MPXM2010D datasheet(5/24 Pages) FREESCALE

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AN1950

Sensors

Freescale Semiconductor

Converting Pressure to Water Level

Hydrostatic pressure being measured is the pressure at the

bottom of a column of fluid caused by the weight of the fluid

and the pressure of the air above the fluid. Therefore, the

hydrostatic pressure depends on the air pressure, the fluid

density and the height of the column of fluid.

P= Pa + ρ g ∆h

where P = pressure

Pa = pressure

ρ = mass density of fluid

g = 9.8066 m/s^2

h = height of fluid column

To calculate the water height, we can use the measured

pressure with the following equation, assuming the

atmospheric pressure is already compensated for by the

selection of the pressure sensor being gauge:

∆h = P \ ρ g

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

depressed. If they are depressed concurrently, it calls a

calibration function Calib. If they are not both pressed, 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 depressed.

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 0 mm of water.

The second calibration point is obtained when the head tube

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

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, press PB1 to take 256 A/D readings at 0 mm 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

160 mm of water, before depressing PB2 to take 256 A/D

readings. The average is taken and stored in a page of flash.

Once the two readings are taken, averaged, and stored in the

flash, a message displays the two A/D values stored.

Level Function

The Level function initializes the graphics characters. Once

this is complete, it continues looping to obtain an average A/D

reading, displaying the Water Level, the Water Flow, and a

Graphical History until simultaneously depressing both PB1

and PB2 to return to the main function.

The function first clears the 40 pressure readings it updates

for the Graphical History. The history then enters the loop first

displaying eight special characters, each containing five 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. It 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.

Level Function

The Level function initializes the graphics characters. Once

this is complete, it continues looping to obtain an

average A/D reading, displaying the Water Level, the Water

Flow, and a Graphical History until simultaneously depressing

both PB1 and PB2 to return to the main function.

The function first clears the 40 pressure readings it updates

for the Graphical History. The history then enters the loop first

displaying eight special characters, each containing five 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. It 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

chosen for this reference design are 8x5 pixels by default.

Therefore, each special character that is created will be able

to display five water level readings. Since the height of the

special character is eight pixels, each vertical pixel position

will represent a water level in increments of 20 mm.

Resolution = (H1 – H0) / D

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