HP 54520 Series Service Manual page 134

Main Assembly Theory
Central Processing Unit (CPU) The CPU is a 25 MHz 68EC020 microprocessor with
addressing capability of 16 Mbytes (24 address lines/32 data lines).
A 25 MHz 68882 floating-point co-processor speeds computation intensive operations such as
signal averaging, measurements and statistics, and waveform math functions.
Clocks Several crystal oscillators provide frequencies within the system control.
e The CPU clock is 25-MHz, derived from a 50-MHz oscillator. It drives the CPU and co-
processor directly and, combined with other signals, clocks ROM, RAM, and other circuitry.
e A 22-MHz crystal oscillator clocks the display circuitry.
e A 20-MHz crystal oscillator clocks the 16-channel DAC directly. The 20 MHz is divided to
5 MHz to clock the HP-IB interface circuitry.
e A 24-MHz oscillator clocks the disk drive interface.
e A 3.6864-MHz oscillator clocks the keyboard and RS-232 interface.
e The real-time clock is clocked by a 32.768-kHz oscillator that is part of the clock IC.
@ On the keyboard, the microcontroller is clocked by a 14.7456-MHz oscillator.
Control Logie The Control Logic provides timing and control for the system control.
Primarily it consists of a programmed array logic (PAL) IC but includes other
miscellaneous logic as well. Chiefly, it arbitrates between the various memories and
peripherals for CPU time. The PAL is synchronized with the 25-MHz clock.
Clicker The clicker is the sound effect circuit. The clicker sounds when warning or
error messages are displayed, when a key on the keypad is pressed, and (with some
functions) when knobs are rotated.
Reset/Preset The reset/preset circuit provides the main assembly with a timeout during
power up and power down. It consists of a voltage divider, reference voltage, and
comparator. The timeout signal is used in critical time and power sensitive circuitry. The
signal goes to the microprocessor, the control logic, decoders, and HP-IB and data
acquisition interfaces.
When power is applied, as the +5 V supply crosses the upper threshold of the comparator, a
timeout signal is generated and applied to the system control circuitry, assuring the board
powers up in a known state. Similarly, when power is removed, as the +5 V supply crosses the
lower threshold of the comparator, the timeout halts the microprocessor and resets all critical
timing before the +5 V supply falls below the valid operating region for TTL.
Memory Memory for the system control is composed of Boot ROM, Flash ROM,
nonvolatile RAM, and System RAM.
e The Boot ROM isa single 128K x 8-bit EPROM. Boot ROM holds the power-up firmware.
* The Nonvolatile RAM is CMOS static RAM organized as two sets of 128 K x 16-bits,
512 Kbytes total. The nonvolatile RAM stores front-panel setups, calibration factors, and
nonvolitile waveforms. The nonvolatile RAM uses a lithium battery as power backup.
When the supply falls below an acceptable voltage level, (during power-down) the lithium
battery is automatically switched on and write protection is unconditionally enabled to
prevent loss of data. Normal power-up of the instrument restores the calibration factors
and menu configurations that were in effect before the last power-down. A key-down
power-up, in which any key is held down during power-up, does not affect stored
calibration factors, but does reset the menu configurations to the default settings.
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