HT56R66 Holtek Semiconductor Inc., HT56R66 Datasheet - Page 47

• Manufacturer Part Number: HT56R66
• Description: Tinypower Tm A/d Type With Lcd 8-bit Otp Mcu
• Manufacturer: Holtek Semiconductor Inc.

Pulse Width Modulator
The devices contains a series of Pulse Width Modula-
tion, PWM, outputs. Useful for such applications such as
motor speed control, the PWM function provides an out-
put with a fixed frequency but with a duty cycle that can
be varied by setting particular values into the corre-
sponding PWM register.
PWM Overview
A register pair, located in the Data Memory is assigned
to each Pulse Width Modulator output and are known as
the PWM registers. It is in each register pair that the
12-bit value, which represents the overall duty cycle of
one modulation cycle of the output waveform, should be
placed. The PWM registers also contain the enable/dis-
able control bit for the PWM outputs. To increase the
PWM modulation frequency, each modulation cycle is
modulated into sixteen individual modulation
sub-sections, known as the 8+4 mode. Note that it is
only necessary to write the required modulation value
into the corresponding PWM register as the subdivision
of the waveform into its sub-modulation cycles is imple-
mented automatically within the microcontroller hard-
ware. The PWM clock source is the system clock f
This method of dividing the original modulation cycle
into a further 16 sub-cycles enables the generation of
higher PWM frequencies, which allow a wider range of
applications to be served. As long as the periods of the
generated PWM pulses are less than the time constants
of the load, the PWM output will be suitable as such long
time constant loads will average out the pulses of the
PWM output. The difference between what is known as
the PWM cycle frequency and the PWM modulation fre-
quency should be understood. As the PWM clock is the
system clock, f
wide, the overall PWM cycle frequency is f
However, when in the 8+4 mode of operation, the PWM
modulation frequency will be f
Rev. 1.10
All devices
Part No.
Modulation
Frequency
f
SYS
PWM
/256
Channels
SYS
4
, and as the PWM value is 12-bits
PWM Cycle
Frequency
f
SYS
Mode
/4096
PWM
8+4
SYS
/256.
Output
PD0~
PD3
Pin
(PWM register
PWM Cycle
value)/4096
Duty
PWM0H~
PWM3H
PWM0L~
PWM3L
Register
Names
SYS
/4096.
SYS
.
47
8+4 PWM Mode Modulation
Each full PWM cycle, as it is 12-bits wide, has 4096 clock
periods. However, in the 8+4 PWM mode, each PWM cy-
cle is subdivided into sixteen individual sub-cycles known
as modulation cycle 0 ~ modulation cycle 15, denoted as
tains 256 clock cycles. In this mode, a modulation fre-
quency increase of sixteen is achieved. The 12-bit PWM
register value, which represents the overall duty cycle of
the PWM waveform, is divided into two groups. The first
group which consists of bit4~bit11 is denoted here as the
DC value. The second group which consists of bit0~bit3
is known as the AC value. In the 8+4 PWM mode, the
duty cycle value of each of the two modulation sub-cycles
is shown in the following table.
The accompanying diagram illustrates the waveforms
associated with the 8+4 mode of PWM operation. It is
important to note how the single PWM cycle is subdi-
vided into 16 individual modulation cycles, numbered
0~15 and how the AC value is related to the PWM value.
PWM Output Control
The four PWM0~PWM3 outputs are shared with pins
PD0~PD3. To operate as a PWM output and not as an
I/O pin, bit 0 of the relevant PWM register bit must be set
high. A zero must also be written to the corresponding
bit in the PDC port control register, to ensure that the
PWM0 output pin is setup as an output. After these two
initial steps have been carried out, and of course after
the required PWM 12-bit value has been written into the
PWM register pair register, writing a 1 to the corre-
sponding PD data register will enable the PWM data to
appear on the pin. Writing a 0 to the bit will disable the
PWM output function and force the output low. In this
way, the Port D data output register bits, can also be
used as an on/off control for the PWM function. Note
that if the enable bit in the PWM register is set high to
enable the PWM function, but a 1 has been written to
its corresponding bit in the PDC control register to con-
figure the pin as an input, then the pin can still function
as a normal input line, with pull-high resistor selections.
i in the table. Each one of these sixteen sub-cycles con-
Modulation cycle i
Parameter
(i=0~15)
8+4 Mode Modulation Cycle Values
HT56R66/HT56R666
AC (0~15)
i AC
i AC
September 8, 2009
DC (Duty Cycle)
DC+1
256
256
DC