Questions tagged [clock]

A digital signal that goes high and low at a specific frequency.

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101
votes
5answers
9k views

Can microcontrollers be run at arbitrarily low clock frequencies?

The datasheet for the ATTiny13A, for instance, lists Min frequency of 0 MHz. Does this mean the clock can be run at any arbitrarily low frequency with no ill effects? I'm assuming it draws lower ...
67
votes
5answers
12k views

Serbia, Kosovo power grid row delays European clocks. Why?

According to this article (and a lot more published today on the same topic), Kosovo electricity net production balance has decreased during the last weeks. This has led to a small deviation of the ...
40
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3answers
8k views

How do GPS satellites refresh their clocks

How do GPS satellites keep their on board clocks accurate? I assume that they need to get update from a base station. But how do you make sure that after the update all the satellites are synchronized,...
38
votes
4answers
12k views

How to synchronize two microcontrollers to micro-second accuracy?

I need to synchronize two micro-controllers so that they can measure the speed of propagating waves. The time delay measurements need have microsecond accuracy (error less that 1/2 of a microsecond). ...
31
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8answers
22k views

Why do Microcontrollers need a Clock

Why do instructions need to be processed at set time intervals (i.e. with the use of a clock)? Can't they be executed sequentially - immediately after the previous instruction has completed? An ...
29
votes
6answers
16k views

How precise is the frequency of the AC electricity network?

If I make a digital clock which first detects the AC frequency roughly (whether it is 50, 60, 100Hz, etc), then uses it as its clock pulse source, how precise would it be? Would it work world wide ...
25
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6answers
8k views

Why are clocks used in computers?

As I know, a clock controls all of the logic operations, but it also limits the speed of a computer because the gates have to wait for the clock to change from low to high or high to low depending on ...
23
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5answers
13k views

How do electronic devices keep track of time without power?

There's laptops, PCs, microcontrollers and a lot of other things that can be plugged out and plugged in without a battery. But how does the system clock still keep track of time without anything to ...
22
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4answers
4k views

How to communicate faster than the system clock

I was reading about the new (ish) Thunderbolt 3 today, and was very impressed by the specced speed of 40Gbps. Then I looked at Intel's latest i9 processor speed... about 4.2GHz max. How can a system ...
19
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6answers
34k views

Using the ATMega328 with the internal oscillator?

I have a project that I think would be best suited for an ATMega328P. However, in every simple project I've seen, people always hook up a 16MHz external oscillator. From what I can see, it should have ...
19
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5answers
30k views

Why edge triggering is preferred over level triggering?

I am trying to understand why edge triggering is preferred over level triggering. In my book it is not explained clearly. After searching online I came to know that edge triggering is insensitive to ...
16
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2answers
4k views

Why use DDR instead of increasing clock speed?

Why would you want to use DDR ram and read/write on every rising and falling edge of the clock instead of just doubling your clock speed and read/write on just one of either the rising or falling edge?...
16
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5answers
36k views

Why in 8085 microprocessor, the clock frequency is divided by two?

Why is it that the produced clock frequency is 6.144 MHz, but internally it (8085 processor) uses only 3.072 MHz. Also what leads to the specific value of 6.144 in a clock. I found an answer at ...
16
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2answers
9k views

How can a CPU dynamically change its clock frequency?

My Intel CPU changes clock speed depending on the usage, but how does it decide what clock speed to run at? Is the clock speed determined by the OS software using an algorithm, or is it hardware based?...
12
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3answers
3k views

How do processors control their clock speed?

I recently came across an STM processor with 2 oscillators on the circuit - I suppose one for high-speed operation and the other for low power. For something like a desktop processor where the clock ...
12
votes
3answers
16k views

I2C minimum clock speed and reliability

Is there a minimum clock rate specified by I2C? I know the most widely used clock rate is 100kHz and there is a "fast" mode of 400kHz supported by some devices, and a faster yet mode supported by ...
12
votes
2answers
2k views

How fast is 1 MHz?

How fast is 1 MHz in an AVR microcontroller? Is it actually 1,000,000 Hz or is it 1,048,576 Hz (1,024 Hz * 1,024)?
11
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5answers
2k views

Can wireless communciation be synchronous?

