Clock Generator Circuit With Low Current Frequency Divider

This invention is in the field of integrated circuits, and is more specifically directed to clock generator circuits for the same.

Due to recent advances in the field, modern semiconductor integrated circuits have the capability of providing a high degree of functionality and performance with low active power dissipation. In particular, complementary metal-oxide-semiconductor (CMOS) technology provides circuits that can have quite fast switching times with extremely low levels of DC power dissipation. These advances have enabled complex electronic systems to now use batteries and other alternative power sources as primary or backup power. Examples of such systems include portable personal computers of the "notebook" or "laptop" class, personal digital assistants (PDAs), and cellular telephones and facsimile machines.

While many digital circuit functions are readily implemented in CMOS in such a manner that the steady-state power dissipation is extremely low (e.g., CMOS inverters and other logic gates), certain necessary circuit functions are not so easily implemented in that manner. An example of such a circuit function is a clock generator circuit, such as those based on a crystal oscillator. Clock generator circuits are inherently in a constant switching mode, and seldom in a DC state, and thus cannot readily take advantage of the extremely low steady-state power dissipation provided by CMOS technology.

Referring now to FIG. 1, clock generator circuit 10 according to the prior art will now be described in detail. Clock generator circuit 10 utilizes crystal oscillator 14 to generate a periodic signal that is amplified by level shift circuit 16. The amplified periodic signal is then presented to frequency divider 18 to produce an output clock signal of the desired frequency at terminal CLK. According to this conventional approach, reference voltages V.sub.pref and V.sub.nref are generated by low-current reference circuit 12 and applied to crystal oscillator 14 to control the voltage swing of the output clock signal. Reference circuit 12 is preferably configured to include a current source operating in the subthreshold region, as is known in the art (and as will be described hereinbelow), such that the steady-state current dissipated by reference circuit 12 is on the order of 40 nA.

Crystal oscillator 14 is a current-based oscillator of conventional construction, and includes a single gain stage with crystal and resistive feedback. Crystal resonator 15 is a conventional quartz crystal, and is connected off-chip to nodes XTAL1, XTAL2 in the conventional manner. Crystal oscillator 14 includes bias feedback resistor R connected across nodes XTAL1, XTAL2, and crystal frequency control capacitors C.sub.1, C.sub.2 connected between nodes XTAL1, XTAL2, respectively, and ground. The single gain stage in crystal oscillator 14 is provided in the conventional manner by a series of MOS transistors 11, 13, 19. P-channel transistor 11 has its source-drain path connected between V.sub.cc and node XTAL1, and has its gate biased by reference voltage V.sub.pref, N-channel transistor 19 has its source connected to ground and its drain connected to the source of n-channel gain transistor 13; the gate of n-channel transistor 19 is biased by reference voltage V.sub.nref. N-channel gain transistor 13 has its drain connected to node XTAL1 and its gate connected to node XTAL2.

According to this arrangement, transistor 11 operates as a current source load, biased by reference voltage V.sub.pref. Transistor 13 operates as a current-starved inverter, as the current therethrough is limited by transistors 11, 19. As a result, gain transistor 13 is self-biased in the weak inversion regions via resistor R and the current source transistors 11, 19. This current-limited self-biased implementation allows crystal oscillator 14 to operate as a class A or class AB oscillator, preventing significant bias point shift between its DC and AC operating points. In addition, the small signal operation of crystal oscillator 14 ensures low signal and low loss operation of crystal resonator 15.

According to this conventional construction, the output of crystal oscillator 14 is applied to level shift circuit 16, under the same bias conditions, to amplify the small signal output of oscillator 14 prior to application of the signal to frequency divider 18. According to this conventional approach, level shift circuit 16 includes a series of MOS transistors 21, 22, 23 having their source-drain paths connected in series between V.sub.cc and ground. P-channel transistor 21 and n-channel transistor 23 have their gates biased by reference voltages V.sub.pref, V.sub.nref, respectively. N-channel gain transistor 22 has its source connected to the drain of transistor 23, its drain connected to the drain of transistor 21 at node OUT, and its gate connected to oscillator node XTAL2. The signal at node OUT will thus be an amplified version of the oscillator output, and at the same frequency.

Frequency divider 18, according to this conventional arrangement, divides down the amplified signal at node OUT to the desired frequency to produce a clock signal at node CLK for distribution elsewhere in the integrated circuit or the electronic system. Frequency divider 18 is conventionally implemented as a series of counter stages, as is well known in the art, with the appropriate feedback stages implemented as necessary to provide the proper output frequency at node CLK.

