The working principle and maintenance of the P4 motherboard clock circuit

The working principle of the clock circuit: after the power supply is given to the diode and L1 (L1 can be replaced by a 0 ohm resistor) into the divider, the divider starts to work. , which oscillates together with the crystal, and the waveform can be seen on both feet of the crystal.

The resistance between the two pins of the crystal is between 450-700. There is a voltage of about 1V on each of its pins, which is provided by the crossover. The sum of the frequencies produced by the crystals is.

The main frequency OSC is sent to the B16 pin of the PCI and the B30 pin of the ISA after the crossover comes out, and these two pins are called OSC test pins. Some were also sent to the South Bridge, with the aim of making the frequency of the South Bridge more stable. There is also a capacitor on the line of the main frequency OSC, and the resistance value of the main frequency line to ground is between 450-700 ohms.

The amplitude of the clock waveform of the total frequency must be greater than 2V.

If the OSC light on the boot digital card does not light up, check the voltage and waveform of the crystal first. There is voltage and waveform, and when the main frequency line is normal, the crossover is bad; No voltage and no waveform, in the case of normal power supply of the frequency divider, the frequency divider is bad; If there is a voltage and no waveform, the crystal is bad.

Without the total frequency, there is no frequency on the south and north bridges, cpu, cache, io and memory. With the total frequency, there is not necessarily a frequency on the south and north bridges, memory, CPU, cache, and Io.

Once the total frequency is normal, the divider starts to divide the frequency, and R2 sends the frequency split by the crossover to the south bridge, and then sends it to the B39 pin (PCICLK) of the PCI and the B20 pin (Sysclk) of the ISA after processing on the surface bridge, which are called the system clock test pins. This pin can reflect whether all the clocks on the motherboard are working properly. The amplitude of the waveform of the system clock must be greater than that, and the resistance of these two pins is between 450-700 ohms, which is provided by Nanqiao.

On the motherboard, both RST and CLK are handled by the South Bridge, and the total frequency is normal, if the RST and CLK are not available, the South Bridge is bad when the power supply of the South Bridge is normal. If the motherboard is not turned on, the RST is abnormal, so check the total frequency first.

The amplitude of the waveform of the clk is not enough: check whether the amplitude of the R3 output is enough, not enough, and the divider is bad. Enough, check whether the voltage of the South Bridge is enough, enough that the South Bridge is bad; Not enough, check the power circuit.

R1 sends the frequency split from the divider to the sixth pin of the CPU (next to the RST pin on the CPU, see drawing), which is the CPU clock pin. If the CPU does not have a clock, it will never work, and the CPU clock may be provided by Northbridge. If there is a CLK signal on the south bridge and not on the CPU, it is possible that the crossover or the south bridge is bad.

R4 provides the frequency for IO.

On motherboards, the clock line is a bit thicker than the AD line and comes with a bend.

The frequency shift is caused by the crystal capacitance, and its phenomenon is that it will crash as soon as it is turned on, and the operation of 98 will be errored.

If the crossover itself is broken, the frequency will not go up. It has nothing to do with crystals.

The two sides of the CPU are the control places, which control the south bridge and the crossover, and will automatically adjust when the frequency is offset.

Let's take a look at the circuit diagram.

Replying is a virtue, and respecting others equals respecting yourself!

Clock circuit problems are divided into: some have no clock, and all have no clock. Part of the no clock note to check the current limiting resistor on a certain clock line to chase the line to the clock IC, whether the clock IC power supply is less than one way, generally there is one way and see if there is a bad power supply.

The most important thing to check whether the working conditions of the clock IC still meet the power supply, PG, the PSON signal sent by the south bridge is not available on some motherboards, whether the crystal oscillator is vibrating, and the clock IC itself is bad. But first make sure the motherboard is powered properly, then the clock, and then reset.

The role of the clock circuit chip in the motherboard: to give the hardware device a running frequency.

A clock chip is a type of clock circuit.

The clock chip is a high-performance, low-power, real-time clock circuit with RAM, the clock chip can run the timer for year, month, day, Sunday, hour, minute, and second, with leap year compensation function, and the working voltage is. It uses a three-wire interface to run synchronous communication with the CPU and can use bursts to deliver multiple bytes of clock signals or RAM values at once.

Data input and output (IO).

At the rising edge of the next SCLK clock after the control instruction word input, data is written to the DS1302 and the data input starts at the low bit 0. Similarly, the data of DS1302 is read out on the descending edge of the next SCLK pulse immediately after the 8-bit control command word, and the data is read from the low 0 bit to the high 7 bit.

The main function of the motherboard clock circuit is to send clock signals to the functional chips on the motherboard and the ISA, PCI and other buses (the shape of the clock generator chip signal line on the motherboard is a serpentine line), so that these chips work together under the control of the clock signal.

Same as above, that is, to provide a unified clock signal for each heart piece.

First, check the rough core of the power supply of the clock IC, which is usually two-way. Check the crystal oscillator again, and there is still a lot of damage to the crystal oscillator. The third is to check the capacitance, including the clock oscillation capacitor and the RC filter capacitor for leakage.

The fourth is the clock IC itself. Fifth, it is more difficult to dig - to see if the periphery has caused hail differences on the clock circuit, such as I2C control, BIOS, etc.

This is a bit difficult to look forward to.

At this time, the crystal oscillator element of the motherboard clock circuit and the resonant capacitor element of the crystal oscillator are normal, if there is any abnormality, it can be replaced directly.

It can be judged by the diagnostic card, and there is a clock reset light on the general diagnostic card, and the clock should be always on for normal, and the reset should be normal for flashing and extinguishing.

But it is not very accurate, it is most accurate to use an oscilloscope, and you can know the test points measured on it with a false load, a resistance card, and an oscilloscope.

The fault phenomena when the motherboard clock circuit is not working and the clock is completely absent are: after the motherboard is turned on, the ATX power supply (not working), the motherboard detection card display (abnormal display), the CPU fan (not moving), the BIOS self-test program (not working), the monitor (will light up), and the motherboard buzzer (alarm).