What is a Countdown Clock?

Clock frequency (also translated: clock rate , English: clock rate ) refers to the fundamental frequency of the clock in the synchronization circuit, it is measured in "several cycles per second", the measurement unit is SI units Hertz (Hz). It is an important indicator for evaluating CPU performance. Generally, the larger the digital value of the main frequency, the better. FSB is the external operating frequency of the CPU and is the reference clock frequency provided by the motherboard. FSB frequency is the frequency of data transmission on the Front Side Bus connecting the CPU and the North Bridge chip in the motherboard chipset. There is a relationship between the CPU's main frequency and the external frequency: main frequency = external frequency × multiplier.

The main frequency of the CPU is the clock frequency of the CPU core (CPU Clock Speed). Generally speaking how many GHz a certain CPU is, and this number of GHz is the "CPU's main frequency". Many people think that the CPU's clock speed is its operating speed, but it is not. The main frequency of the CPU indicates the speed of the digital pulse signal oscillation in the CPU, which has no direct relationship with the actual computing capacity of the CPU. Frequency and actual

Frequency vs. Speed: Generally speaking, a

Overall structure of the frequency synthesizer

A wonderful piece of music will have a main melody, and the main melody of the computer is the clock frequency of the CPU. Main frequency, FSB and multiplier, where do they come from? What is the problem with frequency locking and overclocking?

There are many semiconductor chips in a computer, and each chip works at a specific clock frequency. The clock signal provided by the clock generator to the chip is a continuous pulse signal, and the pulse is equivalent to the pulse of the chip. Each time the pulse arrives, the transistor in the chip changes state once, allowing the entire chip to complete certain tasks.

Most of the chips in the computer are digital logic chips. Many of the transistors in the digital chip all work in the switching state. Their on and off actions do not follow the rhythm of the clock signal. If the clock frequency is too high, it may happen that the state of the transistor is too late to change, resulting in deadlock or random misoperation. Therefore, each chip has its own frequency limit.

What is the clock frequency

Frequency is represented by f, and the basic unit is "1 times / second", which is recorded as Hz (Hertz). 1Hz is once per second, and 10Hz is 10 times per second (Figure 1). However, the unit of Hz is too small in the computer, so the signal frequency is usually expressed in KHz, MHz or GHz. As the frequency rises, I am afraid that THz will need to be used as a unit of frequency for several years (Table 1).

Table 1: Frequency notation
Frequency unit	kHz	MHz	GHz	THz
Conversion relationship	1 × 10 ^ 3Hz	1 × 10 ^ 6Hz	1 × 10 ^ 9Hz	1 × 10 ^ 12Hz
English name	Kilo Hz	Mega Hz	Giga Hz	Tera Hz
Chinese name	Kilohertz	Megahertz	Ghertz	Terahertz

Period and frequency

In computer technology, a common term corresponding to frequency is period. The period is the inverse of the frequency. The higher the frequency, the shorter the period. For example, when the clock frequency is 1 GHz, the clock cycle is 1 nanosecond.

2. Bandwidth and frequency

Another parameter related to frequency is the data transmission rate, also called "bandwidth", which is used to measure the speed of data communication. Generally, bandwidth = clock frequency × (bit width ÷ 8). For example, the clock frequency of the PCI bus is 33.33MHz. Because its bit width is 32bit, its bandwidth is 33.33 × (32 ÷ 8) = 133MB / s.

3. CPU frequency : In 286 and earlier computers, the CPU frequency is the same as the external bus frequency. The Intel 386 computer uses a clock division method. The frequency of the clock signal provided by the clock circuit to the CPU is 66 MHz, and the CPU internally operates at a frequency of 33 MHz. Intel 80486 DX2 uses the frequency doubling method, which allows the CPU to run at 2 or 3 times the speed of the external bus, but still communicates with the outside world at the original clock frequency. After entering the Pentium era, the frequency doubling technology has been widely used, and the processor's frequency doubling has reached 20 times.

System clock frequency : It is also commonly called "FSB"-the clock frequency of the CPU's external bus. The external frequency is provided by a frequency synthesizer chip. The frequency synthesizer chip will be described in detail later. Main frequency: The main frequency is the actual operating frequency of the CPU core (integer and floating-point arithmetic unit) circuit. It is determined by the external frequency (or front-end bus frequency) and the magnification, that is, the main frequency = external frequency × magnification.

Front-side bus frequency : The front-side bus (FSB) frequency is the frequency at which data is exchanged between the CPU and the Northbridge chip. It is both connected and distinguished from the external frequency. The external frequency is the frequency of the front-side bus clock signal, and the front-side bus frequency refers to the frequency of data transmission. For the Pentium 4 processor, because QDR (Quad Data Rate) technology is used, data can be transmitted 4 times in one clock cycle, so the front-side bus frequency is equivalent to 4 times the external frequency: FSB 800MHz Processor, FSB is only 200MHz.

