JPH02181509A - Semiconductor integrated circuit - Google Patents

Abstract

PURPOSE:To change oscillation frequency by comparatively simple constitution by connecting a means for changing the oscillation frequency between the unit elements of a ring oscillator. CONSTITUTION:In the ring oscillator 8 constituted by connecting the odd number of inverters 12(1) to 12(2n+1) in series, switches 10A to 10C consisting of transistors to short-circuit the prescribed number of inverters between the inverters. The number of inverters to be short-circuited by each of the switches 10A to 10C is different from each other. Thereby, the oscillation frequency (f) of the ring oscillator 8 is changed in accordance with the number (2n+1) of elements constituting the ring oscillator 8 and also changed in accordance with respective delay times td1, td2 based upon the shown equality I.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a semiconductor integrated circuit, and particularly to an improvement in a semiconductor integrated circuit having a ring oscillator in which unit elements are connected in series and oscillates at a predetermined frequency.

[Prior Art 1] In a semiconductor integrated circuit, a ring oscillator is sometimes provided on a chip as an oscillation circuit in order to obtain square wave oscillation such as a clock pulse. This ring oscillator is an inverter, N0R1NAN
D, EXNOR (exclusive NOR), is configured by connecting an odd number of unit elements in series, such as a Schmitt trigger circuit 0 For example, if the unit element is NAND6, the ring oscillator 8 is the third
It is configured as shown in the figure. The oscillation frequency f of the ring oscillator is determined by setting the number of unit elements to a predetermined odd number 2n+1 (n=o
, 1.2...), then the following formula (1)% formula % where td+ is the turn-on delay time and td2 is the turn-off delay time. [The problem that the invention tries to solve! ! ! By the way, in semiconductor integrated circuits, the oscillation frequency required of the oscillation circuit may change depending on usage conditions and characteristics. However, in conventional semiconductor integrated circuits, the ring oscillator is built on the chip, and the number and characteristics of unit elements cannot be easily changed, so there is generally a problem that the oscillation frequency cannot be changed arbitrarily. there were. On the other hand, it is conceivable to provide multiple ring oscillators depending on the required frequency, but in this case,
The area occupied by the oscillation circuit on the chip increases significantly, which limits the degree of freedom in circuit design. The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to provide a semiconductor integrated circuit that can change the oscillation frequency of a ring oscillator with a relatively simple configuration.

[Means to achieve the task]

The present invention provides a semiconductor integrated circuit having a ring oscillator in which unit elements are connected in series and oscillates at a predetermined frequency, in which means for changing the oscillation frequency is connected between the unit elements of the ring oscillator. The above-mentioned problem has been achieved. In the present invention, the means for changing the oscillation frequency may be a means for changing the number of stages of unit elements or a means for delaying the output of the unit elements.

[Operation and effects of the invention]

From equation (1) above, the oscillation frequency f of the ring oscillator changes depending on the number 2n+1 of the constituent elements of the ring oscillator, and also changes depending on the respective delay times tdl and td2. As a result of various studies based on this point of view, the inventors found that in order to change the oscillation frequency of the ring oscillator, a switch that short-circuits between the unit elements as shown in FIG. 10A to IOC, or each delay time tdl, t as shown in Fig. 2 below.
The inventors discovered that it was sufficient to connect capacitors 01 to C3 to change d2, and devised the present invention. According to the present invention, the oscillation frequency can be changed arbitrarily simply by connecting a means for changing the oscillation frequency to the ring oscillator. Therefore, since it is not necessary to provide a plurality of ring oscillators when changing the frequency in a semiconductor integrated circuit, the oscillation circuit can be configured without increasing the area occupied by the oscillation circuit on the chip. Therefore, the degree of freedom in circuit configuration is not impaired, and the economic efficiency of integrated circuit design and manufacturing is improved.

