MPXM2102AS datasheet(490/670 Pages) MOTOROLA

490 / 670 page

AN1551

3–344

Motorola Sensor Device Data

www.motorola.com/semiconductors

*WE NEED A SUBROUTINE TO WRITE TO AND READ FROM THE SPI

*TO CALL THIS ROUTINE LOAD ACCUMULATOR A WITH THE COMMAND DATA

*AND JSR WRITSPI. WHEN THE ROUTINE RETURNS, ACCUMULATOR A

*CONTAINS THE DATA RETURNED FROM THE SENSOR

WRITSPI

LDX

#PORTS

BASE ADDRESS OF THE I/O

STAA

SPDR,X

SEND THE COMMAND TO THE SENSOR

WRLOOP

BRCLR

7,SPSR,WRLOOP

LOOP UNTIL THE DATA HAS SHIFTED

OUT OF AND BACK INTO THE SPI

LDAA

SPDR,X

RETRIEVE THE RESULTS OF THE LAST

COMMAND

TO THE SENSOR

RTS

* NOW WE NEED TO CALL THE ABOVE */

START

JSR

INITIO

SET–UP THE I/O

LOOP

LDAA

#$1

1 IS THE COMMAND TO THE SENSOR THAT

REQUESTS PRESSURE

JSR

WRITSPI

SEND THE COMMAND TO THE SENSOR.

...

THE VALUE RETURNED IN ACCUMULATOR A

WILL BE IN THE RANGE 0..0XFF = 0..100%

FULL SCALE PRESSURE THE SECOND TIME

THROUGH THE LOOP. THE INITIAL TIME

THROUGH THE LOOP, THE DATA RETURNED

IS INDETERMINATE DATA FROM THE SENSOR

BRA

LOOP

Table 3. SPI Timing Characteristics

Characteristic

Symbol

Min

Max

Unit

Frequency of Operation

fOP

525

kHz

Cycle Time

tSCLK

—

1920

Clock (SCLK) Low Time

tSCLKL

932

—

Dout Data Valid Time

—

200

Din Setup Time

100

—

Din Hold Time

100

—

On–Bus Delay Time

tD1

—

Off–Bus Delay Time

tD2

—

µs

Chip Select Period

tD3

TBD

—

SERIAL DATA OUTPUT FORMAT

The serial data output is an 8–bit number of value 0–255.

This number represents the current applied pressure as a

percentage of the full–scale pressure rating of the smart

sensor. The master MCU can simply consider an output of “0”

to be zero pressure and “255” to be full–scale pressure. To

convert this number to engineering units, such as inches of

water (

″ H2O), themasterMCUmustmultiplythesmartsensor

output (0–255) by the full–scale pressure of the smart sensor

″ H2O and then divide (normalize) by 255. See equation 2.

The master MCU can either use an absolute number for the

full–scale pressure of the smart sensor (as indicated

previously) or can query each smart sensor that is connected

to the serial bus for its rated pressure range. The latter

technique allows multiple smart sensors of various full–scale

pressure ranges to be communicating with a single master

MCU, without the need for an absolute addressing scheme

that contains full–scale pressure information for each sensor.

CONCLUSION

A smart sensing system that achieves high performance for

low–pressure applications has been presented here. The key

performance advantage of the smart sensor system is that it

takes advantage of the fact that the output of the actual

sensing element is ratiometric (linearly proportional) to the

excitation voltage applied to the sensing element. A sensor

device is pulsed at a much higher than normally specified

voltage and a low duty–cycle for the purpose of increased

sensitivity. Although some of the sensor’s parasitic drawbacks

are increased in magnitude, some of the sensor’s negative

characteristics are lessened, and other sources of error and

noise in the system are reduced. The net effect is that a better

signal–to–noise ratio is obtained. This, combined with several

other performance–enhancing smart features, provides better

pressure resolution and accuracy than inherent in the sensor

device alone.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

MPXM2102AS Datasheet(PDF) 490 Page - Motorola, Inc

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