Circuit for asynchronous reset in current mode logic circuits

A current mode logic (CML) flip flop includes a first CML latch and a second CML latch. A plurality of pull-up switches are responsive to a reset signal. Outputs of the first and second CML latches are pulled up to a supply voltage through the pull-up switches. The first CML latch includes a first pull-up isolation switch driven by the reset signal for resetting the latch.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to current mode logic circuits, and more particularly, to a method and circuit for resetting current mode logic circuit elements that have a memory and/or initial state.

2. Related Art

Current mode logic (CML) is widely used to build high speed logic blocks, such as frequency dividers in PLLs and high speed serial link transceiver circuits. CML logic can operate two to three times faster than CMOS logic. For frequency dividers and counters, there is often a need to implement a reset to enable initialization of a defined state. In CMOS logic, this is done by using the resetable flip flops, which are typically implemented by having pull-up or pull-down transistors enabled in the reset state. Since CMOS logic is a complementary-type logic, where one path is turned ON and the other OFF, it is straightforward to use pull-up, pull-down and pass switch transistors to implement the reset. In CML, latches are realized using biased differential transistor pairs and crossover switches, and, unlike CMOS latches, speed and bias requirements do not easily allow a reset to be done by using pass transistors switches with pull-ups and pull-downs. A reset method commonly used in CML is described below.

In conventional art, which is illustrated by a divide-by-N circuit of FIGS. 1-7 , a multiplexer 101 provides the initial state of the divide-by-N, and the reset mechanism is done sequentially from the input (vip, vin) to the output (vop, von). This is illustrated in FIGS. 1-7 . The conventional CML divide-by-N circuit with an asynchronous reset shown in FIG. 1 consists of three blocks:

CML combination logic 102 (illustrated in detail in FIG. 4 ), includes NAND, NOR and MUX gates, and sets a duty cycle of divide-by-N circuit output.

CML multiplexer 101 (see FIG. 5 ) has NMOS differential transistor pairs M 501 -M 502 , M 503 -M 504 , NMOS transistor switches M 505 , M 506 , NMOS current source M 507 and resistors R 501 , R 502 . Multiplexer 101 with one input connected to (VSS, VDD) provides the initial state and isolates the unknown input signal disturbing the reset process when reset is positive.

FIG. 2 shows a structure of CML flip flop 103 . As shown in FIG. 2 , CML flip flop 103 includes two CML latches 201 A, 201 B connected in series. CML latch 201 A has inputs (vip, vin), and outputs (vop, von) which are inputted into corresponding inputs (vip, vin) of the second CML latch 201 B. Both latches 201 A, 201 B have common clock inputs (clk, clkn).

FIG. 3 shows a structure of a CML latch 201 of FIG. 2 . As shown in FIG. 3 , CML latch 201 has a differential transistor pair M 301 , M 302 , whose gates are driven by (vip, vin) respectively. CML latch 201 also has an NMOS cross coupled transistor pair M 303 , M 304 . Drains of transistors M 301 , M 302 , M 303 and M 304 are tied to VDD through pull-up resistors R 301 , R 302 . Sources of the differential transistor pair M 301 , M 302 are tied to a switch transistor M 305 , whose gate is driven by clock input clkn. Sources of cross-coupled transistor pair M 303 , M 304 are tied to a drain of transistor switch M 306 , whose gate is driven by clock input clk. Current source transistor M 307 , whose gate is biased by voltage vb, supplies current to sources of switch transistors M 305 , M 306 . Latch 201 produces outputs (vop, von) as shown in FIG. 3 .

FIG. 4 shows a circuit diagram of CML combination logic 102 of FIG. 1 . As shown in FIG. 4 , CML combination logic 102 includes four differential transistor pairs (M 401 , M 402 ), (M 403 , M 404 ), (M 405 , M 406 ), (M 407 , M 408 ), and (M 409 , M 410 ) forming NAND and MUX gates. Switches M 411 , M 412 , and M 413 are connected to sources of respective differential transistor pairs, as shown in FIG. 4 . Tail current source transistors M 414 and M 415 supply current to the differential transistor pairs. Power supply voltage VDD is provided through pull-up resistors R 401 -R 402 , R 403 -R 404 , and outputs (vop, von) are connected to the pull-up resistors R 403 -R 404 as shown in FIG. 4 . The sel signal is an internal signal of CML combination logic 102 , and can be connected to power or ground. It sets the first output of (vop, von) to be logic 1 or 0 for duty cycle setting after reset and the sel setting is up to the user. Note that it does not affect the reset operation.

