_CHARGE_PUMP,
            0x10 if self.external_vcc else 0x14,
            SET_DISP | 0x01,  # display on
        ):  # on
            self.write_cmd(cmd)
        self.fill(0)
        self.show()

    def poweroff(self):
        self.write_cmd(SET_DISP)

    def poweron(self):
        self.write_cmd(SET_DISP | 0x01)

    def contrast(self, contrast):
        self.write_cmd(SET_CONTRAST)
        self.write_cmd(contrast)

    def invert(self, invert):
        self.write_cmd(SET_NORM_INV | (invert & 1))

    def rotate(self, rotate):
        self.write_cmd(SET_COM_OUT_DIR | ((rotate & 1) << 3))
        self.write_cmd(SET_SEG_REMAP | (rotate & 1))

    def show(self):
        x0 = 0
        x1 = self.width - 1
        if self.width != 128:
            # narrow displays use centred columns
            col_offset = (128 - self.width) // 2
            x0 += col_offset
            x1 += col_offset
        self.write_cmd(SET_COL_ADDR)
        self.write_cmd(x0)
        self.write_cmd(x1)
        self.write_cmd(SET_PAGE_ADDR)
        self.write_cmd(0)
        self.write_cmd(self.pages - 1)
        self.write_data(self.buffer)


class SSD1306_I2C(SSD1306):
    def __init__(self, width, height, i2c, addr=0x3C, external_vcc=False):
        self.i2c = i2c
        self.addr = addr
        self.temp = bytearray(2)
        self.write_list = [b"\x40", None]  # Co=0, D/C#=1
        super().__init__(width, height, external_vcc)

    def write_cmd(self, cmd):
        self.temp[0] = 0x80  # Co=1, D/C#=0
        self.temp[1] = cmd
        self.i2c.writeto(self.addr, self.temp)

    def write_data(self, buf):
        self.write_list[1] = buf
        self.i2c.writevto(self.addr, self.write_list)


class SSD1306_SPI(SSD1306):
    def __init__(self, width, height, spi, dc, res, cs, external_vcc=False):
        self.rate = 10 * 1024 * 1024
        dc.init(dc.OUT, value=0)
        res.init(res.OUT, value=0)
        cs.init(cs.OUT, value=1)
        self.spi = spi
        self.dc = dc
        self.res = res
        self.cs = cs
        import time

        self.res(1)
        time.sleep_ms(1)
        self.res(0)
        time.sleep_ms(10)
        self.res(1)
        super().__init__(width, height, external_vcc)

    def write_cmd(self, cmd):
        self.spi.init(baudrate=self.rate, polarity=0, phase=0)
        self.cs(1)
        self.dc(0)
        self.cs(0)
        self.spi.write(bytearray([cmd]))
        self.cs(1)

    def write_data(self, buf):
        self.spi.init(baudrate=self.rate, polarity=0, phase=0)
        self.cs(1)
        self.dc(1)
        self.cs(0)
        self.spi.write(buf)
        self.cs(1)

4.3 温度数据类

文件名series_list.py。

想模拟成时序数据库，目前只是保存了屏幕可以显示的数据数量。

class SeriesList:
    
    def __init__(self, maxLen: int, firstVal: float):
        self._maxLen = 1 if maxLen <= 0 else maxLen
        self._list = [firstVal]
        self._len = len(self._list)
    
    def append(self, val: float):
        self._list.append(val)
        if (self._len + 1) > self._maxLen:
            delVal = self._list.pop(0)
        else:
            self._len += 1
    
    def last(self, index: int = 0) -> int:
        return self._list[self._len - index -1]
    
    def histogram(self, maxRange: int) -> list:
        hList = self._list.copy()
        hMax = int(max(self._list))
        hMin = int(min(self._list))
        hRange = hMax - hMin + 1
        rate = 1 if hRange <= maxRange else (maxRange / hRange)
        if rate == 1:
            for i, v in enumerate(hList):
                hList[i] = int(v) - hMin
        else:
            for i, v in enumerate(hList):
                hList[i] = int((int(v) - hMin) * rate)
        return hList

4.4 主程序

文件名main.py。

总结一下：

  • 使用了定时任务（Timer）去探测温度和更新显示，每秒执行一次。
  • 每次执行最大耗时约150毫秒，绘画曲线的代码可以再优化。
  • 绘画曲线时，如果把整个区域置黑，再画曲线，性能比较高，但是会闪屏。目前是逐列置黑，再画，虽然慢了点，但观感良好。
  • 展示的曲线是为了直观看到温度变化，如果显示范围内的温度差超过屏幕区域的分辨率，曲线会按比例压缩。
from micropython import const
import time
from machine import Pin, SoftI2C, Timer
from max6675 import MAX6675
from ssd1306 import SSD1306_I2C
from series_list import SeriesList

UNIT_60 = const(60)
SCREEN_W = const(128)
SCREEN_H = const(64)
HISTOGRAM_X = const(0)
HISTOGRAM_Y = const(10)
HISTOGRAM_W = const(SCREEN_W)
HISTOGRAM_H = const(SCREEN_H - HISTOGRAM_Y)

""" init MAX6675 ################################## """
print('init MAX6675')
so = Pin(10, Pin.IN) # GPIO10
sck = Pin(2, Pin.OUT) # GPIO02
cs = Pin(12, Pin.OUT) # GPIO12
max = MAX6675(sck, cs, so)
time.sleep(1)
curTemp = max.read()

""" init OLED ################################## """
print('init OLED')
i2c = SoftI2C(scl=Pin(5), sda=Pin(4))
oled = SSD1306_I2C(SCREEN_W, SCREEN_H, i2c)

""" init Ticks ################################## """
ticksStart = time.ticks_ms()

""" init Temperature list ################################## """
sList = SeriesList(SCREEN_W, curTemp)

""" init Timer ################################## """
def timerRefresh(t):
    global max, curTemp, oled, timeSec, sList
    
    duration = 0 if ticksStart==None else int(time.ticks_diff(time.ticks_ms(), ticksStart) / 1000)
    dSec = duration % UNIT_60
    dMin = int(duration / UNIT_60)
    
    curTemp = max.read() # Current temperature
    sList.append(curTemp)
    
    oled.fill(0)
    oled.text('{:02d}:{:02d} | {:>6.2f} C'.format(dMin, dSec, curTemp) , 0, 0)
    hList = sList.histogram(HISTOGRAM_H)
    hLen = len(hList)
    hPre = 0
    hCur = 0
    startX = SCREEN_W - hLen
    startY = HISTOGRAM_Y + HISTOGRAM_H - 1
    for i, v in enumerate(hList):
        hPre = v if i == 0 else hCur
        hCur = v
        oled.vline(startX + i, startY - hCur, (hCur - hPre if hPre < hCur else 1), 1)
        if hPre > hCur:
            oled.vline(startX + i - 1, startY - hPre, hPre - hCur, 1)
    oled.show()

timerTemp = Timer(0)
timerTemp.init(period=500, mode=Timer.PERIODIC, callback=timerRefresh) # Every 1 second

print('start run')