10
5 Theory of Operation
Keyboard Circuitry
The CADD‑Prizm
®
PCS II pump is controlled
by a microprocessor. The actions of the
microprocessor are controlled by a program,
which is contained in the memory.
Commands are issued to the microprocessor
from the user via the nine keys on the
keyboard and the Remote Dose cord. The keys
on the keyboard feed individually into the Gate
Array on the microprocessor board. A key
closure applies a ground to the associated
input of the Gate Array. Key debounce
circuitry resident in the Gate Array provides
a clean output signal to the microprocessor
for the duration of the key closure. The
microprocessor reads keyboard status by
accessing special memory locations in the
Gate Array.
The Remote Dose button consists of an SPDT
switch with its own dedicated input to the
microprocessor circuitry. The switch has a
common input line and two output signal
lines. The two signal lines are complementary
such that one line is always logic high and the
other is always low. When the Remote Dose
button is pressed, both signal lines change
to the alternate logic state. This redundancy
prevents a single line failure from starting a
dose delivery.
Data Memory EEPROM
Many settings of the pump’s delivery and
record keeping parameters are stored by the
microprocessor in an Electrically Erasable
Programmable Read Only Memory (EEPROM).
Data to and from the memory is presented
serially. Whenever the microprocessor uses
data from the EEPROM, the data is checked
for validity.
Battery Backed RAM
Additional settings of the pump’s delivery
and record keeping parameters are stored
in a battery backed Random Access Memory
(RAM). Battery backup is provided by two
printed circuit board‑mounted lithium
batteries. These batteries are designed to
provide a minimum of five years of memory
retention during normal pump usage.
Whenever the microprocessor uses data from
the RAM, the data is checked for validity.
Time Base Circuitry
An accurate 3.6864 MHz timebase is provided
by a quartz crystal. The 3.6864 MHz signal is
connected to the microprocessor, where it
is frequency‑divided to access the program
memory at a cycle rate of 921 kHz.
In addition, an accurate 32.768 kHz timebase is
provided by a second quartz crystal. The 32.768
kHz signal is used for the real time clock.
LCD Circuitry
The high‑impedance, low‑power, special
drive signals for the liquid crystal display are
provided by the LCD‑drivers. Each alpha or
numeric character on the LCD is formed by
darkening combinations of dots. Commands to
display dots are issued via data bus commands
to the LCD‑drivers by the microprocessor.
The LCD circuit also contains a power supply
which provides bias voltage to the LCD panel.
This voltage controls the relative brightness of
the characters. Additional circuitry allows the
microprocessor to disable the LCD when not in
use in order to conserve battery power.
A two brightness level LCD backlight is
provided to improve LCD viewing under low
light conditions. When the microprocessor
enables the LCD, it also enables the low
brightness backlight. Low brightness is used
to conserve battery power. If the AC adapter is
connected, the microprocessor will enable the
high brightness backlight since this does not
consume power from the battery.
The backlight automatically shuts off when the
LCD is turned off.
LED Status Indicators
An amber and a green Light Emitting Diode
(LED) are provided under the pump’s front
panel overlay to provide pump status to
the user. Under software control, the LEDs
can either flash at a low duty cycle or be on
continuously. A flashing indicator typically
indicates a normal mode of operation and
a steady “on” indicator typically indicates a
fault condition.
