33291 datasheet(13/27 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

33291

FUNCTIONAL DESCRIPTION

INTRODUCTION

Figure 14. 33291 SPI System Daisy Chain

Multiple 33291 devices can also be controlled in a parallel

input fashion using SPI, illustrated in Figure 15. This figure

shows a possible 24 loads being controlled by only three

dedicated parallel MCU ports used for chip select.

Figure 15. Parallel Input SPI Control

Figure 16, page 14, illustrates a basic method of

controlling multiple 33291 devices using two MCUs. A system

can have only one master MCU at any given instant of time

and one or more slave MCUs. Master control of the system

must pass from one MCU to the other in an orderly manner.

The master MCU supplies the system clock signal (top MCU

designated the master); the lower MCU being the slave. It is

possible to have a system with more than one master;

however, not at the same time. Only when the master is not

communicating can a slave assume the mastership and

communicate. MCU master control is switched through the

use of the slave select (SS) pin of the MCUs. A master will

become a slave when it detects a logic low state on its SS pin.

These basic examples make the 33291 very attractive for

applications where a large number of loads require efficient

control. To this end, the popular Synchronous Serial

Peripheral Interface (SPI) protocol is incorporated to

communicate efficiently with the MCU.

SPI SYSTEM ATTRIBUTES

The SPI system is flexible enough to communicate directly

with numerous standard peripherals and MCUs available

from Motorola and other semiconductor manufacturers. SPI

reduces the number of pins necessary for input/output (I/O)

on the 33291. It also offers an easy means of expanding the

I/O function using few MCU pins. The SPI system of

communication consists of the MCU transmitting, in return it

receives one data-bit of information per system clock cycle.

Data bits of information are simultaneously transmitted by

one pin, Master Out Serial In (MOSI), and received by

another pin, Master In Serial Out (MISO), of the MCU.

Some features of SPI are as follows:

• Full duplex, three-wire synchronous data transfer

• Each microcontroller can be a master or a slave

• Provides write collision flag protection

• Provides end of message interrupt flag

• Four I/Os associated with SPI (MOSI, MISO, SCLK, SS)

• Drawbacks to SPI are as follows:

• An MCU is required for efficient operational control

• In contrast to parallel input control it is slower at performing

pulse width modulating (PWM) functions.

SCLK

Parallel Port

MISO

MOSI

IRQ

MC68XX

Microcontroller

SPI

SCLK

33291

8 Outputs

Parallel

Ports

MC68XX

Microcontroller

SPI

SCLK

33291

SCLK

MOSI

33291 Datasheet(PDF) 13 Page - Freescale Semiconductor, Inc

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