2070LX Controller – User Manual 21
Version 1.0
2.4.2.6 Reset Circuits
Microprocessor supervisory circuit MAX795
This microprocessor supervisory circuit monitors and controls the activities of the Processor. One feature
is μP reset. The threshold voltage range is from 3VDC to 3.15VDC. Its reset signal is an open-drain,
active-low output that drives low for 200ms when triggered and stays low while Vcc is below the
threshold. Once Vcc rises above the threshold level, the output remains low for 200ms.
Power On Reset Circuit / Hardware Reset Circuit
This circuit consists of one AND gate and two input signals; it receives the POWERUP signal and the
reset signal from the MAX795, if any of the signals is asserted, the gate generates an active-low reset
signal that goes to the FLASH memories and to the Processor. The Processor, based on the status of this
signal generates a hardware reset that results in a CPULRESET.
POWERUP: This signal comes from the Power Supply. It is a status signal that indicates whether the AC
power requirements are met. It is high while the requirements are met, otherwise it is low.
MAX795 Reset: This signal comes from the microprocessor supervisory circuit.
CPULRESET Line circuit
The CPULRESET is an active-low, +5VDC output signal provided by the Engine board in order to reset
other devices.
This signal is generated by the CPU RESET Line circuit and responds to two input signals provided by
the Processor, CPURESET and /RSTO.
CPURESET line is asserted when the application program receives a reset command or when /RSTO line
is asserted when the Processor receives a Power On Reset/Hardware reset.
The circuitry consists of two N-Channel MOSFET’s with the Drain connected to the same pull-up resistor
and the Source connected to logic ground. The CPURESET line controls one MOSFET and the other
MOSFET is controlled by an inverter gate that receives /RSTO as an input.
This circuit acts as an OR gate. If any of the two lines are asserted, the CPURESET line is also asserted
by being pulled to logic ground. The CPU Reset line then goes through a buffer to convert it to a 5VDC
signal and then goes to the P2 connector.
Microprocessor supervisory circuit with SRAM backup
The MAX795 microprocessor supervisory circuit monitors and controls the activities of the Processor by
providing some features such as: Processor reset, write protection for the SRAM and backup switchover.
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