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How the SR latch works SR = set/reset
The NOR gate Only ON when BOTH inputs are off
S-R Latch active high Turn on S Both inputs to top gate are off so Q is on Both inputs to lower gate are on so Q’ is off
S-R Latch active high Turn off S No change
S-R Latch active high Turn on R Both inputs to top gate are on so Q’ is on Both inputs to lower gate are off so Q is off
S-R Latch active high Turn off R No change Turning R on or off doesn’t change anything This is a valid state because Q and Q’ are opposites.
S-R Latch active high Turn on both S and R When S and R are both on, the output of the gates change so both Q and Q’ are off. Unstable state because Q and Q’ MUST be opposites of each other
Summary of states SRQQ’ 00NC No Change 0101Latch is RESET 1010Latch is SET 11??Invalid condition
Other variations On the SR latch
Active low NAND is only low when both inputs are high InputsOutputs QComments 00??Invalid Condition 0110Latch is SET 1001Latch is RESET 11NC No change RSRS Since R and S are opposites
Logic symbols for SR latch
The gated latch Requires the enabler to be on to effect a change
Gated D latch InputsOutputs DENQComments 0101Latch is RESET 1110Latch is SET X0NC Latch remains in present state
Gated latch symbols
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