Digital storage element architecture comprising dual scan clocks and gated scan output
A digital storage element (e.g., a flip-flop or a latch) includes a master transparent latch that receives functional data from a data input port and scan data from a scan input port and a slave transparent latch coupled to the master transparent latch. The slave transparent latch includes dedicated functional data and scan data output ports. The digital storage element operates in a functional mode and in a scan mode. While in the scan mode, a first clock signal is used by the slave transparent latch and a second clock signal is used by the master transparent latch. The first and second clock signals are non-overlapping and, as such, avoid the digital storage element from creating hold violations.
1. A digital storage element comprising:
a data input port that provides functional data during a functional mode;
a scan input port that provides scan data during a scan mode;
a first clock input port that provides a first clock signal during the functional mode and a second clock signal during the scan mode, wherein the first and second clock signals are non-overlapping;
a second clock input port that provides a third clock signal;
an enable port that indicates the scan mode and the functional mode;
a data output port;
a scan output port;
a master latch including:
a first inverter that is coupled to the data input port;
a second inverter that is coupled to the scan input port;
a first logic gate that is coupled to the first clock port and the enable port;
a first pass gate that is coupled to the first inverter and the first logic gate, wherein the first logic gate is adapted to activate and deactivate the first pass gate, and wherein, in the scan mode, the first logic gate deactivates the first pass gate;
a second pass gate that is coupled to the second inverter and the second clock input port; and
a third inverter that is coupled to the first and second pass gates; a slave latch including:
a third pass gate that is coupled to the third inverter and the first clock input port;
a fourth inverter coupled between the third pass gate and the data output port;
a fifth inverter that is coupled to the third pass gate; and
a Second logic gate that is coupled to fifth inverter, the enable port, and the scan output port.
2. The digital storage element of claim 1 , wherein the slave latch further comprises:
a fourth pass gate; and
a sixth inverter that is coupled to the fourth pass gate, wherein the fourth pass gate and the sixth inverter are coupled in parallel to the fifth inverter.
3. The digital storage element of claim 1 , wherein the master latch further comprises a fourth pass gate that is coupled in parallel to the third inverter.
4. The digital storage element of claim 3 , wherein the fourth pass gate further comprises:
a first transistor coupled to the third pass gate at its gate;
a second transistor that is coupled to the drain of the first transistor at its source and that is coupled to the second clock terminal at its gate;
a third transistor that is coupled to the drain of the second transistor at its source, that is coupled to the first logic gate at its gate, and that is coupled to the first and second pass gates at its drain;
a fourth transistor that is coupled to the first and second pass gates at its source;
a fifth transistor that is coupled to the source of the fourth transistor at its drain; and
a sixth transistor that is coupled to the source of the fifth transistor at its drain and that is coupled to the third pass gate at its gate.
5. The digital storage element of claim 1 , wherein the first logic gate is a NOR gate.
6. The digital storage element of claim 1 , wherein the second logic gate is a NAND gate.
7. An apparatus comprising:
a data input port that provides functional data during a functional mode;
a scan input port that provides scan data during a scan mode;
a first clock input port that provides a first clock signal during the functional mode and a second clock signal during the scan mode, wherein the first and second clock signals are non-overlapping;
a second clock input port that provides a third clock signal;
an enable port that indicates the scan mode and the functional mode;
a data output port;
a scan output port;
a master latch having a plurality of master latch inverters and a plurality of master latch pass gates, wherein the master latch is coupled to each of the data input port, the scan input port, the first clock input port, the second clock input port, and the enable port; and
a slave latch having:
a slave latch pass gate that is coupled to the master latch and the first clock input port;
a first slave latch inverter coupled between the slave latch pass gate and the data output port;
a second slave latch inverter that is coupled to the slave latch pass gate; and
a NAND gate that is coupled to the second slave latch inverter, the enable port, and the scan output port;
wherein the master latch further comprises:
a first master latch inverter that is coupled to the data input port;
a second master latch inverter that is coupled to the scan input port;
a NOR gate that is coupled to the clock port and the enable port;
a first master latch pass gate that is coupled to the first master latch inverter and the NOR gate, wherein the NOR gate is adapted to activate and deactivate the first pass gate, and wherein, in the scan mode, the NOR gate deactivates the master latch first pass gate;
a second master latch pass gate that is coupled to the first master latch inverter and the second clock input port;
a third master latch inverter that is coupled to the first and second master latch pass gates; and
a fourth master latch pass gate that is coupled in parallel to the third inverter.
8. The apparatus of claim 7 , wherein the slave latch farther comprises:
a second slave latch pass gate; and
a third slave latch inverter that is coupled to the second slave latch pass gate, wherein the second slave latch pass gate and the third slave latch inverter are coupled in parallel to the second slave latch inverter.