Specifications

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4.3 AT MOTHERBOARD LOGIC
Among its many functions, the Little Board/286 contains the equivalent of an entire PC/AT
motherboard. This section briefly describes the motherboard subsystem contained within the
Little Board/286.
4.3.1 CPU
A powerful 80286-type microprocessor (U5) represents the central element in the Little
Board/286. A variety of equivalent devices can be used, including the Intel 80286 standard
device, the AMD 80L286 low power device, and Harris (or other) 80C286 CMOS device.
The Little Board/286 is designed to operate at a variety of clock speeds, determined by the value
of the master clock oscillator, U20. U20 must be selected for twice the desired CPU clock rate.
Standard product is currently available from Ampro with either 16 or 12 MHz CPU clock rates.
Appropriate speed 80286, RAM, delay line, and math coprocessor devices must be present,
depending on the CPU clock rate. In addition, the board must be properly jumpered according to
the clock rate and memory wait state requirements (factory settings, do not alter!).
4.3.2 CPU and Bus Speed Control
To allow for maximum software and hardware flexibility, logic has been included which allows
you to shift the CPU or the bus to half-speed mode, under software control. The CPU and bus
speed together, or the bus speed alone, can be placed at half-speed. For example, a 12 MHz Little
Board/286 can be made to operate at a CPU and bus rate of 6 MHz, or its expansion bus alone can
be slowed down.
When the CPU is running at full speed and the bus is set for half-speed, the CPU operating
frequency automatically shifts to half its normal rate during memory and I/O transactions on the
AT expansion bus. When this occurs, the CLK signal on the expansion bus (J9-B20) goes to 1/2
its normal speed.
Note
The bus cannot run faster than the CPU, so if the CPU speed
option is set for half-speed, the bus will run at half-speed
regardless of the setting of the bus speed option.
Most software is independent of CPU speed. However, occasionally programs may be found
which were intended for older 6 MHz or 8 MHz AT's. In this case, employing the CPU half-speed
option under software control may be useful. In addition, many PC or AT Expansion Bus plug-in
cards have data access times which are too slow for 12 or 16 MHz expansion bus operation, so the
bus half-speed option -- especially switched during system operation via software -- can be useful.
It is also possible to reduce the board's power consumption and heat dissipation by making use of
the CPU half-speed option.