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68000 Motherboard User's Manual
7.1.1
Volt age Regulat ion
Internally the electronics operates on +5VDC. This may be provided externally or
generated by the on-board voltage regulator. The current consumption of the MB68k-
100 motherboard is roughly 400mA, when operating at 10MHz. If the voltage regulator
is used, a minimum input voltage of +8VDC is recommended to allow for the voltage
drop of reversal protection and voltage regulator circuits. Limited by power dissipation
in the voltage regulator, the maximum input voltage recommended is around +12VDC,
where there is no additional load beyond the MB68k-100 board. If the voltage regulator
is bypassed with A1JP121, the input voltage must still compensate for the drop across the
reversal protection FET. This FET's resistance at that voltage is approximately 300m!,
requiring an input voltage of about +5.12VDC minimum at 10MHz.
protection FET may also be bypassed with jumper A1JP120, if that function is not
desired.
7.1.2
Active Reversed Connection Pr ot ection
The power input section circuitry offers protection against reversed polarity. This circuit
employs a P-Channel FET switch in-line with the positive connection that conducts only
under proper polarity. Under reversed polarity this FET switch opens and prevents
current flow.
With proper polarity, a side effect of the FET's operation is that it
introduces a slight voltage drop on the positive supply line as a function of input voltage
and current. At +5VDC, the FET resistance is approximately 300m! of inline resistance.
A bypass jumper, A1JP120, allows the FET to be circumvented.
7.1.3
Discr ete Voltage S uper vis or
The Discrete Voltage Supervisor is designed to provide reset control of the processor. It
controls the processor's reset signals for start-up, supply voltage brown-out and input
from the Reset Button, A1S140. The circuit is crafted for fast and monotonic assertion of
the Reset signals in response to any reset trigger. Both the 68000's /HALT and /RESET
signals are provided as required for processor control. The circuit is enabled with
insertion of the A1JP140 /HALT and A1JP141 /RESET enable jumpers. When these
jumpers are installed, their corresponding pull-up resistors, R151 and R152, may be
populated in the board assembly, but they are not required since the /HALT and /RESET
signals already incorporate pull-ups.
On start-up, the circuit drives a reset period of approximately 1.5s. This is more than
sufficient to meet the 100ms power-up reset requirement of the 68000, described in
section on Reset Operation in the Motorola 68000 User's Manual.
The Discrete Voltage Supervisor also monitors the system's DC supply voltage for
voltage droop and power supply glitches. Under such an event, reset is issued to the
digital logic to avoid errant operation due to insufficient supply voltage. The low voltage
Page 23 of 54
Rev. A
The reversal
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