14

Synchronous Serial Port

The synchronous serial port is used to set and read param-

eters in the sensor, and to read out the motion informa-

tion. The serial port is also used to load SROM data into

the sensor.

The port is a four wire port. The host micro-controller al-

ways initiates communication; the sensor never initiates

data transfers. SCLK, MOSI, and NCS may be driven di-

rectly by a micro-controller. The port pins may be shared

with other SPI slave devices. When the NCS pin is high, the

inputs are ignored and the MISO is tri-stated as long as the

supplies valid.

The lines that comprise the SPI port are:

SCLK:

Clock input. It is always generated by the master

(the micro-controller).

MOSI:

Input data. (Master Out/Slave In)

MISO:

Output data. (Master In/Slave Out)

NCS:

Chip select input (active low).

NCS needs to be low to activate the serial port;

otherwise, MISO will be high Z, and MOSI & SCLK

will be ignored.

NCS can also be used to reset the serial port in case

of an error.

Motion Pin Timing

The motion pin is an active low output that signals the mi-

cro-controller when motion has occurred. The motion pin

is lowered whenever the motion bit is set; in other words,

whenever there is non-zero data in the Delta_X or Delta_Y

registers. Clearing the motion bit (by reading Delta_X and

Delta_Y, or writing to the Motion register) will put the mo-

tion pin high. The motion pin can also return high if either

one or both the Delta_X and Delta_Y registers were non-

zero (motion pin low) and then subsequently both reg-

isters return to zero data (motion pin high). For example

if there is minute motion that causes one or both delta

registers to go to one count (motion pin low) and then

subsequently both registers returning back to zero count

(motion pin high).

Chip Select Operation

The serial port is activated after NCS goes low. If NCS is

raised during a transaction, the entire transaction is abort-

ed and the serial port will be reset. This is true for all trans-

actions including PROM download. After a transaction is

aborted, the normal address-to-data or transaction-to-

transaction delay is still required before beginning the

next transaction. To improve communication reliability,

all serial transactions should be framed by NCS. In other

words, the port should not remain enabled during peri-

ods of non-use because ESD and EFT/B events could be

interpreted as serial communication and put the chip into

an unknown state. In addition, NCS must be raised after

each burst-mode transaction is complete to terminate

burst-mode. The port is not available for further use until

burst-mode is terminated.

Write Operation

Write operation, defined as data going from the micro-

controller to the sensor, is always initiated by the micro-

controller and consists of two bytes. The first byte contains

the address (seven bits) and has a “1” as its MSB to indicate

data direction. The second byte contains the data. The

sensor reads MOSI on rising edges of SCLK.

Figure 8 MOSI Setup and Hold Time

Figure 9. Write Operation

SCLK

MOSI

t

Hold,MOSI

15

7

8

9

10

11

12

13

14

16

2

3

4

5

6

1

SCLK

MOSI

MOSI Driven by Micro-Controller

1

1

1

2

NCS

MISO

PixArt Imaging Inc.

Track-on-Glass Laser Sensor

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PixArt Imaging Inc.