I understand that in synchronous communication, the sender and receiver need a common clock. Is it possible that wireless communication be synchronous? Can some common clocking element be there for ...
11
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2answers
5k views

Sharing an oscillator between two ICs

I have a microcontroller and an FPGA on the same board. If they're both going to run at the same clock speed, can I just use one oscillator to clock them both? There seems like there is something I ...
11
votes
2answers
833 views

Does this circuit have marginal voltage level problem?

As a research for the problem I described here I found this circuit by Maxim: This is clock doubler, and must be a really good fit in my case as input frequency is very well defined. However looking ...
11
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2answers
5k views

When do I need to use a clock buffer IC?

I am designing a circuit and PCB for driving 7 DACs from an FPGA. (DAC is AD9762) Would it be possible to drive the clock inputs on all 7 DACs with a single clock output (from a PLL output pin) of ...
10
votes
2answers
15k views

What generates the clock signal in a fast CPU and how does it work?

Often, for integrated circuits, a quartz crystal is used to generate the clock signal. However, this only reaches speeds in MHz. What component, or what circuit, generates signals up to 5 GHz ...
10
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3answers
29k views

What is clock skew, and why can it be negative?

My HDL compiler (Quartus II) generates timing reports. In it, the nodes have "clock skew" column. The only definition of clock skew I found is in the TimeQuest documentation (see page 7-24): To ...
10
votes
4answers
4k views

timing constraint for bus synchronizer circuits

I've a bus synchronizer circuit for passing a wide register across clock domains. I'll provide a simplified description, omitting asynchronous reset logic. The data is generated on one clock. ...
10
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2answers
5k views

Need help understanding AVR ATMEGA / ATTINY timer mirrored output

I am trying to use Timer1 of Atmel AVR microcontroller, either AtMega328 as used in the Arduino, or the ATTiny85, to output two clock signals which are mirror images of each other. The frequency I am ...
10
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2answers
5k views

ASIC timing constraints via SDC: How to correctly specify a multiplexed clock?

Introduction Having found multiple, sometimes conflicting or incomplete information on the internet and in some training classes about how to create timing constraints in SDC format correctly, I'd ...
9
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6answers
3k views

1 kHz clock over long wire

I will have a wire that will run up to 1000 m with many nodes connected to it. The wire will run along a CAN bus and a ground wire and must transport a 1 kHz square wave 50% duty-cycle clock signal to ...
9
votes
5answers
2k views

Setup and hold time output when violated

Consider a positive edge triggered D flip flop with input signal X with a setup time of 20 ns and a hold time of 0 ns. What will be the output? C is clock signal with a period of 40 ns....
9
votes
3answers
2k views

Routing 30MHz clock thru several boards

I'm using TLC5945 LED driver. Microcontroller (I'm using LPC1343) must provide a clock for it's internal PWM timer/counter. Maximum allowed clock speed is specified as 30MHz. I will have several ...
9
votes
2answers
1k views

Strange extra frequency in crystal oscillator

I have inherited a circuit from another designer using a 12.288 MHz crystal as the source for an audio clock. We recently had supply chain issues and I was asked to approve an alternate part with ...
8
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5answers
5k views

Why is there a PLL in CPU?

I read that PLL are used in CPU to generate the clock, but I can't understand why. I don't really have any guess of why this is.
8
votes
3answers
1k views

Estimate electrical frequency from clock shift

About 6 months ago I installed a solar power system with 2 Tesla Powerwalls for backup. Today, we got to test out the system with a power outage from an ice storm in the southeast. (We are still on ...
8
votes
5answers
2k views

How do I drive 14.3Mhz clock input from 10MHz?

I intend to use an IC which requires 14.3MHz clock input, but want to drive it from a stable 10MHz source - derived from GPS. How do I turn the 10MHz clock into the 14.3MHz that the IC requires?
8
votes
3answers
257 views

Theoretical Clock Question

Just a theoretical question more than anything. Does the clock signal have to have the same width between peeks and troughs? A normal clock signal with data (below): A random clock signal with data (...
8
votes
3answers
700 views

Frequency of a MCU is higher than frequency of a quartz

How is it possible that in a LPC3141 MCU the MCU has frequency 270MHz while the 12MHz quartz crystal is used in oscilator (inside the clock generation unit)? Makes no sense to me... Could anyone ...
8
votes
2answers
1k views

Is it true that the power line frequency is kept accurate by atomic clocks?

