MS5611 Pressure Sensor

The MS5611 [1] is a high resolution altimeter sensor from MEAS Switzerland with SPI and I2C bus interfaces. It has an operating voltage range of 1.8 V and 3.6 V, so you'll need a regulator and logic-level conversion if you want to use it on a 5 V board.

Breakout boards are available from sites such as eBay and Banggood [2]:


To use the I2C interface take the PS pin high. The CSB pin can be used to choose between two I2C addresses; the default I2C address with CSB connected to GND is #x77 or 119. These are the default settings on the breakout board if you leave PS and CSB unconnected.

This sensor requires floating-point calculations, so it's only suitable for 32-bit versions of uLisp. The following routines use the I2C interface.

Initialising the sensor

The routine readprom reads the configuration values C1 to C6 from the PROM into the variable config. You should call this once before using the sensor:

(defvar config nil)

(defun readprom ()
  (let (c)
    (dotimes (n 6)
      (with-i2c (str #x77)
        (write-byte (+ #xA2 (* 2 n)) str)
        (restart-i2c str 2)
        (push (logior (ash (read-byte str) 8) (read-byte str)) c)))
    (setq config (reverse c))))

Reading the temperature and pressure

The routine read calls adc to read the adc values, and converts them to give the temperature and pressure:

(defun adc ()
  (delay 10)
  (with-i2c (str #x77)
    (write-byte #x00 str)
    (restart-i2c str 3)
    (logior (ash (read-byte str) 16) 
            (ash (read-byte str) 8)
            (read-byte str))))

(defun read ()
  (let ((c (lambda (x) (nth (1- x) config)))
        d1 d2)
    ; Convert D1 with OSR = 4096
    (with-i2c (str #x77)
      (write-byte #x48 str))
    ; Read ADC
    (setq d1 (adc))
    ; Convert D2 with OSR = 4096
    (with-i2c (str #x77)
      (write-byte #x58 str))
    ; Read ADC
    (setq d2 (adc))
    ; calculate the temperature and pressure as in datasheet
    (let* ((dt (- d2 (* (c 5) 256)))
           (temp (/ (+ 2000 (* dt (/ (c 6) 8388608))) 100))
           (off (+ (* (c 2) 65536) (/ (* (c 4) dt) 128)))
           (sens (+ (* (c 1) 32768) (/ (* (c 3) dt) 256)))
           (p (/ (/ (- (/ (* d1 sens) 2097152) off) 32768) 100)))
      (list temp p))))

The temperature and pressure are returned as a list of two values, temperature in °C and pressure in hPa; for example:

> (read)
(23.1468 985.334)

Thanks to Ronny Suy for the routines on which these were based.

  1. ^ MS5611 Datasheet on TE Connectivity.
  2. ^ MS5611 GY-63 Atmospheric Pressure Sensor Module on Banggood.