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Flash Microcontrollers Solve Problems
High voltages are always a problem to deal with because they must be carefully handled to prevent both circuit
damage as well as disturbing the FLASH that was just programmed.  Typically, on microcontrollers that require an
external programming voltage to be applied require that the power supply pin and the programming voltage pin to
come up to Vdd.  Then the programming voltage is stepped up to the required value for programming, the FLASH
is programmed and then the programming voltage is reduced back to Vdd.  It is possible when the rise and fall
times of the Vdd and programming voltage sources are significantly different for a voltage differential to develop
between Vdd and the programming voltage as both ramp toward Vdd.  If this differential is of sufficient magnitude,
around 0.4 volts or greater, a possibility exists in some microcontroller designs for the FLASH cells to be disturbed. 
For board designs that do allow for field programmability, care must be taken to appropriately keep any voltage
differentials between Vdd and the programming voltage to a minimum.  By using an onboard charge pump to
generate the programming voltage for the FLASH module, problems with ramp-up times for the various power
supplies is eliminated.
Microcontrollers with integrated FLASH have many advantages. Many of these challenges associated with the early
FLASH-based MCU's have been appropriately addressed as described here.  However, there are other economic
issues that will continue to drive more and more designs toward FLASH.  Specifically, integrated circuit
manufacturers would much rather manufacture a smaller number of different device designs.  High volume
production of a smaller number of designs is inherently more efficient.  Manufacturing a microcontroller where many
different customer ROM patterns are involved requires significant logistical support.   Because of this,
microcontroller manufactures will continue to put a major emphasis on FLASH-based MCU design.  The good part
about all this is that everyone wins.  Integrated circuit manufacturers can build general purpose, low cost devices
and designers using these microcontrollers are presented with more flexible and easy to use microcontrollers in
their end products.  
Charles Melear and Kevin Kilbane
6501 William Cannon Dr. West
Austin, Texas 78735-8598
Phone       512-895-2942
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