-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Hi,
One of the properties of Flash memory is that changing a 1 bit to a 0
bit is very different to changing a 0 bit to a 1 bit. That's where this
issue comes from.

Flash memory can easily change a single bit from 1 to 0, and that's
exactly what the "write" operation does: for each 0 bit in the data
you're writing, it zeros out that bit in Flash; for each 1 bit in the
data, it does nothing. This gives you exactly the outcome you want, IF
the memory starts out containing all 1s. If it doesn't start out with
all 1s, you might see a 0 in Flash where your data to write had a 1.
PICs require you to do a "write" with more than one byte at a time for
power and speed reasons: it takes time and power to write to Flash, so
it's better to do more bytes in parallel (in your case, 8 bytes).

It's a lot harder to change 0s to 1s. You can't do this on a bit-by-bit
basis (that's why a "write" operation does nothing if it sees a 1). You
can only take a large pile of bits (a "row", in your case 64 bytes) and
change them all to 1s together. This is called an "erase".

Using these two operations together, you can get any data you want into
Flash: you first have to erase the area (to make it all 1s) then you
have to write the area (to make some of the bits 0).

So if you want to fill a large region of Flash with data, you would
first erase the region 64 bytes at a time, then you would write the
region 8 bytes at a time.

Chris

Mike snyder wrote:
| I am trying to understand how bootloaders work and was reading the
| PIC18 datasheets for how one would write to program memory.
|
| 1. I understand that you can only erase memory in rows of 64 bytes is
| that correct?
| 2. The bootloader described in AN851 says
|
| "For the PIC18F devices, commands 1 through 3 support operations to
| FLASH program memory. Read operations occur at the byte level. Write
| operations are performed on multiples of 8 bytes (one block). Erase
| operations are performed on 64 bytes (one row)."
|
| Does this mean that program memory can be written in 8 byte chunks?
|
| Any pointers appreciated.
|
| Thanks
| Mike

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2.0.9 (GNU/Linux)
Comment: GnuPT 2.7.2
Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org

iEYEARECAAYFAkhSzYAACgkQiD2svb/jCb4oTACdFIpJdBCCcR00bB21Hm/WbMwE
OUsAn3pMXxLvCZqYZEzHPtG6T2TAhjOI
=afGL
-----END PGP SIGNATURE-----
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist