High-performance flip-flops having low clock load and embedded level shifting
An architecture for high-performance flip-flops having minimal clock-activated transistors is disclosed. The flip-flops operating in a first voltage domain can receive an input signal from a second voltage domain. The flip-flops include a first latch electrically coupled to a second latch. The first latch includes a first output and a second output. The second latch further includes a first and a second keeper pull-up sub-circuit which electrically couples to the first and second output of the first latch. The clock-gating functionality of the first and second keeper pull-up sub-circuits is merged with the first latch to reduce the loading on the clock signal, and thus the operation of the flip-flop is contention-free and fully-static. An embodiment of the second latch includes only one clock-activated transistor for low-power application. Another embodiment includes two clock-activated transistors for high-speed application. The high-performance flip-flops have near-zero setup time and a two-stage propagation delay.
1. A flip-flop circuit, comprising:
a first latch operating in a first voltage domain, wherein the first latch comprises:
four clock-activated transistors comprising at least a first clock-activated transistor and a second clock-activated transistor;
a first output node; and
a second output node;
a second latch operating in the first voltage domain, wherein the second latch comprises:
a first keeper pull-up sub-circuit coupled to the first output node,
wherein the first keeper pull-up sub-circuit is electrically coupled to the first-clock-activated transistor of the first latch and the first clock-activated transistor is a clock-gated device for the first keeper pull-up sub-circuit;
a second keeper pull-up sub-circuit coupled to the second output node,
wherein the second keeper pull-up sub-circuit is electrically coupled to the second clock-activated transistor of the first latch and the second clock-activated transistor is a clock-gated device for the second keeper pull-up sub-circuit;
a first pull-down network; and
a second pull-down network,
wherein the second latch is configured to merge pull-up functionality of the first keeper pull-up sub-circuit with the first clock-activated transistor of the first latch and to merge pull-up functionality of the second keeper pull-up sub-circuit with the second clock-activated transistor of the first latch.
2. The flip-flop circuit of claim 1 , wherein the first pull-down network of the second latch receives a first input from the second output node and the second pull-down network of the second latch receives a second input from the first output node.
3. The flip-flop circuit of claim 1 , wherein the four clock-activated transistors further comprise a third clock-activated transistor and a fourth clock activated transistor, wherein the first clock-activated transistor and the third clock-activated transistor of the first latch are electrically coupled to the first output node and the second clock-activated transistor and the fourth clock-activated transistor of the first latch are electrically coupled to the second output node.
4. The flip-flop circuit of claim 3 , wherein the first latch further comprises a first p-type transistor electrically coupled to a first intermediate node of the first latch to form a first inverter sub-circuit and the first intermediate node is an output of the first inverter sub-circuit, wherein the first latch further comprises a second p-type transistor electrically coupled to a second intermediate node of the first latch to form a second inverter sub-circuit and the second intermediate node is an output of the second inverter sub-circuit.
5. The flip-flop circuit of claim 1 , wherein the first pull-down network is electrically coupled to the second pull-down network and the first pull-down network and the second pull-down network are electrically coupled to a first clock-activated transistor of the second latch.
6. The flip-flop circuit of claim 1 , wherein the first pull-down network receives an input signal from a second voltage domain independent from the first voltage domain and the second pull-down network receives an inverted input signal from the second voltage domain.
7. The flip-flop circuit of claim 1 , wherein the first pull-down network comprises a second clock-activated transistor of the second latch and the second pull-down network comprises a third clock-activated transistor of the second latch, wherein the first pull-down network is electrically decoupled from the second pull-down network.
8. The flip-flop circuit of claim 1 , wherein the second latch further comprises a first keeper pull-down sub-circuit electrically coupled to the first pull-down network, wherein the first keeper pull-down sub-circuit and the first pull-down network are electrically coupled to a first intermediate node of the first latch.
9. The flip-flop circuit of claim 6 , wherein the flip-flop circuit is configured to sample a data bit when an input signal from the second voltage domain transitions before or at a rising edge of a clock that activates the at least four clock-activated transistors.
10. The flip-flop circuit of claim 8 , wherein the first keeper pull-down sub-circuit and the first pull-down network are electrically coupled to a second keeper transistor of the first latch, wherein the second keeper transistor is enabled by the first pull-down network.
11. The flip-flop circuit of claim 8 , wherein the second pull-down network is electrically coupled to a second intermediate node of the first latch.
12. The flip-flop circuit of claim 8 , wherein the second pull-down network is electrically coupled further to a first keeper transistor of the first latch, wherein the first keeper transistor is enabled by the second pull-down network.
13. The flip-flop circuit of claim 1 , wherein the first latch further comprises a level restoring sub-circuit electrically coupled to the first clock-activated transistor and the second clock-activated transistor and coupled to the first output node and the second output node of the first latch.
14. The flip-flop circuit of claim 1 , wherein the propagation delay of the flip-flop circuit is one or two stage delays.
15. A flip-flop circuit, comprising:
a first latch operating in a first voltage domain, wherein the first latch comprises:
four clock-activated transistors,
a first output node,
a second output node,
a first intermediate node,
a second intermediate node,
a first p-type transistor electrically coupled to the first intermediate node to form a first inverter sub-circuit, and
a second p-type transistor electrically coupled to the second intermediate node to form a second inverter sub-circuit;
a second latch operating in the first voltage domain, wherein the second latch comprises:
a first keeper pull-up sub-circuit coupled to the second intermediate node;
a second keeper pull-up sub-circuit;
a first keeper pull-down sub-circuit coupled to the first intermediate node;
a first pull-down network; and
a second pull-down network,
wherein the first intermediate node is an output of the first inverter sub-circuit and the second intermediate node is an output of the second inverter sub-circuit.
16. The flip-flop circuit of claim 15 , wherein the first keeper pull-up sub-circuit is coupled to the output of the second inverter sub-circuit, wherein the first pull-down network is driven by the output of the second inverter sub-circuit and the first pull-down network is activated upon the discharging of the first output node of the first latch.
17. The flip-flop circuit of claim 15 , wherein when the first pull-down network is activated, the first keeper pull-up sub-circuit is deactivated.
18. The flip-flop circuit of claim 15 , wherein the second keeper pull-up sub-circuit is coupled to the second output node of the first latch.
19. The flip-flop circuit of claim 15 , wherein the second keeper pull-up sub-circuit is coupled to the output of the first inverter sub-circuit and the second pull-down network receives an input from the first output node of the first latch.