
AD7863
–
15
–
REV. A
AD7863–MC68000 Interface
An interface between the AD7863 and the MC68000 is shown
in Figure 17. As before, conversion can be supplied from the
MC68000 or from an external source. The AD7863 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 AD7863 is attempted. Because of the nature of its
interrupts, the MC68000 requires additional logic (not shown in
Figure 18) to allow it to be interrupted correctly. For further
information on MC68000 interrupts, consult the MC68000 users
manual.
The MC68000
AS
and R/
W
outputs are used to generate a
separate
RD
input signal for the AD7863.
CS
is used to drive the
68000
DTACK
input to allow the processor to execute a normal
read operation to the AD7863. The conversion results are read
using the following 68000 instruction:
MOVE.W ADC
,
D
0
where
D
0 is the 68000
D
0 register and
ADC
is the AD7863
address.
ADDR
DECODE
EN
ADDRESS BUS
A15
A0
DTACK
AS
D15
D0
CS
A0
RD
DB13
DB0
DATA BUS
MC68000
AD7863*
*ADDITIONAL PINS OMITTED FOR CLARITY
OPTIONAL
CONVST
R/
W
Figure 17. AD7863
–
MC68000 Interface
AD7863–80C196 Interface
Figure 18 shows an interface between the AD7863 and the
80C196 microprocessor. Here, the microprocessor initiates
conversion. This is achieved by gating the 80C196
WR
signal
with a decoded address output (different from the AD7863
CS
address). The AD7863 BUSY line is used to interrupt the mi-
croprocessor when the conversion sequence is completed.
ADDR
DECODE
EN
ADDRESS BUS
A15
A1
WR
D15
D0
CS
A0
BUSY
RD
DB13
DB0
DATA BUS
80C196
AD7863*
*ADDITIONAL PINS OMITTED FOR CLARITY
RD
Figure 18. AD7863
–
80C196 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 cou-
pling effect can reduce the performance of the motor because,
for example, if the torque is increased by increasing the fre-
quency, 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 trans-
forms the three phase drive currents into separate torque and
flux components. The AD7863 is ideally suited for use in vector
motor control applications.
A block diagram of a vector motor control application using the
AD7863 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
can be calculated if two phases are known. V
A1
and V
A2
of the
AD7863 are used to digitize this information.
Simultaneous sampling is critical to maintain the relative phase
information between the two channels. A current sensing isola-
tion amplifier, transformer or Hall effect sensor is used between
the motor and the AD7863. Rotor information is obtained by
measuring the voltage from two of the inputs to the motor. V
B1
and V
B2
of the AD7863 are used to obtain this 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 informa-
tion fed back by the AD7863.
DSP
MICROPROCESSOR
DAC
DRIVE
CIRCUITRY
3
PHASE
MOTOR
I
C
I
B
I
A
V
B
V
A
ISOLATION
AMPLIFIERS
VOLTAGE
ATTENUATORS
TORQUE
SETPOINT
FLUX
SETPOINT
*ADDITIONAL PINS OMITTED
FOR CLARITY
TRANSFORMATION
TO TORQUE &
FLUX CURRENT
COMPONENTS
DAC
DAC
AD7863*
V
A1
V
A2
V
B1
V
B2
TORQUE & FLUX
CONTROL LOOP
CALCULATIONS &
TWO TO THREE
PHASE
INFORMATION
Figure 19. Vector Motor Control Using the AD7863