FPS Series Pressure Sensor

SPI - SERIAL PERIPHERAL INTERFACE

Page 8

Introduction

The FPS serial interface is a high-speed data input

and output communication port that is asynchronous

to the internal conversion cycle. The serial interface

operates in 3-wire SPI mode. In this setup, the MOSI

and MISO pins are tied together, forming a bidirec-

tional data line to the master microcontroller. The

remaining serial interface connections consist of chip

select (/CS) and serial clock (SCLK).

Care should be taken to ensure that the sensor is

properly connected to the master microcontroller.

Refer to the manufacturer's datasheet for more infor-

mation regarding physical connections.

Signal control

The serial interface is enabled by asserting /CS low.

The serial input clock, SCLK, is gated internally to

begin accepting the input data at MOSI, or sending

the output data on MISO. When /CS rises, the data

clocked into MOSI is loaded into an internal register.

Note: If SPI communication is not required (i.e. only

analog output is being used) pins 6 to 8 must be

connected to ground and pin 9 to Vs. Failure to do so

may result in damage to the sensor.

Note: It is important to adhere to the communication protocol in order to avoid damage to the Sensor.

Data read

When powered on, the sensor begins to continuous-

ly measure pressure. To initiate data transfer from

the sensor, the following three unique bytes must be

written sequentially, MSB first, to the MOSI pin (see

Fig. 1):

Step Hexadecimal

Binary

Description

1

0x2D

B00101101 Poll current pressure measurement

2

0x14

B00010100 Send result to data register

3

0x98

B10011000 Read data register

The entire 16 bit content of the FPS register is then

read out on the MISO pin, MSB first, by applying 16

successive clock pulses to SCLK with /CS asserted

low. Note that the value of the FPS is held at zero

for internal signal processing purposes. This is

below the noise threshold of the sensor and thus its

fixed value does not affect sensor performance and

accuracy.

From the digital sensor output the actual pressure

value can be calculated as follows:

So for a 1.0 inH2O sensor (FPSS250B...) with a

scale factor of 120 a digital output of 30,000 counts

(7530’h) calculates to a positive pressure of 1.0

inH2O. Similarly, a digital output of -30,000 counts

(8AD0’h) calculates to a negative pressure of -1.0

inH2O.

Fig. 1 : Data Transfer

SCLK

(CPOL=0)

(CPAH=0)

MOSI

MISO

/CS

0

10

1

1

1

0

0

0

10

1

0

0

0

0

Step 2

Step 1

SCLK

(CPOL=0)

(CPAH=0)

MOSI

MISO

/CS

1

0

1

1

0

0

0

Data from sensor

Step 3

0

15 14 13 12 11 10

98765

4

321

0

MSB

scale factor

Pressure [inH2O] = digital output [counts]

counts

inH2O