Chip select (CS)

Also called: chip enable, CE, memory enable, chip-select signal, chip select line

A memory chip's enable input. When chip select is active the chip responds to reads and writes; when inactive it ignores them and releases the data bus.

Chip select tells one particular chip "this access is for you". It's the memory's enable input.

CS active: the chip reads or writes according to its R/W line.
CS inactive: the chip does nothing. Writes are ignored, and its data outputs go to high impedance so other chips can use the data bus.

In a system built from several chips, the chip selects come from the high address bits. A decoder on those bits activates exactly one chip select for any address, so exactly one chip (or one bank) responds. That's depth expansion.

To read a chip select off a memory map:

The chip's own address bits (log₂ of its size) vary inside its range, so leave them out.
The bits above them are fixed across the range. AND them together: the variable where the bit is 1, its complement where it is 0.

Also called chip enable (CE). On real chips it is usually active low, drawn with a bar: the chip is selected when the pin is 0.

Worked examples

Example

Chip select for a 4K chip at 7000₁₆–7FFF₁₆

16-bit address space, A15–A0.

1.
4K = 2¹², so the chip uses A11–A0. Those bits stay out of the select.
2.
7000₁₆ = 0111 0000 0000 0000, 7FFF₁₆ = 0111 1111 1111 1111. The top four bits are 0111 in both.
3.
CS = .
4.
Test 7A3C₁₆: top bits 0111 → CS = 1 ✓. Test 8000₁₆: A15 = 1 → CS = 0 ✓.

Example

Reading the decoder in the diagram

Four 16K chips cover a 64K space. A15 A14 drive a 2-to-4 decoder whose outputs are CS0–CS3. The CPU accesses C1F0₁₆.

1.
C₁₆ = 1100, so A15 A14 = 11.
2.
CS3 = = 1; the other three are 0.
3.
Chip 3 (C000₁₆–FFFF₁₆) responds.

Common mistakes

Including the chip's own address bits in the select. They change across the range, so the select would only be true for part of it.
Checking too few high bits. The chip then also answers at other addresses and appears more than once in the map.
Thinking a deselected chip outputs 0. It goes to high impedance.

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