JPS6263327A - Oscillation circuit of microcomputer - Google Patents

Abstract

PURPOSE:To reduce electromagnetic-wave disturbances to peripheral equipment from an oscillation circuit used in a microcomputer, by dispersing the spectrum distribution of the higher harmonic component of the electromagnetic waves. CONSTITUTION:Delay circuits 121-125 which are selected by means of switches SW1-SW5 are provided and the selection is made in accordance with outputs of shift registers 13. Then the output of the delay circuits is inputted to a control section 10 after they are combined to an output f1 whose frequency fluctuates in the vicinity of f0 by an OR gate G1. The delaying times TD1-TD5 of the delay circuits 121-125 are set at random and the set delaying times Td1-TD5 are successively selected by the parallel outputs of the shift registers 13 which clock (CK) the f0. However, the shift registers 13 are cleared when read/write signals R/W from the microcomputer controlling section 10 become the write mode and all the outputs SW1-SW4 of flip flops FF1-FF4 of each stage are made zero. As a result, the output of an OR gate G2 and inverter I respectively become '0' and '1' and the flip flop FF1 of the 1st stage is set to '1'.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an oscillation circuit for a microcomputer that reduces radiation noise by adding jitter within a certain range to the oscillation frequency.

[Conventional technology]

In recent years, microcomputers have been widely used to control various types of automobiles, but in this case, high-frequency noise is generated from the microcomputer's crystal oscillation circuit, which can cause electromagnetic interference (EMC) to nearby radio receivers and the like. The conventional solution to this problem is LC.

High frequency noise is often prevented from leaking outside from a 13711 device using a microcomputer by using an RC filter network or a shield plate. FIG. 4 shows an example of this, where 1 is a microcomputer, 2 is a controller using the microcomputer, and L and C are an inductance and a capacitor that constitute a high frequency blocking filter.

[Problem that the invention seeks to solve]

However, the above-mentioned 2MC countermeasure does not prevent the generation of high frequency noise itself, and therefore has many imperfections. The present invention attempts to fundamentally improve this point.

[Means for solving problems]

The present invention is characterized in that, in a fixed frequency oscillation circuit that generates a clock used in a microcomputer, the output of the original oscillation has jitter within a certain range.

[Effect]

FIG. 1 is a diagram explaining the principle of the present invention, and (a) shows the fundamental wave f.

harmonics 2・fo,...n-fo,...
It is a spectrum of... If the frequency of the fundamental wave fa''''- is fixed to a constant value as in this case, specific harmonics become noise. FIG. 6B is a partially enlarged view of the same. If the frequency of the fundamental wave fo is randomly varied within a range that does not affect the control accuracy, the high frequency spectrum will be dispersed as shown in FIG. As a result, the influence of high frequency noise on the outside is reduced. The frequency of the fundamental wave fO can be varied randomly using read/write (R/W) cycles of the microcomputer.

〔Example〕

FIG. 2 is a block diagram showing an embodiment of the present invention, in which 10 is a control unit using a microcomputer, 11 is a frequency f
This is a crystal oscillation circuit of o. The conventional method is to input the output fO of this oscillation circuit (CR may be used) 11 as it is to the control unit 10, but in this example, the switches SW+ to SW!
Delay circuits 12+ to 125 selected by 1 are interposed, and the delay circuits 12+ to 125 are selected by the output of the shift register 13. Then, the signal is combined with the output r1 whose frequency fluctuates around fO by C;+, and then input to the control unit 10. The delay times TD+ to TDs of the delay circuits 121 to 125 are set randomly as shown in FIG.
CK) are selected in order by the parallel outputs of the shift register 13. However, this shift register 13 is cleared when the read/write f word R/W from the microcomputer control unit 10 enters the write mode, and the flip-flop FF+ of each stage is cleared.
~Output SW of FF4 +~Set SW4 to all O (
These outputs turn on the corresponding switch at 1).

As a result, the output of the OR game) 02 becomes 01, and the output of the inverter I becomes 1, and 1 is set in the first stage flip-flop FF1. Therefore, due to the randomness of the read/write cycle of the microcomputer control section 10, the selection of the delay time is also random. For example, the first time is SW I, SW2. SW3
It is turned on until it is cleared, and then SW+, S
Only W2 is turned on and cleared, and so on.

The delay circuit 125 is for outputting a clock during the clearing period of the shift register 13, and its switch SW5 is turned on when the output of the OR gate G2 becomes 0.

The frequency f+ obtained by the above-mentioned circuit is obtained by making the crystal original oscillation fO have a frequency fluctuation of about θ~10% in the short term, and keeping it at a constant average value in the long term. The spectral distribution of harmonics can be dispersed without affecting the control accuracy of the control unit 10. Note that the delay circuit, shift register, etc. may be built into the microcomputer.

〔Effect of the invention〕

As described above, according to the present invention, there is an advantage that electromagnetic interference caused by an oscillation circuit used in a microcomputer to peripheral equipment can be reduced by dispersing the spectrum distribution of its harmonic components.

[Brief explanation of drawings]

Fig. 1 is a diagram explaining the principle of the present invention, Fig. 2 is a block diagram showing an embodiment of the invention, Fig. 3 is a time chart showing its operation, and Fig. 4 is an explanatory diagram of a conventional high-frequency noise countermeasure. be. In the figure, 10 is a control unit using a microcomputer;
．． 1 is a crystal oscillation circuit, 121 to 125 are delay circuits, SW
+ to SW5 are selection switches, and 13 is a shift register.

Claims (1)

[Claims] An oscillation circuit for a microcomputer that is characterized in that, in a fixed frequency oscillation circuit that generates a clock used in a microcomputer, the output of the original oscillation has jitter within a certain range.