SA828
We must calculate the value of
pdy
that will give the required
pulse delay time:
AMP
7
AMP
6
AMP
5
AMP
4
AMP
3
AMP
2
AMP
1
AMP
0
AMPLITUDE
SELECT WORD
AMP
7
=
MSB
AMP0
=
LSB
pdy
f
CARR
x 512
⇒
pdy
=
t
pdy
x
f
CARR
x 512
t
pdy
=
= 5 x 10
–6
x 6 x 10
3
x 512 =
15·4
However, the value of
pdy
must be an integer. As the
purpose of the pulse delay is to prevent ‘shoot-through’ (where
both top and bottom arms of the inverter are on simultaneously),
it is sensible to round the pulse delay time up to a higher, rather
than a lower figure.
Thus, if we assign the value 16 to
pdy
this gives a delay time
of 5·2µs. From Table 6,
pdy
= 16 corresponds to a 6-bit PDY
word of 110000 in temporary register R2.
4. Setting the pulse deletion time
In setting the pulse deletion time (i.e., the minimum pulse
width) account must be taken of the pulse delay time, as the
actual minimum pulse width seen at the PWM outputs is equal
to
t
pd
–
t
pdy
.
Therefore, the value of the pulse deletion time must, in this
instance, be set 5·2µs longer than the minimum pulse length
required
Minimum pulse length required = 10µs
∴
t
PD
to be set to 10µs + 5·2µs = 15·2µs
Now,
pdt
t
pd
=
f
CARR
x 512
Fig.14 Temporary register R2
Amplitude selection
The power waveform amplitude is determined by scaling
the amplitude of the waveform samples stored in the ROM by
the value of the 8-bit amplitude select word (AMP).
The percentage amplitude control is given by:
A
x
100%
Power Amplitude,
A
POWER
=
255
where
A
= decimal value of AMP.
POWER-UP C0NDITIONS
All bits in both the Initialisation and Control registers power-
up in an unidentified state. Holding
RST
low or using the SET
TRIP input will ensure that the PWM outputs remain inactive
(i.e., low) until the device is initialised.
SA828 PROGRAMMING EXAMPLE
The following example assumes that a master clock of
12·288 MHz is used (12·288 MHz crystals are readily available).
This clock frequency will allow a maximum carrier frequency of
24 kHz and a maximum power frequency of 4 kHz.
Initialisation Register Programming Example
A power waveform range of up to 250Hz is required with a
carrier frequency of 6kHz, a pulse deletion time of 10µs and an
underlap of 5µs.
1. Setting the carrier frequency
The carrier frequency should be set first as the power
frequency, pulse deletion time and pulse delay time are all
defined relative to the carrier frequency.
We must calculate the value of
n
that will give the required
carrier frequency:
⇒
pdt
=
f
pd
x
f
CARR
x 512
= 15·2 x 10
–6
x 6 x 10
3
x 512 =
46·7
Again,
pdt
must be an integer and so must be either rounded
up or down – the choice of which will depend on the application.
Assuming we choose in this case the value 46 for
pdt,
this gives
a value of
t
pd
, of 15
µs
and an actual minimum pulse width of 15
– 5·2µs = 9·8µs.
From Table 7,
pdt
= 46 corresponds to a value of PDT, the
7-bit word in temporary register R0 of 1010010.
The data which must be programmed into the three temporary
registers R0, R1 and R2 (for transfer into the initialisation
register) in order to achieve the parameters in the example
given, is shown in Fig. 15.
k
512 x
n
k
12·288 x 10
6
⇒
n
=
=
=
4
512 x
f
CARR
512 x 6 x 10
3
f
CARR
=
From Table 4,
n
= 4 corresponds to a 3-bit CFS word of
010 in temporary register R1.
2. Setting the power frequency range
We must calculate the value of
m
that will give the required
power frequency:
f
f
RANGE
=
CARR
x
m
384
250 x 384
⇒
m
=
f
RANGE
x 384 =
6 x 10
3
f
CARR
=
16
Temporary Register R0
1
CR
1
0
1
0
0
1
0
PDT
6
PDT
5
PDT
4
PDT
3
PDT
2
PDT
1
PDT
0
Temporary Register R1
1
0
0
X
X
X
X
0
1
0
FRS
2
FRS
1
FRS
0
CFS
2
CFS
2
CFS
2
From Table 5,
m
= 16 corresponds to a 3-bit FRS word of
100 in temporary register R1.
3. Setting the pulse delay time
As the pulse delay time affects the actual minimum pulse width
seen at the PWM outputs, it is sensible to set the pulse delay time
before the pulse deletion time, so that the effect of the pulse delay
time can be allowed for when setting the pulse deletion time.
Temporary Register R2
X
X
X
X
1
1
0
0
0
0
PDY
5
PDY
4
PDY
3
PDY
2
PDY
1
PDY
0
Fig. 15
8
MITEL [ MITEL NETWORKS CORPORATION ]
