1. Latches and Flip-Flops 2
©Paul Godin
Created September 2007
Last Edit Aug 2013

2. Review: Steering Gates
The flow of logic can be controlled with a logic gate.
The NAND as a steering gate inverts the input. 1 1
Signal 1
Control 1
ANIMATED

3. Review: Steering Gates
A control input ‘0’ inhibits the signal. 1 1
Signal 1 1
Control
ANIMATED

4. Steering Gates in Pairs1 1
Signal 1 1
Control 1 1
Signal 1
Output Enabled (inverted)
ANIMATED

5. Steering Gates in Pairs1 1
Signal 1 1
Control 1 1 1
Signal 1
Output Disabled
ANIMATED

6. Transparent SR Latch
A Transparent latch has an additional “control” input that enable or disable the input signals.
Control S R Q

7. Transparent SR Latch The Transparent SR Latch is also known as:
Gated SR Latch
Enabled SR Latch
The Control (Enable) input of the Transparent SR Latch
Controls when the SR inputs are read
Places the Latch in a hold state

8. Transparent Latch
The control (steering) gates invert the input. The latch portion is active-low but the input to the transparent latch is active high.
Control S R Q

9. “X” is “don’t care” or “doesn’t matter”
Gated SR Latch
The enable input can be used to disable further SR inputs to the device, leaving it in a hold state. En S R Q Q’
Mode 1
Hold
Reset
Set ?
Invalid X
Disabled S R Q En
“X” is “don’t care” or “doesn’t matter”

10. Exercise 1: Complete the timing diagramQ En En S R Q Q

11. Exercise 2: Complete the timing diagramQ En En S R Q Q

12. D Latch
The problem with the SR Latch is that it requires two inputs to store one value.
A latch is needed where one input is applied to store one value.
A control input is also required to place the device in a hold state.

13. D Latch
This device has one input to store one value (D), and an input to hold that value (Enable).
Enable D Q S R

14. This device is the foundation of digital memory.
Gated D Latch
“When enabled, whatever appears on D appears on Q”. En D Q Q’
Mode 1
Reset
Set X
Disabled D Q En
This device is the foundation of digital memory.

15. Exercise 3: Complete the timing diagramQ En En D Q Q

16. END
©Paul R. Godin
gmail.com