Memsic 2125

bdoc · 2006-02-27 08:51

In using the parallax sample code for basic memsic 2125 interpretation I'm getting g readings over 6g. I'm curious as to why this would be the case? Again, sample code straight from parallax used with a BSIIsx. Any ideas? Thanks

Jon Williams · 2006-02-27 14:30

The problem is that you're using BS2 code with a faster processor, the BS2sx, and the value of PULSIN on the BS2sx returns different timing units.· This is where conditional compilation can help us.· Add this to the top of your program:

#SELECT $STAMP
· #CASE BS2, BS2E, BS2PE
··· Scale······ CON···· $200··················· ' 2.0 us per unit
· #CASE BS2SX, BS2P, BS2PX
··· Scale······ CON···· $0CC·················· ·' 0.8 us per unit
#ENDSELECT

And then we have to modify the subroutine that reads the Memsic pulses like this:

Read_G_Force:
· PULSIN Xin, HiPulse, pulse··················· ' read pulse output
· pulse = pulse */ Scale······················· ' convert to uSecs
· xmG = ((pulse / 10) - 500) * 8··············· ' calc 1/1000 g
· PULSIN Yin, HiPulse, pulse
· pulse = pulse */ Scale
· ymG = ((pulse / 10) - 500) * 8
· RETURN

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Jon Williams
Applications Engineer, Parallax

bdoc · 2006-02-27 15:01

Thanks... but now I'm getting 4g... here's the code i've been using below: Any ideas?

'
' Refer to Memsic documentation (AN-00MX-007.PDF) for details on g-to-tilt
' conversion and considerations.
'
' www.memsic.com


' -----[noparse][[/noparse] Revision History ]------------------------------------------------


' -----[noparse][[/noparse] I/O Definitions ]-------------------------------------------------

Xin             PIN     0                       ' X input from Memsic 2125
Yin             PIN     1                       ' Y input from Memsic 2125


' -----[noparse][[/noparse] Constants ]-------------------------------------------------------

' Set scale factor for PULSIN

#SELECT $STAMP
  #CASE BS2, BS2E
    Scale       CON     $200                    ' 2.0 us per unit
  #CASE BS2SX
    Scale       CON     $0CC                    ' 0.8 us per unit
  #CASE BS2P
    Scale       CON     $0C0                    ' 0.75 us per unit
  #CASE BS2PE
    Scale       CON     $1E1                    ' 1.88 us per unit
#ENDSELECT

HiPulse         CON     1                       ' measure high-going pulse
LoPulse         CON     0

DegSym          CON     176                     ' degrees symbol


' -----[noparse][[/noparse] Variables ]-------------------------------------------------------

xRaw            VAR     Word                    ' pulse from Memsic 2125
xmG             VAR     Word                    ' g force (1000ths)
xTilt           VAR     Word                    ' tilt angle

yRaw            VAR     Word
ymG             VAR     Word
yTilt           VAR     Word

disp            VAR     Byte                    ' displacement (0.0 - 0.99)
angle           VAR     Byte                    ' tilt angle


' -----[noparse][[/noparse] EEPROM Data ]-----------------------------------------------------


' -----[noparse][[/noparse] Initialization ]--------------------------------------------------

Setup:
  PAUSE 250                                     ' let DEBUG window open
  DEBUG "Memsic 2125 Accelerometer", CR,
        "-------------------------"


' -----[noparse][[/noparse] Program Code ]----------------------------------------------------

Main:
  DO
    GOSUB Read_Tilt                             ' reads G-force and Tilt

    ' display results

    DEBUG CRSRXY, 0, 3
    DEBUG "X Input...  ",
          DEC (xRaw / 1000), ".", DEC3 xRaw, " ms",
          CLREOL, CR,
          "G Force... ", (xmG.BIT15 * 13 + " "),
          DEC (ABS xmG / 1000), ".", DEC3 (ABS xmG), " g",
          CLREOL, CR,
          "X Tilt.... ", (xTilt.BIT15 * 13 + " "),
          DEC ABS xTilt, DegSym, CLREOL


    DEBUG CRSRXY, 0, 7
    DEBUG "Y Input...  ",
          DEC (yRaw / 1000), ".", DEC3 yRaw, " ms",
          CLREOL, CR,
          "G Force... ", (ymG.BIT15 * 13 + " "),
          DEC (ABS ymG / 1000), ".", DEC5 (ABS ymG), " g",
          CLREOL, CR,
           "G Force2... ",
          DEC5 (ymG / 10), " g",
          CLREOL, CR,
          "Y Tilt.... ", (yTilt.BIT15 * 13 + " "),
          DEC ABS yTilt, DegSym, CLREOL
          IF ((ymG/10)) > 00090 AND ((ymG/10)) < 00110 THEN
             LOW 11
             LOW 12
             HIGH 0
          ELSEIF ((ymG/10)) > 00110 THEN
             HIGH 11
             LOW 12
             LOW 0
          ELSE
             LOW 11
             LOW 0
             HIGH 12
          ENDIF



    PAUSE 50                                   ' update about 5x/second
  LOOP
  END


' -----[noparse][[/noparse] Subroutines ]-----------------------------------------------------

Read_G_Force:
  PULSIN Xin, HiPulse, xRaw                     ' read pulse output
  xRaw = xRaw */ Scale                          ' convert to uSecs
  xmG = ((xRaw / 10) - 500) * 8                 ' calc 1/1000 g
  PULSIN Yin, HiPulse, yRaw
  yRaw = yRaw */ Scale
  ymG = ((yRaw / 10) - 500) * 8
  RETURN


Read_Tilt:
  GOSUB Read_G_Force
  disp = ABS xmG / 10 MAX 99                    ' x displacement
  GOSUB Arcsine
  xTilt = angle * (-2 * xmG.BIT15 + 1)          ' fix sign
  disp = ABS ymG / 10 MAX 99                    ' y displacement
  GOSUB Arcsine
  yTilt = angle * (-2 * ymG.BIT15 + 1)          ' fix sign
  RETURN


' Trig routines courtesy Tracy Allen, PhD. (www.emesystems.com)

Arccosine:
  disp = disp */ 983 / 3                        ' normalize input to 127
  angle = 63 - (disp / 2)                       ' approximate angle
  DO                                            ' find angle
    IF (COS angle <= disp) THEN EXIT
    angle = angle + 1
  LOOP
  angle = angle */ 360                          ' convert brads to degrees
  RETURN


Arcsine:
  GOSUB Arccosine
  angle = 90 - angle
  RETURN

Memsic 2125

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