AN2690 Freescale Semiconductor / Motorola, AN2690 Datasheet - Page 9

• Manufacturer Part Number: AN2690
• Description: Low Frequency EEPROM Emulation on the MC68HLC908QY4
• Manufacturer: Freescale Semiconductor / Motorola

Conclusions
MOTOROLA
Low Frequency EEPROM Emulation on the MC68HLC908QY4
It is feasible to emulate EEPROM on the MC68HLC908QY family, when the
main application is running from a 32768 Hz external crystal, by forcing a reset
at a specific part of the code, thereby causing a switch to the internal oscillator
and allowing the FLASH to be programmed.
However, in doing so, the MCU is reset and the data registers, internal registers
and modules are put into their default conditions. This could cause problems in
some applications, for example, where outputs change to inputs until initialized.
Another issue is the time required to switch back to the slow 32768 Hz external
crystal clock source, as the source should not be switched before the crystal
clock is stable, which can take up to 4096 cycles (125 ms for a 32 kHz crystal).
This time delay could be a problem in some real-time systems.
Method 2 is the more flexible solution, as it allows the code to return to the
instruction following the forced reset, whereas Method 1 is simpler and easier
to implement.
The following appendix shows a simple application that demonstrates
Method 1.
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Conclusions
AN2690
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