To understand the reset operation of divide-by-N circuit, a reset of divide-by-2 is shown in FIG. 6 and is explained using the timing diagram of FIG. 7 . First, the input (VSS, VDD) of multiplexer 101 is selected by reset. Second, the output (VSS, VDD) of multiplexer 101 is asserted at the input (vip, vin) of CML flip flop 103 , and is read by the NMOS differential transistor pair M 301 , M 302 of first CML latch 201 A at a negative clock (clk) period (clk LOW). Third, the output (VSS, VDD) of first CML latch 201 A is held by NMOS cross-coupled transistor pair M 303 , M 304 of first CML latch 201 A of CML flip flop 103 , and is read by the NMOS differential transistor pair M 301 , M 302 of second CML latch 201 B of CML flip flop 103 to the output (VSS, VDD) at the positive clk period (clk HIGH). Fourth, the output (VSS, VDD) is held by NMOS cross-coupled transistor pair M 303 , M 304 of second CML latch 201 B at the negative elk period (clk LOW). As a result, the output of CML flip flop 103 is reset to the initial state (VSS, VDD) at the second negative elk period (clk LOW).

Since the conventional circuit resets (vip, vin), (qop, qon) and (vop, von) sequentially, its minimum reset duration must take slightly more than one clock period. For convenience, assume that one clock period is required for reset. Thus, the reset operation of a conventional divide-by-N is such that all the outputs of CML flip flops 103 are reset to the defined value (VSS, VDD) at one clock period.

The disadvantages of the conventional CML divide-by-N with asynchronous reset are as follows:

In a convention reset circuit, multiplexer 101 is used to define an initial state of the circuit. Multiplexer 101 introduces a delay when it is not being used (i.e. when no reset is applied to the circuit), however, the multiplexer is always in the signal path, introducing a delay. CML multiplexer 101 with a propagation delay (t d ) requires the minimum pulse width to be t d t setup t hold . For example, t d is 40 ns in the 1.2V 3.125 GHz Serdes standard, and it is 12.5% of full speed clock 3.125 GHz. As operational speed increases, the propagation delay t d becomes a bottleneck.

Extra current consumption I MUX is needed for multiplexer 101 that provides the initial state. There is a dependence between the extra current consumption I MUX and the reset duration. If each CML flip flop 103 has its own multiplexer 101 , then the current consumption is increased by N I MUX , but the reset duration stays at one clock period. Even if only the first CML flip flop 103 has its own multiplexer 101 , the current consumption is slightly increased by I MUX , but the reset duration is increased to N clock periods.

The reset duration takes at least one clock period. For example, if clk is {fraction (1/20)} of full speed (system) clock, then the latency is a significant number of 20 UI (unit intervals).

Thus, conventional asynchronous reset circuits are unsuitable for low-power very-high-speed applications.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a circuit for asynchronous reset in current mode logic circuits that substantially obviates, one or more of the disadvantages of the related art.

There is provided a current mode logic (CML) flip flop including a first CML latch and a second CML latch. A plurality of pull-up switches are responsive to a reset signal. Outputs of the first and second CML latches are pulled up to a supply voltage through the pull-up switches. The first CML latch includes a first pull-up isolation switch driven by the reset signal for resetting the latch.

In another aspect there is provided a CML divide-by-N circuit including N CML flip flops connected in series, each flip flop inputting (vip, vin) signals and outputting (vop, von) signals. Each flip flop includes a first CML latch and a second CML latch. The first CML latch includes a first pull-up isolation switch driven by the reset signal for resetting the latch. A plurality of pull-up switches are driven by a reset signal. Outputs of the first and second CML latches are pulled up to a supply voltage through the pull-up switches. Combination logic inputs the (vip, vin) signals and outputs the (vop, von) signals to set a duty cycle of the divide-by-N circuit.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 8 illustrates a CML divide-by-N-circuit of the present invention. Comparing FIG. 8 with FIG. 1 , multiplexer 101 is not needed in the divide-by-N-circuit of the present invention. The divide-by-N-circuit includes a plurality of CML flip flops 802 A- 802 N connected in a series, and CML combination logic 801 . CML flip flops 802 A- 802 N are driven by (clk, clkn) clock inputs, and a reset signal. Outputs of the CML combination logic 801 and inputs to first CML flip flop 802 A are (vip, vin) and outputs of the CML flip flops 802 A- 802 N are (vop 1 , von 1 ), . . .

