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CDP68HC68W1 Datasheet PDF : 9 Pages
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CDP68HC68W1
Introduction
Functional Description
The digital pulse width modular (DPWM) divides down a
clock signal supplied via the CLK input as specified by the
control, frequency, and pulse width data registers. The
resultant output signal, with altered frequency and duty
cycle, appears at the output of the device on the PWM pin.
Functional Pin Description
VDD and VSS
These pins are used to supply power and establish logic lev-
els within the PWM. VDD is a positive voltage with respect to
VSS (ground).
CLK
The CLK pin is an input only pin where the clock signal to be
altered by the PWM circuitry is supplied. This is the source
of the PWM output. This input frequency can be internally
divided by either one or two, depending on the state of the
CD bit in the control register.
CS
The CS pin is the chip select input to the PWM’s SPI inter-
face. A high-to-low (1 to 0) transition selects the chip. A low-
to-high (0 to 1) transition deselects the chip and transfers
data from the shift registers to the data registers.
VT
The VT pin is the input to the voltage threshold comparator
on the PWM. An analog voltage greater than 0.75V (at VDD
= 5V) on this pin will immediately cause the PWM output to
go to logic “0”. This will be the status until the VT input is
returned to a voltage below 0.4V, the W1 is deselected, and
then one or more of the data registers is written to.
An analog voltage on this pin less than 0.75V (at VDD = 5V)
will allow the device to operate as specified by the values in
the registers.
DATA
Data input at this pin is clocked into the shift register (i.e.,
latched) on the rising edge of the serial clock (SCK), most
significant bits first.
SCK
The SCK pin is the serial clock input to the PWM’s SPI inter-
face. A rising edge on this pin will shift data available at the
(DATA) pin into the shift register.
PWM
This pin provides the resultant output frequency and pulse
width. After VDD power up, the output on this pin will remain
a logic “0”, until the chip is selected, 24 bits of information
clocked in, and the chip deselected.
Serial Port
Data are entered into the three DPWM registers serially
through the DATA pin, accompanied by a clock signal applied
to the SCK. The user can supply these serial data via shift
register(s) or a microcontroller’s serial port, such as the SPI
port available on most CDP68HC05 microcontrollers. Micro-
controller I/O lines can also be used to simulate a serial port.
Data are written serially, most significant bit first, in 8, 16 or
24-bit increments. Data are sampled and shifted into the
PWMs shift register on each rising edge of the SCK. The
serial clock should remain low when inactive. Therefore,
when using a 68HC05 microcontroller’s SPI port to provide
data, program the microcontroller’s SPI control register bits
CPOL, CPHA to 0, 0.
The CDP68HC68W1 latches data words after device
deselection. Therefore, CS must go high (inactive) following
each write to the W1.
Power-Up Initialization
Upon VDD power up, the output of the PWM chip will remain
at a low level (logic zero) until:
1. The chip is selected (CS pin pulled low).
2. 24-bit of information are shifted in.
3. The chip is deselected (CS pin pulled high).
The 24-bits of necessary information pertain to the loading
of the PWM 8-bit registers, in the following order:
1. Control register
2. Frequency register
3. Pulse width register
See section entitled Pulse Width Modulator Data Regis-
ters for a description of each register. Once initialized, the
specified PWM output signal will appear until the device is
reprogrammed or the voltage on the VT pin rises above the
specified threshold. Reprogramming the device will update
the PWM output after the end of the present output clock
period.
Reprogramming Shortcuts
After the device has been fully programmed upon power up,
it is only necessary to input 8 bits of information to alter the
output pulse width, or 16 bits to alter the output frequency.
Altering the Pulse Width: The pulse width may be
changed by selecting the chip, inputting 8 bits, and dese-
lecting the chip. By deselecting the chip, data from the first
8-bit shift register are latched into the pulse width register
(PWM register). The frequency and control registers
remain unchanged. The updated PWM information will
appear at the output only after the end of the previous total
output period.
Altering the Frequency: The frequency can be changed by
selecting the chip, inputting 16 bits (frequency information
followed by pulse width information), and deselected the
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