【Example】

Embodiments of the present invention will be described in detail below with reference to the drawings. First, a first example will be explained. This first embodiment consists of transistors for short-circuiting a predetermined number of inverters in a ring oscillator 8 in which odd-numbered stages of inverters 12(1) to 12(2n+1) are connected in series, as shown in FIG. switch IOA,
IOB and IOC are connected between inverters. The number of inverter stages to which the switches IOA, IOB, and IOC are short-circuited is an even number of stages: 2h, 2i, and 2j (h, 1, and j are natural numbers). This is because the ring oscillator oscillates by having an odd number of stages of unit elements. Further, the number of inverter stages short-circuited for each switch IOA to IOC is different. Therefore, by inputting on/off signals φ1, φ2, φ3,
By turning on any one of the switches IOA, IOB, and 10C, the number of inverter stages contributing to oscillation in the ring oscillator 8 changes, and the oscillation frequency changes according to equation (1) above. In addition, in the first embodiment, the switches IOA, 10B, IO
Since three C are provided, all switches IOA ~ 10
There are four types of frequencies that can be oscillated, including when G is turned off. Furthermore, when implementing the present invention, as in the first embodiment,
The invention is not limited to providing three switches at the positions as shown in FIG. 1 to short-circuit the inverter, but an appropriate number of switches can be provided at appropriate positions depending on the required frequency. Next, a second embodiment will be explained. This second embodiment uses an inverter 12 similar to the first embodiment.
In a ring oscillator 8 in which (1) to 12 (2n+1) are connected in series, capacitors 01 to C3 are connected between predetermined inverters via respective switches 14A to 14C. This capacitor 01~C3
are turn-on delay time td+, turn-off delay time t
This is for changing d2. Note that the capacitors 01 to C3 have different capacitances. In the ring oscillator 8 according to the second embodiment, on/off signals φ1, φ2,
φ3 is input, and the capacitance of the capacitor connected to the ring oscillator 8 is changed by the combination of ON and OFF to change the delay times td+ and tdz. This results in
The oscillation frequency f can be changed from the above equation (1). Furthermore, there are eight types of oscillation frequencies that can be selected by turning on and off the switches 14A to 14C. In other words, this is because in addition to the three ways in which these switches 14A to 14C are turned on individually, 23=8 ways of capacitance can be connected to the oscillator 8, including the case in which the switches are turned on in combination. . Note that when the capacitors C1 to C3 have the same capacitance, four oscillation frequencies can be selected, including the case where all the switches 14A and 14C are turned off. Furthermore, the number of capacitors to be connected to the ring oscillator is not limited to only three as shown in 01 to C3 in Fig. 2; in other words, an appropriate capacitance and number of capacitors may be used depending on the required frequency. can be connected. In the first and second embodiments, the present invention is applied to a ring oscillator whose unit element is an inverter, but the unit element of the ring oscillator for implementing the present invention is limited to an inverter. For example, if the unit element is NOR, NAND, EX
The present invention can be implemented with a ring oscillator using a NOR, Schmitt trigger circuit. Further, in the first and second embodiments, transistors and capacitors were used as elements for changing the oscillation frequency, but the present invention is not limited to the use of only these elements. Instead, the oscillation frequency can be changed using other elements such as resistors as necessary.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing the configuration of a first embodiment according to the present invention, FIG. 2 is a circuit diagram showing the configuration of a second embodiment according to the present invention, and FIG. 3 is a general ring oscillator. FIG. 2 is a circuit diagram showing an example of the configuration. 12(1) to 12<2n+1)...Inverter, 14
A~14C...Capacitor switch, 01~C3・
...Capacitor.

Claims (2)

[Claims] (1) A semiconductor integrated circuit having a ring oscillator in which unit elements are connected in series and oscillates at a predetermined frequency, characterized in that means for changing the oscillation frequency is connected between the unit elements of the ring oscillator. semiconductor integrated circuits. (2) The semiconductor integrated circuit according to claim 1, wherein the means for changing the oscillation frequency is means for changing the number of stages of unit elements or means for delaying the output of the unit elements.