FIG. 9 illustrates the structure of the single CML flip flop 802 of the present invention. As shown in FIG. 9 , CML flip flop 802 includes two CML latches 901 A, 901 B connected in series, with inputs (vip, vin), and outputs (vop, von), as shown in FIG. 9 . CML latches 901 A, 901 B are clocked by (clk, clkn). First CML latch 901 A is reset by the inverted reset signal (resetn), and second CML latch 901 B is reset by the supply voltage VDD. Four pull-up transistors M UP0 -M UP3 are connected to outputs of first and second CML latches 901 A, 901 B, with pull-up transistors M UP0 , M UP1 connected to outputs of the first CML latch 901 A, and pull-up transistors M UP2 , M UP3 connected to outputs of second CML latch 901 B. Pull-up transistors M UP0 -M UP3 pull up the outputs of the CML latches 901 A, 901 B (qop, qon), (vop, von) to the supply voltage (VSS, VDD). Pull-up transistors M UP0 and M UP2 are driven by the resetn signal (inverted reset signal) at their gates, and pull-up transistors M UP1 , M UP3 are driven by the VDD supply voltage at their gates.

Pull-up PMOS transistors M UP0-3 are placed at the outputs of latches 901 A, 901 B (qop, qon), (vop, von) in order to define their initial state. Pull-up transistors M UP0-3 generate the small voltage spur during the reset and (see FIG. 10 ) the positive feedback NMOS cross-coupled transistor pairs M 1003 , M 1004 of latches 901 A, 901 B accelerate the pull-up operation. The size of pull-up transistors M UP0 -M UP3 can be very small.

Pull-up PMOS transistors M UP0 -M UP3 replace the multiplexer 101 to provide the initial state (VSS, VDD), and the propagation delay of multiplexer 101 is no longer exists. As a result, operating speed that depends on t setup t hold can be pushed higher.

All pull-up PMOS transistors M UP0 -M UP3 work only during the reset operation. Therefore, no additional current is consumed during the normal operation. The current consumption is reduced by N I MUX compared to conventional out circuit.

FIG. 10 illustrates a circuit diagram of a CML latch 901 used in CML flip flop 802 of the present invention. As shown in FIG. 10 , CML latch 901 has a differential transistor pair M 1001 , M 1002 , whose gates are driven by (vip, vin) respectively. CML latch 901 also has an NMOS cross-coupled transistor pair M 1003 , M 1004 . Drains of transistors M 1001 , M 1002 , M 1003 and M 1004 are tied to VDD through pull-up resistors R 1001 , R 1002 , as shown in FIG. 10 . Sources of the differential transistor pair M 1001 , M 1002 are tied to a switch transistor M 1005 , whose gate is driven by clock input clkn. Sources of cross coupled transistor pair M 1003 , M 1004 are tied to a drain of transistor switch M 1006 , whose gate is driven by clock input clk. Current source transistor M 1007 , whose gate is biased by voltage vb, supplies current to sources of switch transistors M 1005 , M 1006 . Additionally, pull-up transistor M ISO , driven by the reset signal, connect sources of differential pair M 1001 , M 1002 , to the supply voltage VDD. Latch 901 produces outputs (vop, von) as shown in FIG. 10 . Pull-up PMOS transistor M ISO is placed at the virtual ground of NMOS differential transistor pair M 1001 , M 1002 of first CML latch 901 A in order to isolate the divide-by-N circuit from external signals during the reset. (M ISO may, alternatively, be a bipolar transistor.)

