The Inverter clock

Introduction It is desirable for many logic devices to be triggered by a momentary pulse, of very short duration, that occurs coincident with the positive (usually the leading) edge or negative (usually the trailing) edge of the clock pulse. The pulse itself can be either positive or negative going dependant on whether the device has active Hi or active Lo inputs. This gives a choice of four different combinations of positive or negative edge, and positive or negative pulse.

TheoryIn all four combinations the inherent propagation delay of an inverter is used to produce the desired pulse on the desired edge of the clock pulse. The INVERTER will be used in conjunction with AND, NAND, OR and NOR gates.

OperationPositive or Leading Edge Triggers.

A +ve Edge/Active Hi trigger circuit. This circuit, utilizing an AND and an INVERTER, will momentarily produce a positive pulse on the positive or leading edge of the clock pulse. When the clock pulse is Lo, one input to the AND is Lo and the other I/P is Hi due to the INVERTER. This results in a Lo O/P from the AND. When the clock pulse goes Hi the upper AND I/P goes Hi immediately, but due to the propagation delay in the INVERTER it's O/P remains Hi for a short period of time before going Lo. During this short period both I/Ps to the AND gate are Hi and a positive pulse appears at the AND O/P. This pulse can be used to trigger an active Hi circuit (such as a Flip-Flop) on the positive or leading edge of the clock pulse.

A +ve Edge/Active Lo trigger circuit. This circuit , utilizing a NAND and an INVERTER, will momentarily produce a negative pulse on the positive or leading edge of the clock pulse. When the clock pulse is Lo, one input to the NAND is Lo and the other I/P is Hi due to the INVERTER. This results in a Hi O/P from the NAND. When the clock pulse goes Hi the upper NAND I/P goes Hi immediately, but due to the propagation delay in the INVERTER it's O/P remains Hi for a short period of time before going Lo. During this short period both I/Ps to the NAND gate are Hi and a negative pulse appears at the NAND O/P. This pulse can be used to trigger an active

+ve edge trigger -ve edge trigger

Lo circuit (such as a Flip-Flop) on the positive or leading edge of the clock pulse.

Negative or Trailing Edge Triggers

A - ve Edge/Active Hi trigger circuit. This circuit , utilizing a NOR and an INVERTER, will momentarily produce a positive pulse on the negative or trailing edge of the clock pulse. When the clock pulse is Hi, one input to the NOR is Hi and the other I/P is Lo due to the INVERTER. This results in a Lo O/P from the NOR. When the clock pulse goes Lo the upper NOR I/P goes Lo immediately, but due to the propagation delay in the INVERTER it's O/P remains Lo for a short period of time before going Hi. During this short period both I/Ps to the NOR gate are Lo and a positive pulse appears at the NOR O/P. This pulse can be used to trigger an active Hi circuit (such as a Flip-Flop) on the negative or trailing edge of the clock pulse.

A - ve Edge/Active Lo trigger circuit. This circuit , utilizing an OR and an INVERTER, will momentarily produce a negative pulse on the negative or trailing edge of the clock pulse. When the clock pulse is Hi, one input to the OR is Hi and the other I/P is Lo due to the INVERTER. This results in a Hi O/P from the OR. When the clock pulse goes Lo the upper OR I/P goes Lo immediately, but due to the propagation delay in the INVERTER it's O/P remains Lo for a short period of time before going Hi. During this short period both I/Ps to the OR gate are Lo and a negative pulse appears at the OR O/P. This pulse can be used to trigger an active Lo circuit (such as a Flip-Flop) on the negative or trailing edge of the clock pulse.

Some circuit diagrams will contain a symbolic clock pulse, as shown below, with an arrow indicating the type of trigger used with a device or circuit.

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Summary: The pulse itself can be either positive or negative going dependant on whether the device has active Hi or active Lo inputs. This gives a choice of four different combinations of positive or negative edge, and positive or negative pulse. The INVERTER will be used in conjunction with AND, NAND, OR and NOR gates.

Positive or Leading Edge Triggers.A +ve Edge/Active Hi trigger circuit. (AND and NAND GATE to inverter)

Negative or Trailing Edge TriggersA - ve Edge/Active Hi trigger circuit.(OR and NOR GATE to inverter)

Quiz:

The 1.______________will be used in conjunction with 2.__________, 3.___________, 4.____________and 5. ____________ gates.

Answers:1. Inverter2. AND 3. NAND4. OR5. NOR

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