REM tilt sensor interface
REM ss 11/98

REM   *****************************************************************
REM   This program was written to be compiled with revision 3 of VSTB.
REM   Because of this, it defines AIN() as a subroutine, since it only
REM   became a built-in function in revision 4. If you want to use this
REM   program, you will have to make the appropriate modifications.
REM   *****************************************************************

REM Fluid filled tilt sensors are problematic from a circuit design standpoint
REM because they cannot withstand an applied DC voltage.  AC must be applied
REM to the bridge to prevent electrolytic plating of the liquid.

REM This program interfaces directly to a tilt sensor using no external components.

REM The sensor consists of 4 pins surrounding a single center pin in conductive
REM liquid.  The 4 outer pins are connected to the keypad pins 4-8 and the
REM center pin connected to AIN pin 1.

CONSTANT VTI_ADRES      AS INTEGER = 0x1E
CONSTANT VTI_ADCON0     AS INTEGER = 0x1F
CONSTANT VTI_TRISA      AS INTEGER = 0x85
CONSTANT VTI_ADCON1     AS INTEGER = 0x9F
CONSTANT VTI_Analog_5   AS INTEGER = 2
REM CONSTANT VTI_Analog_3   AS INTEGER = 4
REM CONSTANT VTI_Analog_0   AS INTEGER = 6
CONSTANT VTI_Fosc_by_32 AS INTEGER = 0x81
GLOBAL   VTI_AD_GO      AS BIT = VTI_ADCON0,2
REM simple A/D, SBC2000-074/-062
VITAL FUNCTION AIN(channel AS INTEGER) AS INTEGER
	POKE(VTI_ADCON1,VTI_Analog_5)	:REM set up port a as 5 analog inputs
	POKE(VTI_TRISA, PEEK(VTI_TRISA) OR (2^channel))
	POKE(VTI_ADCON0, VTI_Fosc_by_32 OR (channel * 8))
	VTI_AD_GO = 1			: REM   Start the conversion
	AIN = PEEK(VTI_ADRES)		: REM   Read the result
END


CONSTANT CRLF AS STRING = "\013\010"

REM these two constants correct for any mechanical off center of the sensor
CONSTANT pitchoffset AS INTEGER = 0
CONSTANT rolloffset AS INTEGER = 0

GLOBAL B4 AS BIT = 6,4
GLOBAL B5 AS BIT = 6,5
GLOBAL B6 AS BIT = 6,6
GLOBAL B7 AS BIT = 6,7

GLOBAL temp AS INTEGER
GLOBAL PITCH1 AS INTEGER, PITCH2 AS INTEGER
GLOBAL roll1 AS INTEGER, roll2 AS INTEGER
GLOBAL PITCH AS INTEGER, ROLL AS INTEGER

REM Here is the key to the "no DC" requirement.
REM Two bridge elements are excited + to -, then - to +,
REM then the other two bridge elements are similarly excited,
REM then all electrodes are turned off.
REM Four A/D reading are taken during the four excitation phases.
REM Routine takes ~8mS on an SBC2000-074.
VITAL SUBROUTINE tilt()
	B4 = 0		:REM energize the two pitch electrodes
	B5 = 1
	pitch1 = AIN(0)	:REM measure the result in the center
	B4 = 1		:REM energize the two pitch electrodes
	B5 = 0		:REM in the opposite phase as before
	pitch2 = AIN(0)	:REM measure the result
	temp = B4		:REM - VERY IMPORTANT -
	temp = B5		:REM turn both pitch electodes off!
	B6 = 0		:REM repeat for roll axis
	B7 = 1
	roll1 = AIN(0)
	B6 = 1
	B7 = 0
	roll2 = AIN(0)
	temp = B6		:REM turn both roll electodes off
	temp = B7
END



REM ********** main
POKE(0x81,128 + PEEK(0x81))		:REM turn off PORT B pullups

DO
	tilt()
	PIPE PRINT COMM0
	PRINT CRLF, pitch1,pitch2,roll1,roll2

	PITCH = pitch1-pitch2+pitchoffset
	roll = roll1-roll2+rolloffset

	LCD_COMMAND(128)
	PIPE PRINT LCD
	PRINT PITCH, ROLL,"   "
LOOP UNTIL 0