Thus, during reset, the input signal (vip, vin) is isolated by pull-up PMOS transistor M ISO located at the virtual ground of NMOS differential transistor pair M 1001 , M 1002 (where their sources are connected) of first CML latch 901 A in CML flip flop 802 . It prevents an unknown input signal from interrupting the reset process. Since pull-up PMOS transistor M ISO is not placed at the signal path, the parasitic capacitance (e.g. 5.5 fF) of PMOS transistor M ISO does not degrade circuit performance.

FIG. 11 illustrates a divide-by-2 circuit utilizing the asynchronous reset of the present invention. As shown in FIG. 11 , the single CML flip flop 802 is used to generate a divide-by-2. Note the pull-up transistors M UP0 -M UP3

The asynchronous reset circuit described above is applicable to any CML circuit with a memory element that needs an initial state, examples of such circuits are those utilizing flip flops and latches where an initial state of the flip flop or latch needs to be defined.

Pull-up transistors M UP0 and M UP2 may be used alone, without their complementary pull-up transistors M UP1 , M UP3 , as long as it is acceptable to have different duty cycles for the vop and von. (Note also that pull-up transistors M UP0 and M UP3 can be NMOS transistors, to pull outputs of the CML latches 901 down to LOW(rather than PMOS transistors, to pull the outputs to HIGH, see M DOWN0 -M DOWN3 FIG. 19 , see also NMOS isolation transistor M ISO in FIG. 20 )). Alternatively, bipolar transistors may be used (see M 2101 - 2104 in FIG. 21 or M 2301 -M 2304 in FIG. 23 , see also binolar transistor M ISO in FIG. 22 and FIG. 24 ), or any device that can be used as a switch to pull the output of CML latches 901 A, 901 B to the supply voltage. Thus, M UP1 , M UP3 may be thought of as dummy transistors and, used to match outputs loading of CML latch 901 A, 901 B, outputs and to ensure symmetrical duty cycles. Note also that in some applications, only a single transistor M UP0 (of the four pull-up transistors M UP0 -M UP3 ) may be used.

As with the pull-up transistors M UP0 -M UP3 , pull-up transistor M ISO may be replaced by a switching element to pull the node up to supply voltage VDD (or in some cases down to ground).

Furthermore, while the first CML latch 901 A must have a pull-up transistor M ISO , it is possible to use a conventional CML latch (i.e. without a pull-up transistor M ISO ) as the second latch of flip flop 802 .

The minimum reset duration is less than half clock period if the reset is released at the negative clk period. It is very useful to reduce the latency if a low speed clock is used. For example, if clk is {fraction (1/20)} of full speed clock, then the latency is reduced at least by 10 UI.

The sizes of pull-up PMOS transistors M UP0-3 along the signal path are just big enough to provide the small voltage spur and then the NMOS cross-coupled transistor pairs of CML flip flops 802 accelerate the pull-up operation. Therefore, the parasitic capacitance due to each pull-up PMOS transistor M UP0-3 and routing is roughly 1 fF. This is too small to affect circuit performance.

The reset operation is the same as in a conventional CML divide-by-N in that all CML flip flop 802 outputs are reset to the known value (VSS, VDD). The reset operation of a CML divide-by-2 circuit shown in FIG. 10 is explained using timing diagrams of FIGS. 12-15 below:

As shown in FIG. 12 , the input (vip, vin) is isolated by pull-up PMOS transistor M ISO when the reset is on. When elk and reset are positive, the output (qop, qon) of first CML latch 901 A is pulled up to (VSS, VDD) by pull-up PMOS transistors M UP0-1 and, held and accelerated by NMOS cross-coupled transistor pair M 1001 , M 1002 of first CML latch 901 A. At the same time, it is amplified to the output (vop, von) of flip flop as (VSS, VDD). This is indicated by the lower downward arrow in FIG. 12 .

When the reset is released at the positive clk period, the output (vop, von) of CML flip flop 802 is still (VSS, VDD). When clk is negative again, the initial state (VSS, VDD) of the flip flop output is held by NMOS cross-coupled transistor pair M 1003 , M 1004 of second CML latch 901 B. Its inverted flip flop output (VDD, VSS) through the combination logic 801 is asserted at the input (vip, vin) and read into CML flip flop 802 . As a result, the divide-by-2-circuit starts running with the initial state. This is indicated by the upward arrow in FIG. 12 .

