3

SA828

Fig. 2 SA828 internal block diagram

PULSE

DELETION

PULSE

DELAY

CIRCUIT

RPHT

RPHB

PULSE

DELETION

PULSE

DELAY

CIRCUIT

YPHT

YPHB

PULSE

DELETION

PULSE

DELAY

CIRCUIT

BPHT

BPHB

RED PHASE

YELLOW PHASE

BLUE PHASE

PHASING

AND

CONTROL

LOGIC

WAVEFORM

ROM

TRIP

LATCH

ADDRESS

GENERATOR

24-BIT

INITIALISATION

REGISTER

24-BIT

CONTROL

REGISTER

R0

R1

R2

R3

R4

BUS

DEMULTIPLEXER

BUS

CONTROL

CLOCK

DIVIDER

RST







MOTEL

INTERFACE

SYSTEM

BUS

8

SET

TRIP

TRIP

CS

CLOCK

ZPP

O/Ps WSS

FUNCTIONAL DESCRIPTION

An asynchronous method of PWM generation is used with

uniform or ‘double-edged’ regular sampling of the waveform

stored in the internal ROM as illustrated in Fig. 3.

The triangle carrier wave frequency is selectable up to 24kHz

(assuming the maximum clock frequency of 12.5MHz is used),

enabling ultrasonic operation for noise critical applications. With

12.5MHz clock, power frequency ranges of up to 4kHz are

possible, with the actual output frequency resolved to 12-bit

accuracy within the chosen range in order to give precise motor

speed control and smooth frequency changing. The output phase

sequence of the PWM outputs can also be changed to allow both

forward and reverse motor operation.

PWM output pulses can be ‘tailored’ to the inverter

characteristics by defining the minimum allowable pulse width

(the SA828 will delete all shorter pulses from the ‘pure’ PWM pulse

train) and the pulse delay (underlap) time, without the need for

external circuitry. This gives cost advantages in both component

savings and in allowing the same PWM circuitry to be used for

control of a number of different motor drive circuits simply by

changing the microprocessor software.

Power frequency amplitude control is also provided with an

overmodulationoptiontoassistinrapidmotorbraking.Alternatively,

braking may be implemented by setting the rotational speed to

0Hz. This is termed ‘DC injection braking’, in which the rotation of

the motor is opposed by allowing DC to flow in the windings.

AtripinputallowsthePWMoutputstobeshutdownimmediately,

overriding the microprocessor control in the event of an emergency.

The Waveform Sampling Synchronisation (WSS) output may

be used in conjunction with the ZPP signals to provide feedback

of the actual rotational speed from the rotor. This is of particular

use in slip compensated systems.

Other possible SA828 applications are as a 3-phase waveform

generator as part of a switched-mode power supply (SMPS) or of

an uninterruptible power supply (UPS). In such applications the

high carrier frequency allows a very small switching transformer

to be used.

MICROPROCESSOR INTERFACE

The SA828 interfaces to the controlling microprocessor by

means of a multiplexed bus of the MOTEL format. This interface

bus has the ability to adapt itself automatically to the format and

timingofbothMOTorolaandIntELinterfacebuses(henceMOTEL).

Internally, the detection circuitry latches the status of the DS/RD

line when AS/ALE goes high. If the result is high then the Intel

mode is used; if the result is low then the Motorola mode is used.

This procedure is carried out each time that AS/ALE goes high. In

practice this mode selection is transparent to the user. For bus

connection and timing information refer to the description relevant

to the microprocessor/controller being used.

Industry standard microprocessors such as the 8085, 8088,

etc. and microcontrollers such as the 8051, 8052 and 6805 are all

compatible with the interface on the SA828. This interface consists

multiplexed to carry both the address and data information, 3 bus

control lines, labelled WR,RD and ALE in Intel mode and R/W, DS

and AS in Motorola mode, and a Chip Select input, CS, which

allows the SA828 to share the same bus as other microprocessor

peripherals. It should be noted that all bus timings are derived from

the microprocessor and are independent of the SA828 clock

input.