According to this conventional approach for generating clock signals, however, the first several counter stages of frequency divider 18 draws significant current, due to the necessity of applying full digital signals to conventional frequency divider counter stages, as produced by level shift circuit 16. The first counter stages of frequency divider 18 are thus switching over large signal swings at relatively high frequencies, causing the high level of significant power dissipation which is undesirable for battery-powered systems.

It is therefore an object of the present invention to provide a clock generator circuit that has reduced current drain and power dissipation relative to conventional clock generator circuits.

It is a further object of the present invention to provide such a circuit which utilizes clocked current sources to limit the overall current requirements of the circuit.

It is a further object of the present invention to provide such a clock generator circuit that is readily implementable in CMOS technology.

Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following description in combination with its drawings.

The invention may be implemented by a clock generator circuit based upon a crystal oscillator and a subthreshold reference generator, where the periodic signal is divided down in frequency prior to amplification. A current switch receives the output of the crystal oscillator and, via current mirror output legs, produce current source control signals based upon the oscillator output. The current source control signals are applied to current sources in current source switched latches connected in counter fashion in a frequency divider. This construction of the latches limits significant current drain to the switching intervals only, thus reducing the overall current required over time. The output of the frequency divider is applied to a level shift circuit which amplifies the output clock signal to the desired voltage swing.

FIG. 1 is an electrical diagram, in block and schematic form, of a clock generator circuit according to the prior art.

FIG. 2 is an electrical diagram, in block form, of a clock generator circuit according to the preferred embodiment of the invention.

FIG. 3 is an electrical diagram, in block form, of a first stage of the clock generator circuit of FIG. 2.

FIGS. 4a through 4c are electrical diagrams, in schematic form, of various embodiments of a voltage reference circuit as used in the preferred embodiment of the invention.

FIGS. 5a and 5b are electrical diagrams, in schematic form, of alternative embodiments of a crystal oscillator circuit as used in the preferred embodiment of the invention.

FIGS. 6a and 6b are electrical diagrams, in schematic form, of alternative implementations of a current switch as used in the preferred embodiment of the invention.

FIGS. 7a and 7b are electrical diagrams, in schematic form, of alternative implementations of a current-clocked latch as used in the preferred embodiment of the invention.

FIG. 8 is a timing diagram illustrating the operation of the clock generator circuit of FIG. 2.

Referring now to FIG. 2, clock generator circuit 30 according to the preferred embodiment of the invention will now be described in detail. In this example, clock generator circuit 30 produces an output clock signal, at terminal CLK, that is divided-down in frequency from a periodic signal produced by crystal oscillator circuit 34 using off-chip crystal resonator 35. While clock generator circuit 30 performs a by-eight frequency division, as will be described hereinbelow, it should be apparent to those of ordinary skill in the art clock generator circuit 30 may divide down the frequency to another subfrequency merely by selection of the number of stages in the forward or feedback loops, in the conventional manner. Selection of the proper frequency division factor will, of course, depend upon the particular clock frequency desired and the component selection for crystal oscillator 34, as is well known.

According to this embodiment of the invention, clock generator circuit 30 includes a low-current reference voltage circuit 32 for producing reference voltages V.sub.pref, V.sub.nref, which are applied to crystal oscillator 34. Crystal oscillator 34 is connected to crystal resonator 35 (which is preferably an external component, as is conventional in the art) at nodes XTAL1, XTAL2, and is configured in the conventional manner (such as that described hereinbelow) to produce a periodic signal at nodes XTAL1, XTAL2. Nodes XTAL1, XTAL2 are received at inputs IN.sub.0, IN.sub.0-- of a first divider stage 36.sub.0. The outputs OUT.sub.0, OUT.sub.0-- of divider stage 36.sub.0 are connected to inputs IN.sub.1, IN.sub.1-- of the next divider stage 36.sub.1, which in turn has its outputs OUT.sub.1, OUT.sub.1-- connected to inputs IN.sub.2, IN.sub.2-- of the third divider stage 36.sub.2. As will be described hereinbelow, each divider stage 36 includes a pair of latches biased by current sources that are switched in complementary fashion, such that each divider stage 36 produces a periodic signal at its outputs OUT, OUT.sub.-- at half the frequency of the signal at its inputs IN, IN.sub.--. Outputs OUT.sub.2, OUT.sub.2-- of the final divider stage 36.sub.2 are received by level shift circuit 38 to produce the desired voltage swing at terminal CLK.