Clock frequency who is generating the frequency

We can think of the clock signal generator as a frequency source as the heart of a computer. The computer can only work if the heart beats.

1. Oscillation source: crystal oscillator

The chip itself usually does not have a clock signal source, so the clock signal must be provided by a special clock circuit. A quartz crystal oscillator (Quartz Crystal OSC) is one of the most commonly used clock signal oscillation sources.

Quartz crystal is pure silicon dioxide, which is a single crystal of silicon dioxide, which is often called crystal. There are two types of quartz crystals: Crude crystals and synthetic crystals. Most of the impurity content and morphology of natural quartz crystals are not uniform, so crystal oscillators in electronic circuits mostly use artificial quartz crystals.

Cut a slice (called a "wafer") from a crystal at a certain azimuth angle, apply a thin layer of silver on both surfaces of the wafer, connect a pair of metal plates, solder the pins, and use a metal shell The package constitutes a quartz crystal oscillator.

The reason why a quartz wafer can be used as an oscillator is based on its piezoelectric effect: applying an electric field to the two poles of the wafer will cause mechanical deformation of the crystal; when an alternating voltage is applied to the quartz wafer, the crystal will Generates mechanical vibration, and at the same time mechanical deformation vibration will generate an alternating electric field. Although the voltage of this alternating electric field is extremely weak, its vibration frequency is very stable. When the frequency of the applied alternating voltage is equal to the natural frequency of the wafer (determined by the size and shape of the wafer), the amplitude of the mechanical vibration will increase sharply. This phenomenon is called "piezoelectric resonance".

The establishment and maintenance of the piezoelectric resonance state must be realized by means of an oscillator circuit. A series-type oscillator, a two-stage amplifier composed of transistors T1 and T2, a quartz crystal XT and a capacitor C2 constitute an LC circuit. In this circuit, the quartz crystal is equivalent to an inductor, and C2 is a variable capacitor, and the circuit can be brought into resonance by adjusting its capacity. The power supply voltage of this oscillator is 5V, and the output waveform is a square wave.

The frequency stability of quartz crystal oscillator can reach 10 ^ -9 / day, or even 10 ^ -11. For example, for a 10MHz oscillator, the frequency change within a day is generally not greater than 0.1Hz. Therefore, the crystal oscillator can be regarded as a constant reference frequency source (both quartz watches and electronic watches use quartz crystals as the reference frequency for timing). Since the birth of the PC, a 14.318MHz quartz crystal oscillator has been used as the reference frequency source on the motherboard. As for the reason that the frequency of 14.318MHz is always used, perhaps it is the need to maintain compatibility. However, I also found a 14.318MHz crystal in the graphics card, flash drive and mobile phone, so I don't know why.

In addition to this 14.318MHz crystal on the motherboard, a crystal with a frequency of 32.768KHz can also be found. It is used in real-time clock (RTC) circuits to display accurate time and date.

2.Frequency divider and frequency doubler

Reduce the pulse frequency by n times. This is what the frequency divider does. In the first generation PC, the output frequency of the quartz crystal oscillator was 14.318MHz, while the main frequency of the Intel 8086 processor was 4.77MHz, and the latter was exactly one third of the former. The frequency conversion is done in Intel 8284 (clock generator / driver), because the Intel 8284 chip integrates a three-frequency circuit, which can reduce the pulse signal generated by the crystal oscillator by 3 times and provide it to the CPU and external Assume

With the increase of the CPU's main frequency, it is necessary to increase the crystal oscillator several times to meet the needs of the CPU, so the frequency doubler replaces the position of the frequency divider in the clock circuit. If the frequency divider performs a division operation, the frequency multiplier performs a multiplication operation, which increases the frequency of the crystal oscillator by n times.

Clock frequency clock chip: programmable frequency synthesizer

The integrated clock circuit is a sign of progress in hardware technology. Different devices in the computer have different requirements for the clock frequency. If you find a 286 motherboard from the waste bin, you can see that several crystals are arranged together. The computer CPU, AGP slot, PCI slot, hard disk interface, USB port and PS / 2 port have very different communication speeds, so different clock frequencies need to be provided. For example, PCI requires 33MHz, USB is 48MHz, etc. . However, a quartz oscillator can only provide one frequency, so motherboard manufacturers usually integrate these oscillating circuits scattered around the motherboard into a "Frequency Synthesizer" chip. The generated pulse signal is frequency-divided (or multiplied) to provide the required clock frequency for chips (or equipment) with different operating speeds.

