AN1827 Freescale Semiconductor / Motorola, AN1827 Datasheet - Page 4

• Manufacturer Part Number: AN1827
• Description: Programming and Erasing FLASH Memory on the MC68HC908AS60
• Manufacturer: Freescale Semiconductor / Motorola

Freescale Semiconductor, Inc.
Application Note
The FLASH bit cell consists of two transistors in series, referred to as the
select-gate and control-gate transistors. The floating gate is associated
with the control gate transistor and stores charges which represent the
two different data states of the memory. The high threshold condition of
the bit cell is the erased state and the low threshold condition is the
programmed state. The select gate prevents bit cell leakage of
unselected wordlines during read operations when the floating gate is
programmed.
Although the size and shape of the array is mostly transparent to the
user, it does help when determining the "cared addresses" during the
erase algorithm. The term and function of "cared addresses" are
explained in the
Erase Operation
of this application note. For now, it is
important to know that the cared addresses determine exactly which
block will erase during the erase operation.
More importantly, the size and shape of the memory array may alter the
size of the programming page. The term page refers to the number of
consecutive bytes that are programmed during a page program/margin
read operation. On larger memory arrays, like the two arrays found on
the MC68HC908AS60, one page equals eight bytes. As the array is
scaled down, the page is proportionally scaled to either four, two, or one
byte(s). This will affect the programming algorithm that appears later in
this application note.
The only other obvious difference in implementation of the FLASH array
is the size of the blocks that can be protected against an undesired
program or erase operation. Again, this depends on the size of the
memory on the microcontroller. Check the appropriate documentation
for each specific microcontroller to determine the size of the memory
array, the page program size, and the erase block sizes.
AN1827
4
MOTOROLA
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