Similarly as described above relative to clock generator circuit 10 of FIG. 1, the voltage swing of the periodic clock signal at nodes XTAL1, XTAL2 of clock generator circuit 30 according to the present invention is relatively small, so that crystal oscillator 34 can operate in a low loss, low power, condition. According to this embodiment of the invention, this small voltage swing periodic signal (e.g., on the order of 200 mV) can be applied directly to frequency divider stages 36, prior to amplification by level shift circuit 38. This operation reduces the current drain and power dissipation of clock generator circuit 30 according to the present invention relative to that of conventional clock generator circuits, such as clock generator circuit 10 of FIG. 1. Clock generator circuit 30 according to the present invention is especially advantageous when implemented into power sensitive applications, such as portable computers and other portable systems that operate from batter power either as the primary power source or as a backup in a loss of power event.

Referring now to FIG. 3, the construction of first divider stage 36.sub.0 will now be described in detail. Second and third divider stages 36.sub.1, 36.sub.2 of clock generator circuit 30 are similarly constructed as divider stage 36.sub.0 ; it is therefore contemplated that the description of the construction and operation of first divider stage 36.sub.0 will be adequate to describe each of divider stages 36 according to this preferred embodiment of the invention. First divider stage 36.sub.0 includes current switch 40, having inputs IN.sub.0, IN.sub.0-- connected to nodes XTAL1, XTAL2 of crystal oscillator 34. Current switch 40 provides current source control signals A, A.sub.-- to complementary clock inputs CK, CK.sub.-- of D-type latch 500, and current source control signals B, B.sub.-- to complementary clock inputs CK, CK.sub.-- of D-type latch 50.sub.1. Latches 50, as will be described hereinbelow, are of the current source switched type, in which the complementary clock inputs CK, CK.sub.-- control pull-up and pull-down current sources therein.

Outputs Q, Q.sub.-- of latch 50.sub.0 are connected to inputs D, D.sub.--, respectively, of latch 50.sub.1. Outputs Q, Q.sub.-- of latch 50.sub.1 are cross-connected to inputs D.sub.--, D, respectively, of first latch 50.sub.0. As is known in the art, and as apparent from this description and FIG. 3, the connection of inputs D, D.sub.-- of latch 50.sub.0 to the complementary outputs (Q.sub.--, Q) of latch 50.sub.1 implements latches 50.sub.0, 50.sub.1 as a divide-by-two frequency divider.

As noted above, it is preferable for purposes of minimizing power dissipation that crystal oscillator 34 operate at small voltage swings, which requires proper biasing by reference voltage circuit 32. Referring now to FIGS. 4a through 4c, alternative conventional examples of reference voltage circuit 32 will now be described in detail, to describe the best mode of the present invention.

FIG. 4a illustrates reference circuit 32a, which produces reference voltages V.sub.pref, V.sub.nref. Reference circuit 32a has a first complementary leg of p-channel transistor 31p with its source biased to V.sub.cc, and n-channel transistor 31n with its source biased to ground; the drains of transistors 31p, 31n are connected together at node V.sub.nref. The second leg of reference circuit 32a has p-channel transistor 33p with its source biased to V.sub.cc and n-channel transistor 33n with its source biased to ground through resistor 35; the drains of transistors 33p, 33n are connected together at node V.sub.pref. The gates of transistors 31p, 33p are connected together at node V.sub.pref, and the gates of transistors 31n, 33n are connected together at node V.sub.nref. In this second leg, however, transistor 33n has a channel width to channel length ratio (W/L) that is a multiple of (i.e., larger than) that of the other transistors 31p, 31n, 33p. For example, the W/L of n-channel transistor 33n may be ten times the W/L of transistors 31p, 31n, 33p (which have equal W/Ls relative to one another).

In reference circuit 32a, each of transistors 31p, 31n, 33p, 33n is biased into the sub-threshold region. As reference circuit 32a operates as a current mirror, the drain current through transistors 31p, 31n is equal to that through transistors 33p, 33n and resistor 35; for conventional construction, this drain current will be on the order of 20 nA for each leg. Alternatively, the construction of reference circuit 32a may be altered so that the leg currents are ratioed, if desired. Because of the subthreshold bias condition, the equal drain currents, and the presence of resistor 35, the gate-to-source voltage across larger transistor 33n will be less than that for smaller transistor 31n by an amount determined by the transistor size ratio and the subthreshold current slope. This difference in gate-to-source voltage produces the voltage across resistor 35. The value of resistor 35 and the gate-to-source voltage difference determines the current level in the bias circuit.

Reference circuit 32b shown in FIG. 4b is configured similarly as reference circuit 32a, but includes additional transistors 37p, 39p to produce an additional reference voltage V.sub.pref '. P-channel transistor 37p has its source connected to the drain of p-channel transistor 31p and has its drain connected to the drain of n-channel transistor 31n at node V.sub.nref. P-channel transistor 39p has its source connected to the drain of p-channel transistor 33p at node V.sub.pref and has its drain connected to the drain of n-channel transistor 33n. The gates of p-channel transistors 37p, 39p are connected together, and to the drains of p-channel transistor 39p and n-channel transistor 33n at node V.sub.pref '. In this example, p-channel transistors 37p, 39p also have a large W/L relative to that of transistors 31p, 33p, 31n, similarly as transistor 33n.

In operation, as in the case of reference circuit 32a, the drain current through transistors 31p, 37p, 31n will be equal to the drain current through transistors 33p, 39p, 33n, and resistor 35. The subthreshold bias of all transistors in reference circuit 32b, along with the differing W/L ratios, cause the voltage drops across the larger devices 37p, 39p, 33n to be lower than across the smaller transistors 31p, 33p, 31n, thus generating reference voltages V.sub.pref, V.sub.nref, V.sub.pref '.

FIG. 4c illustrates reference circuit 32c, which produces reference voltages V.sub.pref, V.sub.nref in a manner that is under the control of input reference voltages PREFIN, NREFIN. Input reference voltages PREFIN, NREFIN may be inputs to the integrated circuit, or alternatively may be reference voltages produced by another on-chip reference voltage generator circuit. Reference circuit 32c includes transistors 31p, 31n, 33p, 33n, and resistor 35 implemented into two legs, as before, with transistor 33n having a substantially larger W/L ratio than transistors 31p, 31n, 33p.

In addition, however, the first leg of reference circuit 32c includes p-channel transistor 41p and n-channel transistor 41n with their source-drain paths connected in series between the drain of transistor 31p and the drain of transistor 31n. Similarly, the second leg of reference circuit 32c includes p-channel transistor 43p and n-channel transistor 43n with their source-drain paths connected in series between the drain of transistor 33p and the drain of transistor 33n. Transistors 41p, 41n, 43p, 43n preferably have a large W/L ratio, similar to that of transistor 33n. The gates of transistors 41p, 43p are connected together and receive input reference voltage PREFIN, while the gates of transistors 41n, 43n are connected together and receive input reference voltage NREFIN. The gates of p-channel transistors 31p, 33p are connected together and to output node V.sub.pref at the drains of transistors 43p, 43n; similarly, the gates of n-channel transistors 31n, 33n are connected together and to output node V.sub.nref at the drains of transistors 41p, 41n.

Reference circuit 32c operates in the subthreshold mode, so long as input reference voltages PREFIN, NREFIN are approximately one p-channel threshold voltage below V.sub.cc and approximately one n-channel threshold voltage above ground, respectively.

Referring back to FIGS. 2 and 3, clock generator circuit 30 includes crystal oscillator 34 which receives one or more of the reference voltages V.sub.pref, V.sub.nref, V.sub.pref ' produced by the appropriate reference circuit 32. FIG. 5a illustrates crystal oscillator 34a according to first implementation useful in clock generator circuit 30 of the present invention.

Crystal oscillator 34a is similar to crystal oscillator 14 of FIG. 1, and as such receives reference voltages V.sub.pref, V.sub.nref from reference circuit 32. Oscillator 34a includes p-channel bias transistor 42p with its source biased to V.sub.cc and its gate receiving reference voltage V.sub.pref, and n-channel bias transistor 42n with its source biased to ground and its gate receiving reference voltage V.sub.nref. Active n-channel transistor 44 has its drain connected to the drain of transistor 42p at node XTAL1, its source connected to the drain of transistor 42n, and its gate connected to node XTAL2. Feedback resistor R and external crystal resonator 35 (see FIGS. 2 and 3) are connected in parallel between nodes XTAL1, XTAL2; nodes XTAL1, XTAL2 are forwarded to divider stage 36.sub.0 of FIG. 2.

In operation, as is conventional in the art, crystal resonator 35 serves as a feedback LRC resonant circuit between nodes XTAL1, XTAL2 in crystal oscillator 34a, in parallel with feedback resistor R. Nodes XTAL1, XTAL2 will therefore oscillate periodically over a relatively small voltage swing (e.g., a range of 200 mV) defined by reference voltages V.sub.pref, V.sub.nref. This small-signal operation maintains crystal resonator 35 in the preferred low loss condition.

FIG. 5b illustrates crystal oscillator 34b according to an alternative implementation which utilizes the three reference voltages V.sub.pref, V.sub.nref, V.sub.pref ' produced by reference circuit 32b of FIG. 4b. Crystal oscillator 34b is constructed similarly as crystal oscillator 34a, with the addition of p-channel transistor 45 having its source-drain path connected between the drain of transistor 42p and node XTAL1 at the drain of transistor 44, and having its gate receiving reference voltage V.sub.pref '. As a result, p-channel transistor 45 is connected in a cascode configuration to current source load transistor 42p, increasing the output resistance of the current source at node XTAL1 in the conventional manner for cascode circuits. Crystal oscillator 34b thus allows reference voltage V.sub.pref ' to more finely control the voltage swing at nodes XTAL1, XTAL2, as well as the drain current through transistors 42p, 45, 44, 42n.

According to the preferred embodiment of the invention, the small voltage swing periodic signal produced by crystal oscillator 34 is divided down in frequency prior to its amplification to a large voltage swing signal, so that the current consumed by clock generator circuit 30 is minimized. As such, referring to FIG. 3, the periodic signal at nodes XTAL1, XTAL2 is applied to current switch 40 in first divider stage 36.sub.0, for the first frequency division in clock circuit 30.

Referring now to FIG. 6a, the construction and operation of a first embodiment of current switch 40 will now be described in detail. Current switch 40 is primarily constructed as a current divider with two output current mirror legs. In this example, n-channel transistor 46 with its source connected to ground has its gate biased by reference voltage V.sub.nref, and serves as a current source. A first divider leg of current switch 40 has p-channel load transistor 48p with its source biased to V.sub.cc and its gate tied to its drain. The gate-drain node of transistor 48p is connected to the drain of n-channel transistor 48n, which has its source connected to the drain of current source transistor 46 and its gate connected to node XTAL1 of oscillator 34. The second divider leg similarly has an n-channel transistor 49n with its source connected to the drain of current source transistor 46 and its gate connected to node XTAL2 of oscillator 34. In this second divider leg, p-channel load transistor 49p has its source biased to V.sub.cc and its gate connected to its drain, and to the drain of n-channel transistor 49n.

Referring to FIG. 2, inputs IN.sub.0, IN.sub.0-- of first divider stage 36.sub.0 thus correspond to the gates of transistors 48n, 49n, respectively, in current switch 40, which receive nodes XTAL1, XTAL2 of crystal oscillator 34 thereat.

As shown in FIG. 6a, a first current mirror output leg is formed by p-channel transistor 51p and n-channel transistor 51n which have their drains connected together and their sources biased to V.sub.cc and ground, respectively. The gate of transistor 51p is connected to the gate of transistor 48p, and thus to common drain node A of transistors 48p, 48n to produce a current source control signal. The gate of transistor 51n is connected to the common drain node A.sub.-- of transistors 51p, 51n, at which a complementary current source control signal is produced. Similarly, the second current mirror output leg is formed by p-channel transistor 52p and n-channel transistor 52n with their drains connected together at node B.sub.-- and their sources biased to V.sub.cc and ground, respectively; the gate of transistor 52n is connected to the common drain node at node B.sub.--. The gate of transistor 52p is connected to the gate of transistor 49p and to the common drain node B of transistors 49p, 49n, producing a current source control signal thereat which is complementary to that produced at node B.sub.--.

In operation, the periodic oscillating signal at nodes XTAL1, XTAL2 controls transistors 48n, 49n to conduct in a complementary fashion relative to one another, such that the bulk of the current of current source 46 alternates between the divider leg of transistors 48p, 48n on one hand, and the divider leg of transistors 49p, 49n, on the other. The current conducted through each divider leg is mirrored in its current mirror output leg. Specifically, the current conducted through transistor 48p (under the control of transistor 48n) is mirrored through transistors 51p, 51n; conversely, the current through transistor 49p is mirrored through transistors 52p, 52n. In this configuration, when the bulk of the current in the current divider is conducted by transistors 48p, 48n, node A at the drain of transistor 48p will be at a relatively small offset below V.sub.cc, causing transistors 51p, 51n to conduct an amount of current ratioed by the relative size of transistor 51p to that of transistor 48p, as is conventional in current mirrors. Node A.sub.-- is pulled to a small offset above V.sub.ss during this time. The low current through transistors 49p, 49n (relative to that through transistors 48p, 48n) will be mirrored through transistors 52p, 52n, placing node B substantially at V.sub.cc and node B.sub.-- substantially at V.sub.ss. Switching of the signal at nodes XTAL1, XTAL2 from oscillator 34 will cause the opposite divider leg (transistors 49p, 49n) to conduct the bulk of the current, resulting in complementary states at nodes A, A.sub.--, B, B.sub.-- from that described above. Accordingly, the signal at node B is complementary to the signal at node A, with the complementary nodes A.sub.--, B.sub.-- also being complementary to one another (as well as to nodes A, B, respectively). The signal swing of these complementary signals is relatively low, however, for example on the order of 200 mV.

Referring to FIG. 6b, current switch 40' according to an alternative implementation is shown. Current switch 40' is substantially identical to current switch 40 of FIG. 6a, except that it further includes small keeper transistors 53p, 53p.sub.--, 53n, 53n.sub.--. P-channel keeper transistor 53p has its source biased to V.sub.cc, its drain connected to node A and its gate connected to node A.sub.--, while p-channel keeper transistor 53p.sub.-- has its source also biased to V.sub.cc, its drain connected to node A.sub.-- and its gate connected to node A. Similarly, n-channel keeper transistors 53n, 53n.sub.-- each have their sources biased to ground; transistor 53n has its drain connected to node B and its gate connected to node B.sub.--, while transistor 53n has its drain connected to node B.sub.-- and its gate connected to node B. In operation, keeper transistors 53p, 53p.sub.--, 53n, 53n.sub.-- serve to maintain the state of nodes A, A.sub.--, B, B.sub.-- during transitions of current switch 40'. The operation of current switch 40' is thus cleaner than that of current switch 40, particularly at higher frequencies.

Referring back to FIG. 3, nodes A, A.sub.-- are connected to the complementary clock inputs of first latch 50.sub.0 in first divider stage 36.sub.0, and nodes B, B.sub.-- are connected to the complementary clock inputs of second latch 50.sub.1 of first divider stage 36.sub.0. As noted above, latches 50 are of the current source switched type, including cross-coupled CMOS inverters. FIG. 7a schematically illustrates latch 50.sub.0 which receives data inputs D, D.sub.-- and nodes A, A.sub.-- from current switch 40 as clock inputs, specifically clock inputs in the form of current source control signals. Latch 50.sub.1 will, of course, be similarly constructed as latch 50.sub.0.

Latch 50.sub.0 includes p-channel pull-up current source transistor 54p with its source biased to V.sub.cc, its drain at node L1, and its gate controlled by the current source control signal at node A of current switch 40. Similarly, n-channel pull-down current source transistor 54n has its source biased to ground, its drain at node L2, and its gate controlled by complementary current source control signal at node A.sub.-- of current switch 40. The latching mechanism of latch 50.sub.0 is implemented in the conventional manner by CMOS inverters, cross-coupled at nodes C1, C2. A first CMOS inverter is formed by p-channel transistor 56p with its source connected to node L1 and its drain connected to the drain of n-channel transistor 56n at node C2, and by n-channel transistor 56n with its source connected to node L2, and its gate connected to the gate of p-channel transistor 56p at node C1. P-channel transistor 58p and n-channel 58n form the opposing CMOS inverter, with their drains connected together at node C1, their gates connected together at node C2, and their sources at nodes L1, L2, respectively. Latch 500 also has complementary output stages, one formed by p-channel transistor 55p and n-channel transistor 55n with their drains connected together to drive output Q.sub.--, their gates connected together at node C1, and their sources at nodes L1, L2, respectively; the other output stage is formed by p-channel transistor 57p and n-channel transistor 57n having their drains connected together to drive output Q, their gates connected together to node C2, and their sources at nodes L1, L2, respectively. Latch 50.sub.0 receives input D at node C1 and input D.sub.-- at node C2.

In operation, the state of outputs Q, Q.sub.-- of latch 50.sub.0 depend upon the states of inputs D, D.sub.--, as clocked by current source transistors 54p, 54n. In this example, both of complementary current source transistors 54p, 54n will be more conductive when node A is at a low level (and node A.sub.-- high), and less conductive when node A is high (and node A.sub.-- low). It is contemplated that transistors 54p, 54n are sized to conduct on the order of 30 nA in their more conductive condition and conduct on the order of picoamperes in their less conductive state. These currents are selected so that latch 50.sub.0 will latch in the data state at its D, D.sub.-- inputs when transistors 54p, 54n are made more conductive. When transistors 54p, 54n are less conductive (i.e., nodes A, A.sub.-- are in their relatively high and low states, respectively), the small current sourced therethrough may be sufficient to maintain the stored state of latch 50.sub.0 ' for a short time, but is inadequate to drive the outputs Q, Q' regardless of the logic levels at the D, D.sub.-- inputs. In the limit of the less conductive condition, transistors 54p, 54n are effectively off, and the states of nodes C1, C2 are temporarily held by the capacitance on the respective nodes.

Referring now to FIG. 7b, the construction of latch 50.sub.0 ' according to an alternative implementation will now be described. The construction of latch 50.sub.0 ' is substantially identical to that of latch 50.sub.0 described hereinabove, except that latch 50.sub.0 ' also includes clamping transistors 61, 63. N-channel latching transistor 61 has its drain and gate connected to node C2, and its source connected to node C1; conversely, n-channel latching transistor 63 has its drain and gate connected to node C1 and its source connected to node C2. Transistors 61, 63 thus limit the voltage swing of cross-coupled nodes C1, C2 to a voltage approximately on the order of an n-channel threshold voltage, ensuring small signal operation of latch 50.sub.0 '.

Latches 50 are thus constructed to operate as D-type latches, where the clocking action is based upon the switching of current source transistors 54p, 54n connected between V.sub.cc and node L1, and between node L2 and ground, respectively. As a result, not only is the current drawn by latches 50 controlled to a certain level, but the time during which this current is drawn is reduced. Latches 50 can therefore operate with relatively small voltage swing signals at nodes A, A.sub.--, B, B.sub.--, and at inputs D, D.sub.--, such as on the order of 200 mV. The voltage swings at outputs Q, Q.sub.-- also are relatively small, such as on the order of 200 mV, allowing succeeding latch stages to similarly operate responsive to small signal swing.

Referring back to FIG. 3, first divider stage 36.sub.0 includes two latches 50.sub.0, 50.sub.1 which are connected to one another in such a manner as to form a two-stage counter, and thus a divide-by-two frequency divider. In this example, outputs Q, Q.sub.-- of latch 50.sub.0 are connected to inputs D, D.sub.--, respectively, of latch 50.sub.1. Outputs Q, Q.sub.-- of latch 50.sub.1 are connected to inputs D.sub.--, D, respectively, of latch 50.sub.0, to provide the negative feedback necessary to operate as a clocked counter. The sequential operation of first stage divider 36.sub.0 is shown in the following table, with the "phase" column indicating the current source control signal A or B that is active in each cycle, and with the results of the latch shown at the end of the stated phase:

This operational sequence thus repeats itself every two cycles of the current source control signals at nodes A, B.

Referring to FIG. 8, which illustrates the operation of clock generator circuit 30 of FIG. 2, attention is directed to the first three lines which illustrate the operation of first divider stage 36.sub.0. Lines A, B illustrate the periodic signals at nodes A, B of current switch 40; as is evident from the description hereinabove, the states at nodes A, B are complementary to the states at terminals XTAL1, XTAL2, respectively, of crystal oscillator 34. Line Q.sub.0 illustrates the signal at output Q of latch 50.sub.0, and line OUT.sub.0 illustrates the signal at output Q of latch 50.sub.1, which serves as output OUT.sub.0 of first divider stage 36.sub.0 of FIG. 2. As is evident from the foregoing table and as illustrated in FIG. 8, the frequency of the signal at output Q of latch 50.sub.1, and thus at outputs OUT.sub.0, OUT.sub.0-- of first divider stage 36.sub.0, is one-half that of the signal at outputs A, B of current switch 40.

