SA828
MICROPROCESSOR BUS TIMING
Intel Mode (Fig. 4 and Table 1)
The address is latched by the falling edge of ALE. Data is
written from the bus into the SA828 on the rising edge of
WR.
RD
is not used in this mode because the registers in the SA828
are write only. However, this pin must be connected to
RD
(or
tied high) to enable the SA828 to select the correct interface
format.
Power frequency range
This sets the maximum power frequency that can be carried
within the PWM output waveforms. This would normally be set
to a value to prevent the motor system being operated outside
its design parameters.
Pulse delay time ('underlap')
For each phase of the PWM cycle there are two control
signals, one for the top switch connected to the positive
inverter DC supply and one for the bottom switch connected to
the negative inverter DC supply. In theory, the states of these
two switches are always complementary. However, due to the
finite and non-equal turn-on and turn- off times of power
devices, it is desirable when changing the state of the output
pair, to provide a short delay time during which both outputs
are off in order to avoid a short circuit through the switching
elements.
Pulse deletion time
A pure PWM sequence produces pulses which can vary in
width between 0% and 100% of the duty cycle. Therefore, in
theory, pulse widths can become infinitesimally narrow. In
practice this causes problems in the power switches due to
storage effects and therefore a minimum pulse width time is
required. All pulses shorter than the minimum specified are
deleted.
Counter reset
This facility allows the internal power frequency counter of
the SA828 to be set to zero, disabling the normal frequency
control and giving a 50% output duty cycle.
Motorola Mode (Fig. 5 and Table 2)
The address is latched on the falling edge of the AS line. Data
is written from the bus into the SA828 (only when R/W is low) on
the falling edge of DS (providing
CS is
low).
CONTROLLING THE SA828
The SA828 is controlled by loading data into two 24-bit
registers via the microprocessor interface. These registers are
the initialisation register and the control register.
The initialisation register would normally be loaded before
motor operation (i.e., prior to the PWM outputs being activated)
and sets up the basic operating parameters associated with the
motor and inverter. This data would not normally be updated
during motor operation.
The control register is used to control the PWM outputs (and
hence the motor) during operation e.g., stop/start, speed,
forward/reverse etc. and would normally be loaded and changed
only after the initialisation register has been loaded.
As the MOTEL bus interface is restricted to an 8-bit wide
format, data to be loaded into either of the 24-bit registers is first
written to three 8-bit temporary registers R0, R1 and R2 before
being transferred to the desired 24-bit register. The data is
accepted (and acted upon) only when transferred to one of the
24-bit registers.
Transfer of data from the temporary registers to either the
initialisation register or the control register is achieved by a write
instruction to a dummy register. Writing to dummy register R3
results in data transfer from R0, R1 and R2 to the control
register, while writing to dummy register R4 transfers data from
R0, R1 and R2 to the initialisation register. It does not matter
what data is written to the dummy registers R3 and R4 as they
are not real registers. It is merely the write instruction to either
of these registers which is acted upon in order to load the
initialisation and control registers.
AD
2
0
0
0
0
1
AD
1
0
0
1
1
0
AD
0
0
1
0
1
0
Register
R0
R1
R2
R3
R4
Comment
Temporary register R0
Temporary register R1
Temporary register R2
Transfers control data
Transfers initialisation data
Initialisation Register Programming
The initialisation register data is loaded in 8-bit segments into
the three 8-bit temporary registers R0-R2. When all the initialisation
data has been loaded into these registers it is transferred into the
24-bit initialisation register by writing to the dummy register R4.
FRS
2
FRS
1
FRS
0
X
X
CFS
2
CFS
2
CFS
2
FREQUENCY
RANGE
SELECT WORD
FRS
2
=
MSB
FRS
0
=
LSB
DON’T
CARE
Fig. 6 Temporary register R1
Carrier frequency selection
The carrier frequency is a function of the externally applied
clock frequency and a division ratio
n,
determined by the 3-bit
CFS word set during initialisation. The values of
n
are selected
as shown in Table 4.
CFS word
Value of n
101 100 011 010 001 000
32
16
8
4
2
1
Table 3 SA828 register addressing
Initialisation Register Function
The 24-bit initialisation register contains parameters which,
under normal operation, will be defined during the power-up
sequence. These parameters are particular to the drive circuitry
used, and therefore changing these parameters during a PWM
cycle is not recommended. Information in this register should
only be modified while
RST
is active (i.e. low) so that the PWM
outputs are inhibited (low) during the updating process.
The parameters set in the initialisation register are as follows:
Carrier frequency
Low carrier frequencies reduce switching losses whereas
high carrier frequencies increase waveform resolution and can
allow ultrasonic operation.
Table 4 Values of clock division ratio n
The carrier frequency,
f
CARR
,
is then given by:
k
f
CARR
=
512 x
n
where
k
= clock frequency and
n
= 1, 2, 4, 8, 16 or 32 (as set
by CFS)
Power frequency range selection
The power frequency range selected here defines the maximum
limit of the power frequency. The operating power frequency is
controlled by the 12-bit Power Frequency Select (PFS) word in
the control register but may not exceed the value set here.
CARRIER
FREQUENCY
SELECT WORD
CFS
2
=
MSB
CFS
0
=
LSB
5
MITEL [ MITEL NETWORKS CORPORATION ]