Ordinary frequency divider is an integer frequency divider. The relationship between the output frequency and the input frequency is an integer multiple. It can only adjust the frequency in sections and cannot meet the requirements of precision adjustment. The frequency synthesizer is a "fractional frequency divider" that allows fine adjustment of the output frequency. R & D engineers are free to design various frequencies in the circuit and are no longer limited by the fixed frequency specifications of quartz oscillator crystals. The clock chip in the computer generally has the "fractional frequency division" capability, and the adjustment step size can be designed to 1% or even 0.1% according to needs. In order to guide and standardize the design and application of frequency synthesizers, Intel has developed design guidelines for frequency synthesizers, such as CK97, CK40X, etc. The specification for the latest Pentium 4 processor is CK410.

1. Frequency adjustment principle

The frequency synthesizer is a clock signal system with negative frequency feedback (Figure 7), which uses two frequency dividers, Mdl is used to reduce the reference frequency, and Ndl is used to divide the VCO. The frequency fi generated by the crystal oscillator (OSC) is obtained by the M frequency divider to obtain the reference frequency fref, and it and the feedback frequency ffd are sent to the two reverse input terminals of the Frequency Detector (FD), and the frequency discriminator output A DC voltage reflecting the quotient of the two is filtered by a low-pass filter (LPF) to provide an AC component and is then provided to a voltage controlled oscillator (VCO) to output a frequency signal fout.

The relationship between the output frequency fout of the frequency synthesizer and the input frequency fin can be expressed by the formula fout = fin × (N + k / M), where N, M, and K are integers, and K can be any value between 0 and M. Integer. Non-integer values N + k / M are usually written as NF, where the dots represent the decimal point, N represents the integer part of the frequency, and F = k / M represents the decimal part of the frequency. Under the condition that the input frequencies fin, N, and M are not changed, as long as the k value is modified, the required frequency value fout can be obtained.

In the frequency synthesizer chip, there is a special SMBus interface circuit, which is the way for the register of the chip to communicate with the outside. With it, the register can be rewritten through the BIOS or software. There are two possibilities for each bit of data in the frequency register, "0" or "1", then when these bits are combined in different states, a variety of FSB outputs can be obtained.

The frequency adjustment accuracy of the frequency synthesizer is related to the number of bits in the frequency register. For example, if the frequency register is 5 bits, the adjustment step size is 1MHz. The greater the number, the higher the adjustment accuracy. In a practical frequency synthesizer, both Mdl and Ndl dividers are programmable. As long as the user sets the corresponding fout value, the BIOS can automatically give the values of N, M, and K, and write them through the SMBus bus. Into the corresponding register.

2.PLL automatic control principle of phase synchronization

The clock chip is the heart of the computer, and its performance and stability directly determine the performance of the entire hardware system. Using a frequency synthesizer on the one hand can save costs and motherboard space. The more important purpose is to maintain a strict synchronization relationship between the clock signals of the motherboard's chips and external devices and the clock signals of the CPU to ensure the correct exchange of data. The FS chip not only has the function of frequency doubling / frequency division, but also has the main feature of phase lock function-the phase of the output signal is forced to be consistent with the phase of the reference signal. Therefore, although the various clock signals output by the frequency synthesizers have different frequencies, they are completely identical in phase, and they are all in phase synchronization with the reference signal source.

In order to achieve phase locking, the clock signal output by the VCO is compared with the reference frequency signal in the phase detector. If the two phases are different, an error voltage proportional to the phase difference is output; the polarity of the error voltage determines The current source in the charge pump absorbs or sends current, so the charge will flow into or out of the capacitor in the filter, and the amount of charge flow is proportional to the magnitude of the phase difference. The voltage controlled oscillator is a voltage controlled oscillator. When the voltage across the internal varactor diode changes, its capacitance will change accordingly, thereby changing the frequency of the oscillator.

The voltage-controlled oscillator is the core unit of the PLL circuit. The phase control process is achieved by changing the input voltage (that is, the tuning voltage) of the voltage-controlled oscillator. The size and polarity of the tuning voltage determine whether the phase adjustment is lagging or leading, thereby The phase error is corrected.

3.Other functions of the frequency synthesizer

Using a frequency synthesizer chip in the motherboard design can easily achieve clock frequency adjustment and phase locking. In addition to these functions, the frequency synthesizer also allows motherboard design engineers to fine-tune the clock delay between various interface clocks to keep the components of various related interfaces in sync, facilitating design and debugging (Figure 8).

In addition, the frequency synthesizer chip can also make a difference in terms of system stability and security. On the one hand, you can lock the frequency that does not need to be adjusted to prevent other devices from failing due to overclocking by the CPU; on the other hand, some frequency synthesizer chips also have a "watchdog" function. Once the overclocking fails, it will crash This function can clear the frequency register to make the system start normally according to the default frequency of the CPU.

Nowadays, the application of frequency synthesizer chips is very common. Common brands are ICS, Cypress, IDT, Realtek and Winbond. However, in the nForce2 motherboard, no frequency synthesizer can be found, because the frequency synthesis function has been integrated into the IGP / SPP chip.

