In recent years, generation of a high-precision (for example, within ±1.5%) clock signal has been requested for a communication protocol for a vehicle. When a crystal oscillator or a ceramic oscillator is used to generate a high-precision clock signal in a vehicle, vibration may damage the oscillator. For example, solder, which holds the oscillator in a fixed state, may separate from the oscillator and cracking may occur in the oscillator. Further, costs should be reduced. Accordingly, there is a demand for oscillation circuits that are incorporated in a silicon device. In order to realize a high-precision oscillation circuit on a silicon device, various proposals have been made.
FIG. 1 illustrates a conventional CR oscillation circuit. The CR oscillation circuit of FIG. 1 includes inverters INV101, INV102, and INV103, a capacitor C101, and a resistor R101. FIG. 2 illustrates waveforms at nodes in an operating state of the CR oscillation circuit in FIG. 1. As illustrated in FIG. 2, waveforms at nodes ND101, ND102, and ND103 are rectangular waves. Capacitive coupling with the node ND102 changes voltage at a node ND104 in the same direction as that at node ND102 when voltage at the node ND102 changes. Then, the voltage at node ND104 is charged and discharged by a voltage at node ND103 via the resistor R101 and gradually changes. The broken line in FIG. 2 indicates a threshold value Vth of the inverter INV101.
An oscillation frequency of the CR oscillation circuit in FIG. 1 generally has fluctuations of −50% to +100% due to the power supply voltage, temperature, resistance of the resistor R101, capacitance of the capacitor C101, and fluctuation in these factors.
FIG. 3 illustrates another conventional CR oscillation circuit. The CR oscillation circuit in FIG. 3 includes inverters INV31, INV32, and INV33, capacitors C31 and C32, current sources IP31 and IN31, PMOS transistors MP31, MP32, and MP33, NMOS transistors MN31 and MN32, an operational amplifier AMP31, a resistor R31, a reference voltage generation circuit 31, a setting register 32, a bias generation circuit 33, and a constant voltage circuit 34.
In the CR oscillation circuit in FIG. 3, an oscillation frequency of a signal output from an output terminal OUT is determined based on an output voltage VREG of the constant voltage circuit 34 and current values of the current sources IP31 and IN31. When the constant voltage circuit 34 supplies the output voltage VREG at a constant level, a signal width of a node ND31 becomes constant without depending on a power supply voltage. Current values of the current sources IP31 and IN31 that charge and discharge the capacitors C31 and C32 are determined based on a reference voltage output by the reference voltage generation circuit 31. Due to process fluctuations, the reference voltage and the current values of the current sources IP31 and IN31, as illustrated in FIG. 4, have linear temperature dependencies that change upward and downward in accordance with the temperature. The setting register 32 trims the temperature dependency of the reference voltage output by the reference voltage generation circuit 31 to make the current values of the current sources IP31 and IN31 constant independently of the temperature.
The following documents are related to the background art described above.    Japanese Laid-Open Patent Publication No. 2008-252414    Japanese Laid-Open Patent Publication No. 2007-299294    Japanese National Phase Laid-Open Patent Publication No. 2009-522661    Japanese Patent No. 7-22253