Datex-Ohmeda Aestiva 7900 SmartVent Technical Reference Manual page 30

2 Theory of Operation
Serial communications module
Memory and I/O decoding
Timer modules
2-14
System clock
An external 32.768 kHz crystal is used with the internal clock synthesizer to
generate a 24.12 MHz system clock.
Periodic interrupt timer
The periodic interrupt timer is the time base for the Real Time Operating
System.
Software watchdog timer
The software controlled internal watchdog timer guards against program
execution going astray.
External bus interface
The external bus interface handles the transfer of information between the
CPU32, external memory and peripherals.
Timing reference
An external 3.6864 MHz clock oscillator is used as the timing reference for the
Baud Rate Generator.
Configuration
Both of the serial communication channels are configured as full-duplex
asynchronous RS232C ports. The internal Baud Rate Generator establishes
the communication baud rate, with a higher limit of 19.2k Baud.
Special operating modes
The serial channels are capable of operating in various looping modes for self
testing as well as for remote testing of serial communications. These tests
include automatic echo, local loop-back, and remote loop-back.
Microprocessor Chip Selects
The four programmable chip selects from Motorola 68340 access external
memory and peripheral circuits, providing handshaking and timing signals as
well as a wait state generation, watchdog logic and ventilation control signals.
I/O lines
Spare I/O lines are used for digital control and/or sense lines.
Counter/timer #1
The first counter/timer module is used to monitor the MC68340 system clock
frequency. The external 3.6864 MHz baud rate clock is the time base for this
measurement.
Counter/timer #2
The second counter/timer module can be used as desired by the application
software. The time base for this timer is the internal 24.12 MHz system clock.
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