AN1950

Sensors

6

Freescale Semiconductor

where H1 and H2 are the maximum and minimum water levels

respectively and D is the possible datapoints available per

character.

Resolution = (160mm– 0mm) / 8.0 = 20 mm / data point.

The graphical history is displayed using the eight 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 carried to the first column of the next

character. This column shifting is continued until all 40 data

points have been updated in the eight 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 Register A before this function is called. Using the

A/D value 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 0 mm H20

N2 is the A/D Value at 160 mm 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 Four, Flash Memory from

the MC68HC908QY4/D Databook.

ALLINIT Function

The Allinit function disables the COP for this version of

software, sets the data direction bits, and disables the data to

the LCD and turns off the LCD enable line. It also sets up the

microcontroller's internal clock to half the speed of the bus

clock. See Section 15, Computer Operating Properly, of the

MC68908QT4 datasheet for information on utilizing the COP

module to help software recover from runaway code.

WARMUP Function

The Warmup function alternates the blinking of the two

LEDs ten times. This gives the LCD some time to warm up.

Then the function warmup calls the LCD initialization function,

lcdinit.

bintasc Function

The binasc function converts a binary value to its ascii

representation.

A/D Functions

The A/D functions are used to input the amplified voltage

from the pressure sensor from channel 0 of the A/D converter.

The function adcbyti will set the A/D control register, wait for

the A/D reading and load the data from the A/D data register

into the accumulator. The function adcbyta is used to obtain an

averaged A/D reading by calling adcbyti 256 times and

returning the resulting average in the accumulator.

LCD Functions

The LCD hardware is set up for multiplexing three pins from

the microcontroller using an eight-bit shift register. Channels

three, four, and five are used on port A for the LCD enable (E),

the LCD reset (RS), and the shift register clock bit,

respectively. The clock bit is used to manually clock data from

channel four into the eight-bit shift register. This is the same

line as the LCD RS bit because the MSB of the data is low for

a command and high for data. The RS bit prepares the LCD

for instructions or data with the same bit convention. When the

eight bits of data are available on the output pins of the shift

register, the LCD enable (E) is toggled to receive the data.

The LCD functions consist of an initialization function lcdinit

which is used once when the system is started and five output

functions. The functions lcdcmdo and lcdchro both send a

byte of data. The function shiftA is called by both lcdcmdo and

lcdchro to manually shift eight bits of data into the shift register.

The function lcdnibo converts the data to binary before

displaying. The lcdnibo displays a byte of data by calling

lcdnibo for each nibble of data. The function lcdnibo enables

strings to be easily added to the software for display. The

function accepts a comma- delimited string of data consisting

of 1–2 commands for clearing the screen and positioning the

cursor. It then continues to output characters from the string

until the @ symbol is found, signally the end of the string.

CONCLUSION

The water level reference design uses a MPXM2010GS

pressure sensor in the low cost MPAK package, the low cost,

eight-pin microcontroller, and a quad op-amp to amplify the

sensor output voltage. This system uses very few

components, reducing the overall system cost. This allows for

a solution to compete with a mechanical switch for water level

detection but also offer additional applications such as

monitoring water flow for leak detection, and the other

applications for smart washing machines.