In many six transistor static ram cell diagrams, it mentions a so called "precharge" that holds both bit lines at positive voltage. What is the use of this? wouldn't this constant voltage just screw up all your data? And why does it require transistors if they are always on? The transistors in question are circled in red

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    \$\begingroup\$ Doesn't that timing diagram pretty clearly show it's not always on? \$\endgroup\$ – user253751 May 3 '16 at 5:24
  • \$\begingroup\$ Possible duplicate: electronics.stackexchange.com/questions/116590/… \$\endgroup\$ – Peter Smith May 3 '16 at 7:21
  • \$\begingroup\$ @PeterSmith: That question is about the sense amplifiers, but doesn't really talk about the need to precharge the bit lines. \$\endgroup\$ – Dave Tweed May 3 '16 at 11:42

When the word line is asserted during a read cycle, a weak differential signal is induced on the pair of bit lines. Precharging those bit lines to the same voltage makes it possible to amplify that differential signal to a useful logic level.

As your timing diagram shows, the precharge transistors are only active for a brief time before the word line is active.

In other words, the low-level details of the internal design of an SRAM is very much an analog circuit design problem, in which the sizes of the various transistors are carefully optimized to balance the requirements for bit density, read/write speed and noise immunity.

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