5 Theory of Operation

Keyboard Circuitry

The CADD-Prizm®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 batter-

ies. 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.

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