AD7862

–14–

REV. 0

Some applications may require that the conversion be initiated

by the microprocessor rather than an external timer. One option

is to decode the AD7862 CONVST from the address bus so

that a write operation starts a conversion. Data is read at the

end of the conversion sequence as before. Figure 18 shows an

example of initiating conversion using this method. Note that

for all interfaces, it is preferred that a read operation not be

attempted during conversion.

AD7862–MC68000 Interface

An interface between the AD7862 and the MC68000 is shown

in Figure 17. As before, conversion can be supplied from the

MC68000 or from an external source. The AD7862 BUSY line

can be used to interrupt the processor or, alternatively, software

delays can ensure that conversion has been completed before a

read to the AD7862 is attempted. Because of the nature of its

interrupts, the 68000 requires additional logic (not shown in

Figure 18) to allow it to be interrupted correctly. For further

information on 68000 interrupts, consult the 68000 user’s manual.

The MC68000 AS and R/W outputs are used to generate a

separate RD input signal for the AD7862. CS is used to drive

the 68000 DTACK input to allow the processor to execute a

normal read operation to the AD7862. The conversion results

are read using the following 68000 instruction:

MOVE.W ADC,D0

where D0 is the 68000 D0 register, and ADC is the AD7862

address.

A0

A15

D15

D0

EN

ADDR

DECODE

ADDRESS BUS

OPTIONAL

DATA BUS

CONVST

CS

DB11

DB0

RD

AD7862*

MC68000

*ADDITIONAL PINS OMITTED FOR CLARITY

DTACK

R/W

AS

A0

Figure 17. AD7862–MC68000 Interface

AD7862–80C196 Interface

Figure 18 shows an interface between the AD7862 and the

80C196 microprocessor. Here, the microprocessor initiates

conversion. This is achieved by gating the 80C196 WR signal

with a decoded address output (different to the AD7862 CS

address). The AD7862 BUSY line is used to interrupt the

microprocessor when the conversion sequence is completed.

D15

D0

ADDR

DECODE

ADDRESS BUS

ADDRESS/DATA BUS

CONVST

CS

DB11

DB0

RD

AD7862*

80C196

*ADDITIONAL PINS OMITTED FOR CLARITY

WR

RD

A0

A15

A1

Figure 18. AD7862–8086 Interface

Vector Motor Control

The current drawn by a motor can be split into two compo-

nents: one produces torque, and the other produces magnetic

flux. For optimal performance of the motor, these two compo-

nents should be controlled independently. In conventional

methods of controlling a three-phase motor, the current (or

voltage) supplied to the motor and the frequency of the drive are

the basic control variables; however, both the torque and flux

are functions of current (or voltage) and frequency. This

coupling effect can reduce the performance of the motor

because, if the torque is increased by increasing the frequency,

for example, the flux tends to decrease.

Vector control of an ac motor involves controlling phase in

addition to drive and current frequency. Controlling the phase

of the motor requires feedback information on the position of

the rotor relative to the rotating magnetic field in the motor.

Using this information, a vector controller mathematically

transforms the three phase drive currents into separate torque

and flux components. The AD7862, with its four-channel

simultaneous sampling capability, is ideally suited for use in

vector motor control applications.

A block diagram of a vector motor control application using the

AD7862 is shown in Figure 19. The position of the field is

derived by determining the current in each phase of the motor.

Only two phase currents need to be measured because the third

AD7862 are used to digitize this information.

Simultaneous sampling is critical to maintain the relative phase

information between the two channels. A current sensing

isolation amplifier, transformer or Hall effect sensor is used

between the motor and the AD7862. Rotor information is

obtained by measuring the voltage from two of the inputs to the

information. Once again, the relative phase of the two channels

is important. A DSP microprocessor is used to perform the

mathematical transformations and control loop calculations on

the information fed back by the AD7862.