Re: In search of bad 4164, 41256 DRAM

On 17.09.2019 12:27, smf wrote:
> Can you explain why empty is read as 1 or 0 though? As far as I know
> dram cells are either empty or full and it checks if the cell is half
> full to work out the 0 or 1. So unless they randomly put inverters in
> there, an empty cell is an empty cell.
Hi,

The output sense is often differential, and bit-read line twisting is 
used to prevent interference (whether sram or dram). It also makes sense 
to invert the polarity of the cells for the same reason. Whether it 
helps in reality is another thing.

In SRAM reads two bit-read lines are precharged up, and the cell tries 
to drive one of them towards ground. The differential sense then decides 
which bit it was.

In SRAM the power-up state is more random from chip to chip, but quite 
consistent for individual chip. Each cell powers up to 0 or 1 depending 
on the mismatch of that cell's transistors, each individual bit tends to 
always power up to the same state. (With only some of the bits so well 
matched transistors that they can power up in either state.)

-Pasi