MPXM2102AS datasheet(545/670 Pages) MOTOROLA

545 / 670 page

AN1950

3–399

Motorola Sensor Device Data

www.motorola.com/semiconductors

Software Function Descriptions

Main Function

The main function calls an initialization function “ALLINIT”,

calls a warm–up function “WARMUP” to allow extra time for

the lcd to initialize, then checks if buttons PB1 and PB2 are

being pressed.

If they are both pressed, then it calls a

calibration function “CALIB”. If they are not both pressed, then

it enters the main function loop. The main loop displays the

menu, moves the cursor when the PB1 is pressed and enters

the function corresponding to the highlighted menu option

when PB2 is pressed.

Calibration Function

The calibration function is used to obtain two calibration

points. The first calibration point is taken when the head tube

is not placed in water to obtain the pressure for 0mm of water.

The second calibration point is obtained when the head tube

is placed at the bottom of a container with a height of 160mm.

When the calibration function starts, a message appears

displaying the A/D values for the corresponding calibration

points currently stored in the flash. To program new calibration

points, the user must press PB1 to take 256 A/D readings at

0mm of water. The average is calculated and stored in a page

of flash. Then the user has the option to press PB1 to exit the

calibration function or obtain the second calibration point. To

obtain the second calibration point, the head tube should be

placed in 160mm of water and then the user should press PB2

to take 256 A/D readings. The average is taken and stored in

a page of flash. Once the two readings have been taken,

averaged, and stored in the flash, a message displays the two

A/D values that were stored.

Level Function

The Level function will initialize the graphics characters.

Once this is complete, it will continue looping to obtain an

average A/D reading and display the Water Level, the Water

Flow, and a Graphical History until the user presses and holds

both PB1 and PB2 to return to the main function.

The function first clears the 40 pressure readings that it will

be updating for the Graphical History. It then enters the loop

which first displays 8 special characters, each containing 5

data points of water level history. The function “adcbyta” is

called to obtain the current averaged A/D value. The function

“LfNx” is called to convert the A/D value to a water level, which

is then compared to the Calibration points, the maximum and

minimum points, to determine if the container is full or empty.

If true, then it displays the corresponding message.

The

current water level is compared to the previous read and

displays the message “filling” if it has increased, “emptying” if

it has decreased, and “steady” if it has not changed.

The water level calculation has to be converted to decimal

in order to display it in the LCD. To convert the water level

calculation to decimal, the value is continually divided with the

remainder displayed to the screen for each decimal place. To

display the Rate of Water Flow, the sign of the value is first

determined. If the value is negative, the one’s complement is

taken, a negative sign is displayed, and then the value is

continually divided to display each decimal place.

If the

number is positive, a plus sign is displayed to maintain the

display alignment and the value is continually divided to

display each decimal place.

The most complicated part of this function is updating the

graphics history display. The characters for the 16x2 LCD that

were chosen for this reference design are 8x5 pixels by

default. Therefore, each special character that is created will

be able to display 5 water level readings. Since the height of

the special character is 8 pixel, each vertical pixel position will

represent a water level in increments of 20mm.

Resolution = (H1 – H0) / D

where H1 and H2 are the maximum and minimum

water levels respectively and D is the possible

datapoints available per character.

Resolution = (160mm – 0mm) / 8 = 20mm / data point.

The graphical history is displayed using the 8 special

characters. To update the graphics, all the characters have to

be updated. The characters are updated by first positioning

a pixel for the most recent water level reading in the first

column of the first character. Then the four right columns of

the first character are shifted to the right. The pixel in the last

column of that character is then carried to the first column of

the next character. This column shifting is continued until all

40 data points have been updated in the 8 special characters.

LfNx Function

The LfNx function calculates the water level from the current

A/D pressure reading. The A/D Pressure value is stored in

and the calibration values stored in the flash, the water level

is calculated from the following function:

RBRA: = (NX – N1) * 160 / (N2 – N1),

where NX is the current A/D Value

N1 is the A/D Value at 0mm H20

N2 is the A/D Value at 160mm H20

To simplify the calculation, the multiplication is done first.

Then the function “NdivD” is called to divide the values.

NdivD Function

The “NdivD” function performs a division by counting

successive subtractions of the denominator from the

numerator to determine the quotient. The denominator is

subtracted from the numerator until the result is zero. If there

is an overflow, the remainder from the last subtraction is the

remainder of the division.

wrflash and ersflsh Functions

The “wrflash” and “ersflsh” functions are used to write to and

erase values from the flash. For more information regarding

flash functionality, refer to Section 4. Flash Memory from the

MC68HC908QY4/D Databook.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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