I've heard rumors that the power line frequency is kept stable and accurate by syncing it with atomic clocks. Is this true? What kind of accuracy does it have? Is this true everywhere?
8
votes
1answer
15k views

Why use STM32 HSE?

Why do we use an external oscillator for STM32??? Why not just use the microchip oscillator? I am using STM32F2xx it clocks at 120Mhz But, an external oscillator of 12Mhz is latch into the microchip,...
8
votes
2answers
1k views

Does VGA have to be drawn at 60hz?

I'm wanting to generate a VGA signal hopefully as painlessly as possible. I have an odd question though. Everywhere I see that VGA requires a 25MHz clock for the sync signals. Is it possible to ...
7
votes
5answers
3k views

How can I estimate the speed of this code section for this microcontroller?

I'm using an ATmega328P to read the state of a digital input by using the following code section written in C (there can be alternative ways, but this is an example). ...
7
votes
6answers
5k views

What is the purpose of a tri-state pin in a Oscillator

I'm trying to interface this clock (32.768kHz Ceramic Surface Mount Crystal Oscillator datasheet), but I'm confused as to what to do with the tri-state pin. What is it's purpose in the oscillator and ...
7
votes
5answers
2k views

Why is this clock signal connected to a capacitor to gnd?

I am trying to understand the following circuit: My problem is to understand why the CLK signal is connected to the capacitor (C7). The bottom side of C7 is connected with a resistor to GND. This ...
7
votes
5answers
4k views

Why aren't 1Hz crystals used to measure seconds?

According to Wikipedia: Many clocks use a 32.768KHz crystal. Is this because the crystal is smaller than a 1Hz crystal? If 1.0 Hz == 1.0 second. Then, why the need for the division?
7
votes
4answers
7k views

Is there a preferred placement of termination resistor for a differential clock line?

Looking at a reference design board from Xilinx, we noticed the placement of the termination resistor for a differential clock line (~300 MHz): The resistor is placed not at the closest possible ...
7
votes
2answers
5k views

What is an efficient way to divide clock by 1000 or more?

Context: This is a home brew project that incorporates NO microcontrollers as a learning exercise. I need to time something up to 20 seconds long, down to the nearest 100th of a second, and display ...
7
votes
1answer
3k views

Why does this rising edge detector using a capacitor and a resistor work?

So in this video from Ben Eater he makes a rising edge detector using a capacitor and a resistor like the one below. simulate this circuit – Schematic created using CircuitLab In the video Ben ...
7
votes
2answers
8k views

What is the best way to layout a PCB with crystal oscillators and MCU?

I have heard that the clock circuit is a noise source and that the clock circuit is also sensitive to noise. I believe that keeping the clock circuit close to the MCU and keeping other routes or ...
7
votes
5answers
3k views

When do you say two clocks are asynchronous?

I have a situation where reference clock of PLL_0 is coming from some clock source and giving out a clock (named C0) with freq0 and and C0 is going as reference clock to PLL_1 and giving out clock C1 ...
7
votes
3answers
28k views

How do I calculate needed pixel clock frequency?

If I want to have a resolution of X * Y pixels, updating in frequency f. How do I calculate the pixel clock speed? Example: 1280 x 1024 @ 85Hz usually have a pixel clock of 157.5 MHz, but how do I ...
7
votes
1answer
9k views

ASIC timing constraints via SDC: How to correctly specify a ripple-divided clock?

Introduction Having found multiple, sometimes conflicting or incomplete information on the internet and in some training classes about how to create timing constraints in SDC format correctly, I'd ...
6
votes
3answers
945 views

Accessing odd address memory locations in 8086

I am a beginner in microprocessors. Apologies if my question is too naive. The memory section of the 8086 processor is divided into two segments: even and odd to allow the CPU to fetch 16 bits in one ...

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