Welch Allyn GSI 38, Service Manual

Page: 30 / 96

The Microprocessors
The GSI 38 operation is controlled by two MC68HC1 1E1 microcontrollers. The
microcontrollers have an 8 bit CPU and additional built in peripheral devices. It was
designed using HCMOS technology which combines smaller size and higher speeds
with the lower power and high noise immunity of CMOS. On chip
memory includes 512
bytes of
RAM
and 512 bytes
of EEPROM. The built in peripheral functions include:
An eight channel/8 bit analog to digital convertor
A 16 bit timer system
An 8
bit
pulse accumulator circuit
A real time interrupt circuit
Parallel input/output ports
A computer operating properly (COP) watch dog system which protects against
software failures.
A serial peripheral interface (SPI)
A serial communications interface (SCI)
Power saving wait and stop modes
The many functions incorporated by the microcontroller helps to reduce board space
requirements and the need for additional support circuitry.
The audio microprocessor (U34) is the master of the system and has control over the
tymp microprocessor (U7) which functions as a slave.
THE AUDIO MICROPROCESSOR CONTROLS THE FOLLOWING FUNCTIONS:
Communication
to the
Tymp Microprocessor: Asynchronous serial communications
data is transmitted and received using the processors built in serial communications
interface. The interface is located within port D of the microprocessor. PDO is the
receive data (ARXD) line and PD1 is the transmit data (ATXD) line. The bi-directional
communication allows the transfer of information, test results, parameters and control
functions to occur between the two processors.
Display Drive Information: Display information to and from the liquid crystal display
(LCD) display board RAM is latched via the bi-directional latch (U29). After the
information is loaded into the LCD board RAM the display board drives the associated
pixels and the audioprocessor is free to perform other tasks. The display area is 240
pixels wide by 64 pixels high.
Monitoring of the Switch Matrix Kevs:
The processor routinely checks the status of
the front panel keys for a change in state. The key panel switch inputs buffers U26,
U27, U28 and U31 are read every 24 msec. If a switch change has occurred the
processor executes the commands associated with the key function. The +10dB
(extended range) key and the reflex control keys (Ipsi, Contra, 500, 1K, 2K, 4K) are
active toggle controls (ie., push on/push off).