The multi-frequency signal workflow of AMD Athlon series processors. When the RESET # signal arrives, the processor sends the FID signal to the logic signal conversion chip, and the chip generates a SIP (Serialization Initialization Packet) to the system. The bus is initialized and set.

Some connection lines called golden bridges are set on the CPU. The level of the FID signal can be set by changing the on-off of the golden bridge. The golden bridge is low when it is on, and high when it is off. The FID signal is generated in the built-in frequency doubling control unit, and after the signal is amplified by the built-in FID driving circuit, it is sent from the FID pin to the logic signal conversion chip, and the generated SIP data packet is returned to the CPU from the BP_FID pin. In this way, the internal frequency synthesizing circuit of the CPU can synthesize the core frequency of the CPU together with the frequency doubler and the external frequency two signals.

3. Setting of memory frequency

The memory bus clock signal on the early motherboard was also generated by the frequency synthesizer, but the newer motherboards have set aside the frequency synthesizer chip on the motherboard, and the north bridge chip completes the setting of the memory bus clock frequency, which is known in the industry as "Memory Asynchronous".

Similar to the principle of the automatic setting of the CPU frequency, the frequency synthesizer in the Northbridge chip also automatically sets the frequency by certain means. The frequency of the memory is provided by the SPD (Serial Presence Detect Memory Sequence Memory Chip) on the memory module. The SPD is similar to the BIOS on the motherboard. It stores the memory capacity, operating frequency, delay time (CAS, tRCD, tRP, tCA), operating voltage, and manufacturer information of the memory chip. The Northbridge chip reads each SDA pin through the SMBus bus. The parameters in the SPD of each DIMM, and the information of the SPD chip will be recorded in the register of the PLL circuit in the Northbridge chip.

The frequency of the memory bus clock and the system clock are often different. For example, when the system clock is 133MHz and the memory clock is 200MHz, there is a 67MHz difference between the two. This frequency difference is called "memory asynchronous" . However, in order to achieve synchronous communication between the memory and the CPU, the two buses still need to be synchronized in phase. It is not technically difficult to achieve phase synchronization, as long as the PLL circuit in the Northbridge chip and the PLL circuit in the frequency synthesizer use the same reference frequency fref.

In fact, the frequency setting of the plug-and-play external device is basically the same as the automatic setting of the memory frequency. The host reads the characteristic parameters including the frequency in the ROM chip in the device, and then automatically allocates system resources and configures the driver automatically. So that the device can work properly.

Clock frequency for computer pulse

1. Test the actual frequency and unleash the potential of the equipment

Intel Processor Frequency ID Utility is the CPU detection software released by Intel Corporation. The software lists two data, "reporting frequency" and "expected frequency". The former item indicates the current operating speed of the tested CPU, and the latter item indicates the Test the maximum operating speed designed for the CPU when it leaves the factory. If the data is the same, the CPU is not overclocked. If the reported frequency is lower than expected, the processor's capabilities are not being used.

Using the test software to get an overview of the working status of each device is important to optimize system performance. There are many similar to the above detection software, and all the frequencies to be tested can be displayed by the test software. However, some advanced players doubt whether the frequency measured by the software is accurate. This suspicion is not unfounded, because the platform on which the detection software runs is based on the reference frequency fref. If the reference frequency itself is not accurate, it is difficult to guarantee the frequency value measured by the software. However, to accurately measure the frequency of a clock signal, you can use an oscilloscope.

There is a close relationship between the performance of external equipment and the operating frequency of the interface circuit. Taking the hard disk as an example, if the software uses AIDA32 and other software to detect the highest UDMA transmission mode of the hard disk is UDMA 6 (ATA-133), but the current UDMA transmission mode is UDMA 1 (ATA-33). That is, the hard disk could work at a frequency of 133MHz, but the interface only exchanges data at a frequency of 33MHz. This will greatly reduce hard drive performance. In this case, it means that the device's potential has not been realized. You should check whether the interface mode selection in the BIOS is correct or solve the problem by installing the relevant IDE driver.

2. Prevent heart rate

If the frequency is too low, the performance of the equipment will be low. On the contrary, if the frequency is too high, the equipment will be unstable and even strike completely. Usually, after we overclock the display chip and video memory, the display screen appears to be a typical case. The situation that the equipment fails to work because of the heart rate accounts for a considerable proportion of computer failures. Due to various reasons, the operating frequency of some equipment cannot actually reach the nominal frequency. Regarding the handling of such problems, the author has introduced them in detail in the article "Computer Fault Down Frequency Diagnosis Method" ("Microcomputer" 2003 No. 17). It should be added here that if the CMOS fails to start after the CPU overclocking fails, the CMOS will be discharged, and the BIOS will start in a safe mode of 100MHz FSB, without serious consequences. ^[3]