From f3b00e2c2e8b42fd0a778443e9b5626dda755ea6 Mon Sep 17 00:00:00 2001
From: lshprung <lshprung@yahoo.com>
Date: Mon, 19 Oct 2020 20:18:13 -0700
Subject: Post-class 10/19

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+[\<- Overflow, comparison, and the design of an ALU](11.md)
+
+---
+
+# Latches
+
+## A circuit that remembers
+
+### Sequential Circuits
+
+- Circuits that have "memory"
+	- A subset of the inputs control whether the output is "allowed" to change
+	- Otherwise the output will hold its "state"
+- In the circuit below, we want On to assert when Set asserts, and to stay asserted even if Set goes back to 0; Reset will clear
+
+![diagram](12.1.png)
+
+### Inverters in a feeback loop
+
+- Wires will hold onto value forever => memory
+
+![diagram](12.2.png)
+
+- But how to get a value onto one of the wires?
+
+### Controlling the feedback loop
+
+- The control signal affects the output
+	- If control = 0, NOR gate acts like an inverter
+	- If control = 1, output is 0
+
+![diagram](12.3.png)
+
+---
+
+## The SR latch
+
+- Qa is primary output (Qb is the inverse)
+	- (S)et Qa to 1, or (R)eset to 0
+
+![diagram](12.4.png)
+
+---
+
+## Synchronizing with a clock signal
+
+### Using sequential circuits
+
+- A latch can hold one bit of info (state)
+- Typically we want to hold onto multiple bits of info
+	- A counter is a typical example
+- A 2-bit counter cycles thru 00, 01, 10, 11, then back to 00, 01, etc.
+- Generally we want the two bits to change at the same time
+	- How to do this?
+
+### Using a Clock for Control
+
+- Most designs use a clock to "synchronize" when the sequential elements can change state
+- In the circuit below, the output can only be changed when Clk "enables" S and R
+
+![diagram](12.5.png)
+
+### Use a D input instead of SR
+
+- If S and R happen to assert at the same time, the output becomes a function of which deasserts last
+	- This may be unknown or unintentional
+- Don't really need both S and R
+	- If we're trying to change the output, we generally know if it should be 0 or 1
+- Define a single input, D, that is passed to the output
+
+### D Latch
+
+![Circuit](12.6.png)
+![Characteristic table](12.7.png)
+![Graphical symbol](12.8.png)
+![Timing diagram](12.9.png)
-- 
cgit