MPXM2102AS datasheet(516/670 Pages) MOTOROLA

516 / 670 page

AN1586

3–370

Motorola Sensor Device Data

www.motorola.com/semiconductors

THE INTEGRATOR

As shown in Figure 1. , the integrator consists of a single

resistor and single capacitor. A programmable duty cycle

pulse train from the microcontroller is input to the integrator.

Assuming that the RC time constant of the integrator is

sufficiently long compared to the pulse train’s frequency, the

resulting output which is input to the inverting terminal of the

comparator is a dc voltage that is linearly proportional to the

pulse train’s duty cycle, i.e.:

DC Output Voltage = Pulse Train’s Duty Cycle (%)

• 5 V

Where the Pulse Train’s Duty Cycle is multiplied by the

pulse train’s logic–level one voltage value which is typically

the same voltage as the microcontroller’s 5 V supply.

Table 3 shows a few examples of Pulse Train Duty Cycles

and the corresponding DC Output Voltage assuming a typical

pulse train logic–level one value of 5 V.

Table 3. Example Pulse Train Duty Cycles and the

Integrator’s Corresponding dc Voltage Output

Pulse Train’s Duty Cycle (%)

100

DC Output Voltage (V)

1.25

2.5

3.75

To establish a stable constant dc voltage at the integrator’s

output, its time constant must be sufficiently long compared to

the frequency of the pulse train. However, the system

resolution and thus performance are directly related to the

pulse train’s frequency. The design of the time constant and

choice of the resistor and capacitor values is discussed in

System Design: Defining and designing for a desired signal

resolution.

COMPARATOR

The LM311 chip is designed specifically for use as a

comparator and thus has short delay times, high slew rate,

and an open–collector output. A pull–up resistor (R6 = 5 k

Ω)

at the output is all that is needed to obtain a rail–to–rail output.

As Figure 1. shows, the pressure sensor’s amplified output

voltage is input to the non–inverting terminal of the op amp and

the integrator’s dc output voltage is input to the inverting

terminal. Therefore, when the pressure sensor’s output

voltage is greater than the integrator’s dc output voltage, the

comparator’s output is high (logic–level one); conversely,

when the pressure sensor’s output voltage is less than the

integrator’s dc output voltage, the comparator’s output is low

(logic–level zero).

An optional resistor, RH is used as positive feedback around

U2 in Figure 1 to provide a small amount of hysteresis to

ensure a clean logic–level transition (prevents multiple

transitions (squegging)) when the comparator’s inputs are

similar in value. The amount of hysteresis increases as the

value of RH decreases. For this design, the value of RH is not

critical but should be on the order of 100 k

Ω.

THE MC68HC05P9 MICROCONTROLLER

The microcontroller for this application requires an output

compare timer channel and one general I/O pin. The output

compare pin is programmed to output the pulse train that is

input to the integrator, and the general I/O pin is configured as

an input to monitor the logic–level of the comparator’s output.

The remainder of this paper discusses the system and

software requirements.

SYSTEM DESIGN: HOW THE SYSTEM WORKS

For any analog sensor voltage output, there’s a pulse train

with a duty cycle that when integrated will equal the sensor’s

output. Therefore, by incrementing via software the pulse

train’s duty cycle from 0% to 100%, there’s a duty cycle that

when integrated will be larger than the sensor’s current

voltage output. When the integrated pulse train voltage

becomes larger than the sensor’s output voltage, the

comparator’s output will change from a logic–level one to a

logic–level zero. This logic–level, in turn, is monitored on the

general I/O pin. The pulse train’s duty cycle creating the

integrated voltage that caused the comparator’s logic–level

transition is the digital representation of the sensor’s voltage.

Thus every sensor analog output voltage is mapped to a

specific duty cycle. This design inherently has outstanding

performance (very stable and accurate) since the digital

representation of the sensor signal is created by the

microcontroller’s digital time base. Also the pressure

measurement, made via software that first increments the

pulse train’s duty cycle and then determines if an edge

transition occurred on the general I/O pin, is straightforward

and easy.

In a calibration routine (discussed below) the sensor’s

output at two known pressures (e.g. zero and full–scale

pressure) can be mapped to two corresponding pulse train

duty cycles. Since the pressure sensor’s output voltage is

linear with the applied pressure, and the integrator’s dc output

voltage is linear with the input pulse train duty cycle, then the

pulse train’s duty cycle that causes the logic–level transition

at the comparator’s output will also be linear with the applied

pressure. Thus by knowing the duty cycles for two known

pressures, a linear interpolation of any duty cycle gives an

accurate measurement of the current pressure. The following

equation is used to interpolate the pressure measurement

where the pressure units are in kPa:

Duty Cycle @ Full–Scale Pressure – Duty Cycle @

Zero Pressure

Current Duty Cycle – Duty Cycle @ Zero Pressure

• Full–Scale Pressure in kPa

Current Pressure =

For example:

At zero pressure, if the pulse train’s duty cycle required to

cause a logic–level transition at the comparator’s output is

25% and at full–scale pressure the pulse train’s duty cycle is

75%, then the current pressure that corresponds to a duty

cycle of 50% (required to obtain the logic–level one to

logic–level zero transition at the comparator’s output) is

Current Pressure

+ 50% – 25%

75% – 25% •

100 kPa = 50 kPa

Until now, the pulse train has been defined in terms of duty

cycle. However, in practice duty cycle is calculated from the

ratio of the high time to the total period of the pulse train.

Therefore, there is a high time (typically in

µs) of the pulse train

that causes the logic–level transition of the comparator’s

output. The interpolation of the current pressure can then be

calculated directly from the high time of the pulse train that is

programmed

the

user

generated

the

Freescale Semiconductor, Inc.

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