Use a chip select to address larger flash size?

Thread Starter


Joined Jan 12, 2010

I am currently addressing a 128Mbit flash using 24 address lines, 8 data lines, and 1 chip select line (the processor has 2 chip selects).

What does the external logic look if I want to use a second chip select line to create a high address line? I want to use a 256Mbit flash.



Joined Nov 6, 2005
You need to enable the Flash ROM when either select line is active, then use the line that is high when you want to access the upper half as the high address line.

If both selects are Active LOW, you could use two small schottky diodes to connect the two select lines to the rom /CS, with a (eg.) 4k7 pullup from /CS to VCC, and connect the low half Select line to the rom A24.

That way, either Select enables the rom and the rom high address is driven to match the low/high half being accessed.