Program memory content after erasing

[bejoy_ak]

Why during erasing 1's are written to pic's program memory? Why not 0's are not written?

[Ttelmah]

Nothing is 'writen'. It is just that erasing gives a '1'.
It is a bit like why does a 'blackboard' 'erase' to black, while a 'whiteboard' erases to white?.
It is down to what type of memory 'cell' is used in the hardware.

Best Wishes

[Wayne_]

Although Ttelmah is technically correct there is another reason.

Some devices (Flash) require alot more effort in changing a bit from 0 to 1 but changing a 1 to a 0 is easy. Either a complete erase or slower page erases are required and usually it is not possible to set the bits of an individual byte to 1 without an erase.

This becomes important when you look at the number of erase cycles a flash device can handle, it is not infinate. To reduce the number of erases you can write your code so that if you are changing a byte and you only need to swap bits in the curent value from 1 to 0 you can just overwrite that memory location e.g.

addr1 = 0xFF (0b11111111 erased)
you can directly change any of the bits to 0 so you can write any value
addr1 = 0x77 (0b01110111)
You can now write any value to this location which only changes the bit from 1 to 0 without an erase
addr1 = 0x03 (0b00000011)

Why you can change a bit from 1 to 0 with a write and need an erase to change from 0 to 1 is proberbly down to the physical design of the chip. Basically I don't know without looking it up

Disclaimer: If any of this is incorrect blame someone else Please no flames or derogatory (speelling) remarks

[FvM]

Of course, the assignment of logical "1" to erased state is arbitray. But it's a common standard for all programmable memories since decades, also expected by any tool dealing with memory contents.

[bkamen]

it's a good idea since a blown device might return all 0's while a working device that's erased would return 1's.

I think the 1 vs 0 is also based on the type of cell. I read about it someplace but have forgotten the details.

-Ben
_________________
Dazed and confused? I don't think so. Just "plain lost" will do. :D

[Ttelmah]

Basically, it is historical, since there is nothing to stop you adding an inverter after the cell. On the early EEPROMs, erase was done using UV, rather than electically, which 'set' which way the charge had to go in the cell, and most electrical designs, have kept to the same standard.

Best Wishes

[bkamen]