SA828

10

PRODUCT DESIGNATION

Two standard options exist, defining waveform shape. These

are designated SA828-1 and SA828-2 as follows:

SA828-1

Sine + third harmonic at one-sixth the amplitude of the

fundamental:

x(t) = A [sin (

ωt) + sin 3(ωt)]

SA828-2

Pure sinewave:

x(t) = A [sin (

ωt)]

Additional wave shapes can be implemented to order, provided

they are symmetrical about the 90

°, 180° and 270° axes.

Contact your local Mitel Semiconductor Customer Service

Centre for further details.

1

6

HARDWARE INPUT/OUTPUT FUNCTIONS

Set Output Trip (SET TRIP input)

The SET TRIP input is provided separately from the

microprocessor interface in order to allow an external source

to override the microprocessor and provide a rapid shutdown

facility. For example, logic signals from overcurrent sensing

circuitry or the microprocessor ‘watchdog’ might be used to

activate this input.

When the SET TRIP input is taken to a logic high, the output

trip latch is activated. This results in the TRIP output and the

six PWM outputs being latched low immediately. This condition

can only be cleared by applying a reset cycle to the RST input.

It is essential that when not in use SET TRIP is tied low and

isolated from potential sources of noise; on no account should

it be left floating.

SET TRIP is latched internally at the master clock rate in

order to reduce noise sensitivity.

Output Trip Status (TRIP output)

The TRIP output indicates the status of the output trip latch

and is active low.

Reset (RST input)

The RST input performs the following functions when active

(low):

1. All PWM outputs are forced low (if not already low) thereby

turning off the drive switches.

2. All internal counters are reset to zero (this corresponds to 0

°

for the red phase output).

3. The rising edge of RST reactivates the PWM outputs

resetting the output trip and setting the TRIP output high –

assuming that the SET TRIP input is inactive (i.e. Iow).

A sixth register, R5, located at A2:O = 101 is used to place

the device into a factory test mode. This is achieved by writing

dummy data to R5 immediately after RST goes high. Care

must be exercised to ensure that the microprocessor/controller

cannot write to this register.

Zero Phase Pulses (ZPPR, ZPPY and ZPPB outputs)

The zero phase pulse outputs provide pulses at the same

frequency as the power frequency with a 1 : 2 mark-space

ratio. When in the forward mode of operation the falling edge

of ZPPR corresponds to 0

° for the red phase, the falling edge

of ZPPY to 0

° for the yellow phase and the ZPPB falling edge

to 0

° for the blue phase. In the reverse mode, the rising edge

of a zero phase pulse corresponds to 0

° for the relevant phase

PWM output.

Waveform Sampling Synchronisation (WSS output)

This output provides a square wave signal of 50% duty

cycle at a frequency 1536 times higher than the fundamental

of the power waveform. Each successive pulse of WSS

corresponds to the SA828 reading the next location of the

waveform ROM. It may be used in conjunction with the ZPP

signals to monitor the position of the machine rotor and may

form part of a closed loop control system such as slip

compensation.

Clock (CLK input)

The CLK input provides a timing reference used by the

SA828 for all timings related to the PWM outputs. The

microprocessor interface, however, derives all its timings from

the microprocessor and therefore the microprocessor and the

SA828 may be run either from the same or from different

clocks.

WAVEFORM DEFINITION

The waveform amplitude data used to construct the PWM

output sequences is read from the internal 384X8 ROM. This

contains the 90

° span of the waveform as shown in Fig. 18.

Each successive 8-bit sample linearly represents the

instantaneous amplitude of the waveform. It is assumed that

the waveform is symmetrical about the 90

°, 180° and 270° axes.

The SA828 reconstructs the full 360

° waveform by reading the

0

°-90° section held in ROM and assigning negative values for

the second half of the cycle.

These samples are used to calculate the instantaneous

amplitudes for all three phases, which will be 120

° transposed

in the normal R-Y-B orientation for forward rotation or B-Y-R for

reverse rotation. The 384 8-bit samples are regularly spaced

over the 0

° to 90° span, giving an angular resolution of

approximately 0·23

°.

Waveform segment

0°- 30°

30·23°- 60°

60·23°- 89·77°

Sample number

0 - 127

128 - 255

256 - 383

Table 8 90

° of the 360° cycle is divided into 384 8-bit

samples

Fig. 18 90

° sample of typical power waveform

VALUE

OF

8-BIT

SAMPLE

0

°

45

°

90

°

255

0

PHASE (384-BIT RESOLUTION)

POWER

WAVEFORM