Semiconductor integrated circuit comprising an oscillator and a selectable frequency divider

Abstract: A semiconductor integrated circuit of this invention includes an oscillation circuit (1) formed on a semiconductor substrate (19), a frequency dividing circuit (9) formed on the semiconductor substrate (19), for dividing a frequency of an oscillation output from the oscillation circuit (1), clocked inverters (CI1, CI2, CI3, CI4) for selectively permitting one of an original oscillation frequency signal of the oscillation circuit (1) and outputs of the frequency dividing circuit (9) to pass therethrough, an output circuit (11) for outputting a signal selected by the clocked inverters (CI1, CI2, CI3, CI4), and a frequency dividing circuit controlling NAND circuit (21) for interrupting the operation of the frequency dividing circuit (9) while the original oscillation frequency signal of the oscillation circuit is being output from the output circuit (11).

Description: This invention relates to a semiconductor integrated circuit, and more particularly to a semiconductor integrated circuit having a CMOS crystal oscillator circuit and a frequency dividing circuit formed on the same semiconductor substrate. As shown in Fig. 1, a crystal oscillator circuit having a CMOS crystal oscillator circuit 1 and a frequency dividing circuit 9 for dividing the frequency of the crystal oscillation output formed on the same semiconductor substrate selects a signal having one of the original oscillation frequency fo and a divided frequency fo/2 n and outputs the selected signal as an output in the prior art. In Fig. 1, a reference numeral 10 denotes a decoder, 11 an output circuit. Fig. 2 shows an example of the circuit. In this example, the original oscillation circuit 1 is formed of a CMOS inverter 2, a feedback resistor 3 and an external circuit 7 which is constructed by a crystal oscillator 4 and pull-down capacitors 5 and 6. The original oscillation signal (frequency fo) of the circuit is input to a clocked inverter CI1 via a CMOS inverter 8 and to a frequency divider 9 (which is formed with a three-stage construction in this example) via the inverter 8 and signals of frequencies fo/2, fo/4 and fo/8 are output from the frequency divider 9. A desired one of the signals respectively having the original oscillation frequency fo, divided frequencies fo/2, fo/4 and fo/8 may be selected by inputting control signals A and B to the decoder 10, activating one of outputs C, D, E and F thereof and setting one of the clocked inverters CI1 to CI4 into the operative state by the activated output. The selected signal is derived from an output section via the 3-state output circuit 11. The potential of an oscillation interrupting terminal 13 is generally set to an "H (high)" level or power source level V DD by means of a pull-up resistor 14. If a signal of "L (low)" level or ground level is supplied to the oscillation interrupting terminal 13, the low level signal is supplied to an N-channel MOS transistor 16 for oscillation interruption via a CMOS inverter 15 to turn on the transistor 16 and set the potential of a node 17 to the "L" level, thereby interrupting the oscillation of the oscillation circuit 1. Further, the signal on the terminal 13 is input to a reset terminal R of the frequency divider 9 via the CMOS inverters 15 and 18 to interrupt the frequency dividing operation of the frequency divider 9. The signal is also input to a control terminal Co of the 3-state output circuit 11 via the CMOS inverters 15 and 18 to set the 3-state output circuit 11 to a high impedance state. Since the frequency dividing circuit 9 is operated even when the oscillation output is selected by the clocked inverter CI1 in the circuit of Fig. 2, noise contained in the oscillation output is increased by the switching noise generated from the frequency dividing circuit 9. The reason is that the power source current is varied in the switching operation caused during the frequency dividing operation of the frequency dividing circuit 9 and the power source V DD tends to be influenced by the generated noise. Particularly, in the integrated circuit, since the frequency dividing circuit 9, oscillation circuit 1 and inverter 8 connected between the frequency dividing circuit 9 and the oscillation circuit 1 commonly use the power source V DD , the power source noise generated from the frequency dividing circuit 9 may influence the power source voltage of the oscillation circuit 1 and the like and the noise may be superposed on an oscillation signal output via the inverter 8, clocked inverter CI1 and output circuit 11. However, since the frequency dividing circuit 9 itself has a wave forming function, the noise contained in the output of the frequency dividing circuit 9 is extremely small. US-4,223,268 discloses a frequency dividing circuit of a variable frequency dividing ratio type. The circuit contains first and second cascade-connected frequency dividers with signal controlling gates connected between the signal source and the first frequency divider, between the first and second frequency dividers, and between the signal source and the gate connected between the first and second frequency dividers. The signal controlling gates can be selectively controlled by a control switch to define a first signal path that includes both the first and the second frequency dividers to effectively divide the input signal by both frequency dividers, or the second signal path that includes only the second frequency divider to effectively divide the input signal by the second frequency divider only. The latter case, i.e. having only one of the frequency dividers operating, is achieved by stopping the input to the first frequency divider with the help of said signal controlling gates. An object of this invention is to provide a semiconductor integrated circuit which has an oscillator circuit and a frequency dividing circuit formed on the same semiconductor substrate and in which noise generated from the frequency dividing circuit can be suppressed and an output containing less noise can be obtained. The above object can be attained by a semiconductor integrated circuit comprising a semiconductor substrate having an oscillation circuit and a frequency dividing circuit connected to an output line of the oscillation circuit formed thereon; signal selection means for selectively permitting one of an original osicllation frequency signal of the oscillation circuit and outputs of the frequency dividing circuit to pass therethrough; an output circuit for outputting a signal selected by the selection means; and operation interrupting means for interrupting the operation of the frequency dividing circuit while the original oscillation frequency signal of the oscillation circuit is being output from the output circuit, where said frequency dividing circuit comprises a reset terminal to which the output of said operation interrupting means is supplied. With the above construction, when the original oscillation frequency signal is selected and output, the operation of the frequency dividing circuit which may cause noise is interrupted so that noise contained in the original oscillation frequency signal can be significantly reduced. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a perspective view of a semiconductor integrated circuit formed on a semiconductor chip formed of the same semiconductor substrate; Fig. 2 is a circuit diagram showing the conventional semiconductor integrated circuit; and Fig. 3 is a circuit diagram showing a semiconductor integrated circuit according to one embodiment of this invention. There will now be described an embodiment of this invention with reference to the accompanying drawings. Fig. 3 is a circuit diagram showing a semiconductor integrated circuit according to one embodiment of this invention and corresponding to that of Fig. 2, and like portions are denoted by the same reference numerals as those used in Fig. 2. In the circuit of Fig. 3, an original oscillation frequency signal (frequency fo) is generated from an original oscillation circuit 1 which is formed of a CMOS inverter 2, a feedback resistor 3 and an external circuit 7 which is constructed by a crystal oscillator 4 and capacitors 5 and 6. The original oscillation signal is amplified by a succeeding stage inverter 8 and then input to a clocked inverter CI1 and a frequency dividing circuit 9. Outputs of the frequency dividing circuit 9 respectively having a divided-by-2 frequency, divided-by-4 frequency and divided-by-8 frequency are respectively supplied to clocked inverters CI2, CI3 and CI4. The clocked inverters CI1 to CI4 are used to select one of the signals having the original oscillation frequency, divided-by-2 frequency, divided-by-4 frequency and divided-by-8 frequency according to output signals C, D, E and F which are generated from a decoder 10 in response to control signals A and B and supplies the selected signal to an output circuit 11 which in turn outputs the same as an output signal from a terminal 12. In a case where the original oscillation frequency signal is selected by the clocked inverter CI1, the output C of the decoder 10 is set to the "H" level and the outputs D, E and F thereof are set to the "L" level so that only the clocked inverter CI1 can be operated as a normal inverter, thus permitting the original oscillation frequency signal to be output. At this time, the output C of the decoder 10 which is at the "H" level and a signal of "H" level supplied from an oscillation interrupting terminal 13 via inverters 15 and 18 are input to a NAND circuit 21. The NAND circuit 21 outputs an "L" level output which is in turn supplied to the reset terminal R of the frequency dividing circuit 9. The operation of the frequency dividing circuit 9 is interrupted. That is, when the original oscillation frequency signal is derived from the terminal 12 as an output signal, the operation of the frequency dividing circuit 9 is interrupted and no switching noise occurs, thereby stabilizing the power source voltage and preventing noise from being generated from the frequency dividing circuit 9. As a result, substantially no noise is superposed on the original oscillation frequency output and an erroneous operation by the noise will not occur. This invention is not limited to the above embodiment and various modifications may be made. For example, the CMOS crystal oscillation circuit used in the above embodiment may be replaced by another oscillation circuit, and the NAND circuit 21 used as the frequency dividing operation interrupting means may be replaced by another suitable circuit means. As described above, according to this invention, since the operation of the frequency dividing circuit is interrupted when the original oscillation frequency output is derived from the oscillation circuit, no noise is generated from the frequency dividing circuit, thereby making it possible to obtain an original oscillation frequency output having extremely less noise.

Claims: A semiconductor integrated circuit comprising: an oscillation circuit (1) formed on a semiconductor substrate (19); a frequency dividing circuit (9) formed on said semiconductor substrate (19), for dividing a frequency of an oscillation output from said oscillation circuit (1); signal selection means (CI1, CI2, CI3, CI4) for selectively permitting one of an original oscillation frequency signal of said oscillation circuit (1) and outputs of said frequency dividing circuit (9) to pass therethrough; an output circuit (11) for outputting a signal selected by said selection means (CI1, CI2, CI3, CI4); and operation interrupting means (21) for interrupting the operation of said frequency dividing circuit (9) while the original oscillation frequency signal of said oscillation circuit is being output from said output circuit (11); characterized in that said frequency dividing circuit (9) comprises a reset terminal (R) to which the output of said operation interrupting means (21) is supplied. A semiconductor integrated circuit according to claim 1, characterized in that said oscillation circuit (1) and said frequency dividing circuit (9) use the same power source. A semiconductor integrated circuit according to claim 1, characterized in that a CMOS integrated circuit is formed on said semiconductor substrate (19). A semiconductor integrated circuit according to claim 1, characterized in that said oscillation circuit (1) is a CMOS crystal oscillation circuit. A semiconductor integrated circuit according to claim 1, characterized in that said output circuit (11) is a 3-state type output circuit.

Inventors: INOUE TOSHIHISA, ITO TAKAHIRO, INOUE, TOSHIHISA, ITO, TAKAHIRO, Inoue, Toshihisa, c/o Intell.Prop.Div.K.K.Toshiba, Ito, Takahiro, c/o Intell.Prop.Div.K.K.Toshiba
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1992
Main classification: H03B19
IPC codes: H03K3, G06F1, H03K23, H01L21, H03B19, H01L27