7

SA828

Fig. 10 The effect of the pulse deletion circuit

PWM SIGNAL

BEFORE

PULSE DELETION

PWM SIGNAL

AFTER

PULSE DELETION

PULSE

DELETED

PULSE

DELETED

where

power frequency range set in the initialisation register.

POWER

FREQUENCY

SELECT WORD

BITS 0-7

Fig. 11 Temporary register R0

F/R

OM

INH

X

POWER

FREQUENCY

SELECT WORD

BITS 8-11

DON’T

CARE

OVERMOD-

ULATION

BIT

0 = DISABLED

1 = ACTIVE

OUTPUT

INHIBIT BIT

0 = OUTPUTS DISABLED

1 = OUTPUTS ACTIVE

FORWARD/

REVERSE BIT

0 = FORWARD

1 = REVERSE

Fig. 12 Temporary register R1

Power frequency selection

The power frequency is selected as a proportion of the power

frequency range (defined in the initialisation register) by the 12-

bit power frequency select word, PFS, allowing the power

frequency to be defined in 4096 equal steps. As the PFS word

spans the two temporary registers R0 and R1 it is therefore

essential, when changing the power frequency, that both these

registers are updated before writing to R3.

The power frequency (

Control Register Programming

The control register should only be programmed once the

initialisation register contains the basic operating parameters of

the SA828.

As with the initialisation register, control register data is

loaded into the three 8-bit temporary registers R0 - R2. When all

the data has been loaded into these registers it is transferred

into the 24-bit control register by writing to the dummy register

R3. It is recommended that all three temporary registers are

updated before writing to R3 in order to ensure that a conformal

set of data is transferred to the control register for execution.

4096

x

pfs

OVERMODULATION BIT NOT SET

(100% MODULATION)

OVERMODULATION BIT SET

(200% MODULATION)

V

V

0

0

t

t

Fig. 13 Current waveforms as seen at the motor terminals,

showing the effect of setting the overmodulation bit

Forward/ reverse selection

The phase sequence of the three-phase PWM output

waveforms is controlled by the Forward/Reverse bit F/R. The

actual effect of changing this bit from 0 (forward) to 1 (reverse)

is to reverse the power frequency phase counter from

incrementing the phase angle to decrementing it. The required

output waveforms are all continuous with time during a forward/

reverse change.

In the forward mode the output phase sequence is red-

yellow-blue and in the reverse mode the sequence is blue-

yellow-red.

Output inhibit selection

When active (i.e., Iow) the output inhibit bit INH sets all the

PWM outputs to the off (low) state. No other internal operation

of the device is affected. When the inhibit is released the PWM

outputs continue immediately. Note that as the inhibit is asserted

after the pulse deletion and pulse delay circuits, pulses shorter

than the normal minimum pulse width may be produced initially.

Overmodulation selection

The overmodulation bit OM is, in effect, the ninth bit (MSB)

of the amplitude word. When active (i.e., high) the output

waveform will be controlled in the 100% to 200% range by the

amplitude word.

The percentage amplitude control is now given by:

Overmodulated Amplitude =

where