As shown in FIG. 13 , the input (vip, vin) is isolated by pull-up PMOS transistor M ISO when the reset is on. When clk is negative and reset is positive, the output (vop, von) of CML flip flop 802 is pulled up to (VSS, VDD) by pull-up PMOS transistors M UP2-3 and, held and accelerated by NMOS cross-coupled transistor pair of second CML latch 901 B. This is indicated by the upward arrow in FIG. 13 .

When the reset is released at the negative clk period, the initial state (VDD, VSS) of inverted flip flop output through combination logic 801 is asserted at the input (vip, vin) and read into CML flip flop 802 . As a result, the divide-by-2-circuit starts running with the initial state. This is indicated by the downward arrow in FIG. 13 .

As shown in FIG. 14 , the input (vip, vin) is isolated by pull-up PMOS transistor M ISO when the reset is on. When clk and reset are positive, the output (qop, qon) of first CML latch 901 A is pulled up to (VSS, VDD) by pull-up PMOS transistors M UP0 -M UP1 , and held and accelerated by NMOS cross-coupled transistor pair M 1001 , M 1002 of first CML latch 901 A. At the same time, it is amplified to the output (vop, von) of flip flop as (VSS, VDD). This is indicated by the downward arrow in FIG. 14 .

When the reset is released at the falling edge of clk, the initial state (VSS, VDD) of CML flip flop 802 output is held by NMOS cross-coupled transistor pair M 1003 , M 1004 of second CML latch 901 B. Its inverted flip flop output (VDD, VSS) appears at the input (vip, vin) through the combination logic and read into CML flip flop 802 . As a result, the divide-by-2-circuit starts running with the initial state. This is indicated by the upward arrow in FIG. 14 .

As shown in FIG. 15 , the input (vip, vin) is isolated by pull-up PMOS transistor M ISO when the reset is on. When elk is negative and reset is positive, the output (vop, von) of flip flop is pulled up to (VSS, VDD) by pull-up PMOS transistors M UP2 -M UP3 , and is held and accelerated by NMOS cross-coupled transistor pair M 1003 , M 1004 of second CML latch 901 B.

When the reset is released at the rising edge of clk, the output (qop, qon) of first CML latch 901 A is pulled up to (VSS, VDD) by pull-up PMOS transistors M UP0-1 until the reset drops below the threshold voltage V th of M UP0 . Although the output (qop, qon) cannot reach the value (VSS, VDD) at the end of reset, the output (qop, qon) is kept pulling up to (VSS, VDD) by the positive-feedback NMOS cross-coupled transistor pair M 1003 , M 1004 of first CML latch 901 A and amplified to the output (vop, von) of CML flip flop 802 as (VSS, VDD). This is indicated by the downward arrow.

When clk is negative again, the initial state (VSS, VDD) of CML flip flop 802 output is held by NMOS cross-coupled transistor pair M 1003 , M 1004 of second CML latch 901 B. Its inverted flip flop output (VDD, VSS) is asserted at the input (vip, vin) through combination logic 801 , and is read into CML flip flop 802 . As a result, the divide-by-2 circuit starts running with the initial state. This is indicated by the upward arrow in FIG. 15 .

Thus, in FIGS. 12-15 , the case illustrated in FIG. 13 show the shortest reset time, and the case illustrated in FIG. 12 shows the longest reset time.

Unlike conventional CML divide-by-N, the reset in the circuits illustrated in FIGS. 8-11 is done asynchronously but not sequentially. The minimum reset duration can be less than the half of clock period if the reset is released at the negative clock period. It shortens the latency by 50%.

Considering FIG. 16 , the divide-by-4 is simulated with N 2 in the circuit of FIG. 8 . Here, VT(ckxpcml) corresponds to vop 2 , VT(ckxncml) corresponds to von 2 and VT(reset) corresponds to reset. VT(reset) has 4 pulses in order to simulate the four possible cases of FIGS. 12-15 . It is observed that, when VT(reset) is on, the initial states of VT(ckxpcml) & VT(ckxncml) are 0 and 1 respectively. This corresponds to the cases shown in FIGS. 12-15 .

FIGS. 16-17 show the simulation results under different simulation conditions, e.g., different temperatures, VDD, and so on. These figures confirm that the asynchronous reset of the invention works properly.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the appended claims. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.