Page 341 - Embedded Microprocessor Systems Real World Design

What happens if the input is changing while the clock is rising? This results in a race
condition, and the output will be indeterminate. Actual devices have a minimum setup time,
measured in nanoseconds, that the input must be stable at before the clock changes to
guarantee a valid output.
A latched device has a latch input (commonly called G), and it will pass the input to the
output as long as the latch is high. This is called the transparent mode. Any changes on the
inputs will be reflected at the output while the latch input is high. When the latch goes low,
the input is captured. The output does not change as long as the latch remains low. Dtype
latches typically are used to capture the address on a multiplexed microprocessor data bus.
Like the registered device, the latched device requires that the data be stable for some

D-TYPE REGISTERED DEVICE
D INPUT
CLOCK INPUT

DINPUT
CLOCKINPUT n I
OUTPUT 1 1 I

OUTPUT ONLY CHANGES
ON RISING EDGE OF
CLOCK. D-INPUT
CHANGES AT OTHER
TIMES DO NOT AFFECT
OUTPUT.

D-TYPE LATCHED DEVICE
D INPUT
LATCH INPUT

DINPUT
LATCH INPUT I
n
OUTPUT I
AS LONG AS LATCH STAYS HIGH,
OUTPUT FOLLOWS INPUT.
WHEN LATCH GOES LOW,
OUTPUT STOPS CHANGING.

Figure C.3
Registered Devices.

322 Appendix C