Referring back to FIG. 2, outputs OUT.sub.0, OUT.sub.0-- of first divider stage 36.sub.0 are connected to inputs IN.sub.1, IN.sub.1--, respectively of second divider stage 36.sub.1. According to this embodiment of the invention, second divider stage 36.sub.1 is similarly constructed as first divider stage 36.sub.0 described hereinabove, and thus contains a current switch 40 followed by two D-type latches 50, with current switch 40 in second divider stage 36.sub.4 also receiving reference voltage V.sub.nref in the manner shown in FIG. 6a. As in the case of first divider stage 36.sub.0, inputs IN.sub.1, IN.sub.1-- of second divider stage 36.sub.1 correspond to the gates of transistors 48n, 49n in its current switch 40. Second divider stage 36.sub.1 thus operates in the same manner as first divider stage 36.sub.0, such that the periodic signal appearing at its outputs OUT.sub.1, OUT.sub.1-- is at one-half the frequency of the periodic signal applied to its inputs IN.sub.1, IN.sub. 1--, and thus at one-fourth the frequency of the oscillator frequency at terminals XTAL1, XTAL2 (i.e., at nodes A, B of current switch 40 in first divider stage 36.sub.0), as illustrated by line OUT.sub.1 in FIG. 8. Since second divider stage 36.sub.1 is constructed similarly as first divider stage 36.sub.0, the signals at outputs OUT.sub.1, OUT.sub.1-- of second divider stage 36.sub.1 also have a small voltage swing, also on the order of 400 mV.

As noted above, current switch 40 of second divider stage 36.sub.1 receives reference voltage V.sub.nref at its current source transistor (corresponding to transistor 46 of FIG. 6a). Alternatively, the current source in current switch 40 of second divider stage 36.sub.1 could be controlled by one of the nodes A, A.sub.--, B, B.sub.-- from the current switch 40 of the prior divider stage 36.sub.0, so as to also operate in a switched manner, further reducing the current drawn by clock generator circuit 30.

In similar fashion, inputs IN.sub.2, IN.sub.2-- of third divider stage 36.sub.2, similarly constructed as first and second divider stages 36.sub.0, 36.sub.1, receives the signals generated at outputs OUT.sub.1, OUT.sub.1-- of second divider stage 36.sub.1. Reference voltage V.sub.nref is also provided to third divider stage 36.sub.2, specifically to the current source in current switch 40 therein; alternatively, as noted above, the current source may be controlled by the output of current switch 40 in a prior divider stage 36.sub.0, or 36.sub.1. Third divider stage 36.sub.2 produces a periodic signal at its outputs OUT.sub.2, OUT.sub.2-- that is at half the frequency of that at outputs OUT.sub.1, OUT.sub.1 --, as shown in FIG. 8 relative to line OUT.sub.2. Accordingly, the frequency of the signal at outputs OUT.sub.2, OUT.sub.2-- is at one-eighth the frequency of that produced by crystal oscillator 34 at nodes XTAL1, XTAL2 (as evident in FIG. 8 from lines A, B), yet is a small voltage swing signal as before.

Outputs OUT.sub.2, OUT.sub.2-- are presented to level shift circuit 38 shown in FIG. 2 for amplification into the desired voltage swing for clock signal CLK. Level shift circuit 38 is similarly constructed as level shift circuit 16 of FIG. 1, utilizing reference voltages V.sub.pref, V.sub.nref and including a single gain stage. As a result, the output voltage swing of clock signal CLK is rail-to-rail, from V.sub.cc to ground. Of course, alternative conventional amplifier or level shift circuitry may be used to amplify the differential signal to the voltage swing desired at terminal CLK. Line CLK of FIG. 8 illustrates the amplified signal, which is at one-eighth the frequency of that of oscillator 34.

Clock generator circuit 30 according to the preferred embodiment of the invention thus draws significantly less current, and thus dissipates significantly less power, than prior clock generator circuits, as it is able to operate without requiring the amplification of high-frequency signals such as at the output of the crystal oscillator. This reduced power dissipation is provided by latches in the frequency divider that are of the current source switched type, where the oscillator output controls the clocking of current sources therein. As such, limited current is drawn by the latches only during short durations (i.e., during switching periods). Further, this clocked latch operation allows the input and output signals to be of small voltage swings, further reducing the power dissipation of the clock generator circuit. The clock generator circuit of the present invention thus requires significantly less current than that drawn according to prior techniques, such as shown in FIG. 1, which require level shifting of the oscillator output prior to frequency division, which results in signals of large voltage swing that switch at high frequencies. The present invention is therefore advantageous for use in low-power applications such as portable digital systems and computers.

While the invention has been described herein relative to its preferred embodiments, it is of course contemplated that modifications of, and alternatives to, these embodiments, such modifications and alternatives obtaining the advantages and benefits of this invention, will be apparent to those of ordinary skill in the art having reference to this specification and its drawings. It is contemplated that such modifications and alternatives are within the scope of this invention as subsequently claimed herein.