MPXM2102AS datasheet(518/670 Pages) MOTOROLA

518 / 670 page

AN1586

3–372

Motorola Sensor Device Data

www.motorola.com/semiconductors

Important Note:

Very small and very large high times (assuming a fixed

period) are typically unattainable due to the finite amount of

time it takes to generate the pulse train on the output compare

pin. This amount of time will vary depending on the

microcontroller’s clock speed and the latency of the actual

software routines implemented. Thus the sensor’s analog

voltage to which the integrator’s dc voltage is compared must

be within the possible ranges of voltages created by the

integrator’s input pulse train — i.e. the sensor’s zero pressure

offset voltage must be greater than the smallest voltage

created by the integrator (corresponding to the pulse train’s

smallest possible high time) and the sensor’s full scale output

voltage must be less than the largest voltage created by the

integrator (corresponding to the pulse train’s largest possible

high time).

After establishing the frequency of the pulse train, the RC

time constant for the integrator can be determined and the

resistor and capacitor value can be chosen. The RC time

constant should be long compared to the period of the pulse

train so that a stable dc voltage (very little ripple due to the

capacitor’s charging and discharging) is obtained at the output

of the comparator.

Follow these steps to design the RC time constant and

integrator’s component values. The design example’s

calculations are presented simultaneously.

For the resolution desired, determine the number of volts

(typically mV) that corresponds to the least significant bit (one

timer count). For this design example, 8–bit resolution (256

timer counts) over the desired pressure sensor span

corresponds to

#of

timer count

Desired Pressure Sensor Span (V)

Number of Timer Counts

256 timer counts

+ 15.6 mV

timer count

Therefore the stability of the integrator’s output voltage

should be less than 15.6 mV (least significant bit). Choosing

an RC time constant that allows a ripple of approximately

one–fourth

the

least

significant

bit

sufficient

(approximately 3.9 mV).

The most ripple occurs at a 50% duty cycle pulse train. For

this design the entire period is 640

µs. 50% duty cycle

indicates a high time (and low time) of 320

µs. Furthermore,

the capacitor should discharge no more than approximately

3.9 mV (defined as

∆V) over the 320 µs. The following

equation is used to calculate the value for RC:

V(t) = Vinitial – ∆V = Pulse Train Logic–level one value •

Duty Cycle

• e

where Vinitial = Pulse Train Logic–level one value • Duty Cycle

and

∆V is the voltage discharge of the capacitor.

Solving for RC:

V(t)

Pulse Train Logic–level one value

Duty Cycle

•

RC = –

320

ln 2.5V–3.9 mV

5 V 50%

+ 0.205 s

•

Finally, choose the values of the resistor and capacitor. A

typical resistor value is on the order of a tens of k

Ω. The

resistor’s value can be higher (hundreds of k

Ω) but care must

be taken to avoid increased thermal noise.

For this design, the resistor value is chosen to be 49.9 k

Ω

(1% resistor). The capacitor’s value is readily calculated to be

+ 0.205 s

49.9 k

W +

4.1

Choose the values of the resistor and capacitor so that the

actual time constant is equal to or greater than the calculated

time constant.

Note: Be aware that temperature variations can create

errors in the system (thus reducing system performance);

therefore, be sure to use low temperature coefficient resistors,

capacitors, etc.

SYSTEM DESIGN: STEP–BY–STEP PROCEDURE

FOR PRESSURE MEASUREMENT AND

CALIBRATION

To measure pressure (note: there are other measurement

algorithms that can be performed that in some cases may be

more acceptable (see below, Additional notes)):

1. Start with a pulse train with the minimum high time

feasible with the system’s microcontroller. Pulse train

should run at a frequency equal to or less than the

frequency calculated above.

2. Make sure the general I/O pin’s input is high (sensor’s

output voltage is greater than the integrator’s output

voltage).

3. Increment the high time of the pulse train by one timer

count.

4. Check the general I/O pin to see if its input is low

(sensor’s output voltage has become less than the

integrator’s output voltage).

5. If the general I/O pin is reading a logic–level zero, store in

memory the high time of the pulse train as the current

pressure high time reading that created the logic–level

transition in the comparator’s output.

6. If the general I/O pin is reading a logic–level one, go back

to step 3 and repeat.

7. Using the equation “Current Pressure = .......” shown

above, calculate the current pressure (assuming the

system has already been calibrated).

8. Repeat steps 1 through 7 for additional pressure

measurements.

To calibrate the system:

At zero and full scale pressures, perform the above 8 step

pressure measurement routine. Store the appropriate pulse

train high times corresponding to zero and full scale pressure.

These high times will be used to calculate the current pressure

as mentioned in Step 7 above.

SOFTWARE EXAMPLES TO GENERATE PULSE

TRAIN ON OUTPUT COMPARE TIMER CHANNEL

The following software examples are written in assembly

language for the MC68HC05P9 (the code is applicable to any

HC05 series microcontroller with TCMP pin).

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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