Built-In Oscillator
There is an oscillator built in the MPC82x54A which can be used as the oscillating source
replacing the external crystal oscillator in some specific applications.
To enable the built-in oscillator, an user must configure the device by clearing(enable) the bit
ENROSC in NVM register OR2 via a general writer.
Making use of the built-in oscillator saves the cost of a crystal oscillator.
Typically, the frequency of the built-in oscillator is designed as 6 MHz at 25 ℃. Dealing with
temperature variation, the frequency could vary from 4.2 MHz to 7.8 MHz (~30%). It is
designed for applications which don’t ask very precise oscillating frequency.
Power-Up and Low Voltage Detector and Reset
This device will never start to work until the power supply reach about 3.3V/1.9V(Operate in
the 5V/3V); however, that is not stable voltage supply to provide well writing for the embedded
flash.
There is a Low-Voltage detector(LVD) built in this device. While the voltage supply is dropping
cross about 3.7V/2.3V(Operate in the 5V/3V) for Fosc=12MHz, the LVD will set bit
LVF(PCON.5). Initially the bit LVF will be set after power-up, and the user should clear it for
further detecting.
There is another bit ENLVFI(AUXR.2) designed to decide if to enable an interrupt to the MCU
while the bit LVF(PCON.5) is set. This mechanism is convenient to inform the MCU take some
emergent action.
The user can configure the device to inhibit the flash read and write actions from ISP and IAP
statements by clearing bit LVFWP(OR0.7) while the voltage level falls under
3.7V/2.3V(Operate in the 5V/3V).
If the user never likes to let the device work under power supply less than 3.7V/2.3V (Operate
in the 5V/3V), he can configure the device to automatically go to reset state by clearing bit
ENLVR(OR0.6). It is named Low-Voltage-Reset.
SPECIAL NOTE ON LOW-VOLTAGE-DETECTOR: The Low-Voltage-Detector is not a
precise detector. The threshold voltage depends on temperature change. Lower the
temperature goes, higher the threshold voltage rises. During temperature scope (-40℃,
85℃), the threshold voltage falls between scope (2.7V, 1.8V) for MPC82L54, and (4.2V,
3.2V) for MPC82E54. To control the low-voltage detection and reset, also the use must
read another document “Initial Configuration.pdf” from Megawin which describes the
initial option register settings.
56
MPC82x54A Data Sheet
MEGAWIN
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