How does the smartphone CPU Work? Part 2

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In this post we will Continue Part 2 and provide you full information about the topic – How does the smartphone CPU Work?

Q: what happens when you take a picture on your smartphone?

A: first the photons from the scene enter the camera’s lenses flow through a color filter array and hit the sensor’s photodiode pixels these color filtered photons are then absorbed by each photodiode and converted into an analog electrical current which is then converted into a digital 12-bit binary value.

( Amazing Fact ) A 12-megapixel camera’s image has 12 million of these 12-bit binary values note that each pixel is either red or green or blue and thus the overall image is considered raw and still has to undergo a number of image processing steps to turn it from a raw image into a recognizable picture where the pixels each have a red and green and blue value but before these processes take place this image data must be stored somewhere and as a result, it’s sent to the smartphone’s working memory or dram to do this the data travels from the camera and enters the soc through a mobile industry processor interface or MIPI which can send or receive data at around 5 to 8 Gigabits per second.

This image data is then routed by the network un chip arbitrator through the soc to the memory controller and into the dram which as mentioned before is located on the die above the soc there are two quick things to note

• First, the data path between the soc and the dram is shared by everything in this scenario it’s the job of the network arbitrator to prioritize the incoming sensor data of the uncompressed raw image so that no data from the camera is lost and thus the arbitrator streams the raw image directly into the dram

• Second, a 12-megapixel uncompressed raw image takes up around 24 megabytes however the memory cache on the soc is typically four to eight megabytes and is shared among all the processes on the soc therefore when the image signal processor or the video processor works on a recently taken picture in order to make it viewable or to compress it they can only work on small subsections of the overall image leaving the entire image temporarily stored on the dram this also happens while watching a video playing a video game or using pretty much any app. okay let’s get back to the raw image data of the picture that was taken by the camera and then sent to the MIPI routed through the soc and stored in the dram the

• The next step is for the image signal processor or isp on the soc to read the raw uncompressed image data and perform a number of image processing steps

these steps involve first correcting for darker pixels on the edge of the sensor due to lens shading then the isp performs the demosaicing process which involves taking the image data and the pattern on the camera’s color filter and calculating a red green and blue value for every pixel next to the image signal processor denoises sharpens enhances the image and color correct it because the red green and blue filters on the camera don’t directly match the hue of the red green and blue pixels in the display. and finally the image signal processor tone maps the image after all this we have a picture with 12 million pixels each with an 8-bit red green and blue value to get the picture to promptly appear on your screenanother series of steps are involved the rgb image data is taken from the image signal processor sent to the gpu where it gets overlaid into the graphics of the camera app and scale to fit the screen the resulting rgb values get sent to the display processor and then the image is routed to the display where it’s converted into intensities of current in order to light up the corresponding pattern of red green and blue pixels and there you go a picture taken by the camera and whoosh displayed on the screen but we’re not done yethow is the picture saved on your smartphone well first the picture needs to be compressed and to do that the uncompressed image currently residing in dram is sent to a dedicated video coding processor where it first gets converted from red green and blue values into yuv or luminance blue chrominance and red chrome in its values next this data undergoes a series of algorithmsin order to remove information that is undetectable to the human eye and compresses the image into approximately a three megabyte jpeg format this compressed image is sent back to the dram and then routed to the smartphone’s flash memory for long term storage now if you were to send this picture to a friend the compressed image would get brought back into the dram  and then routed to the modem where it’s divided and assembled into packets then sent to the 4g 5g or wi-fi microchipconverted into electromagnetic waves and sent to a cellular or wi-fi network.

One thing we’ve talked a lot about is the movement of data through the soc all of this data movement between the blocks runs over connecting wires and what is part of the network on a chip this network has routers and switches for shared access to routes and targets like the dram the routers and switches arbitrate the flow of data and act like digital traffic lights and the pathways act as a digital highway these highways have different widths of wires based on what they’re talking to and the amount of information that needs to be sent depending on required data rates there are typically 128 or 256 wires running parallel to one another carrying one bit at a time and operating between 500and 1500 megahertz in order to avoid wasting power from your smartphone’s battery the frequency and resulting data transfer rates across the network on a chip ramp up and down depending on the requirements of the applications you’re running and this feature is called dynamic frequency scaling 

So thus far we’ve glossed over the functionality of each of these blocks in the soc but now let’s quickly take a look into the central processing unit or CPU section of the soc as you see there are multiple cores and each core can run part of a program by executing instructions if we focus on just one of these cores we can see that inside it there is another incredibly complex set of blocks that depict the different functional sections and the data flow between them there is a ton of information in this block diagram and we’re planning on making an entire series to explain it all but for now here are the CPU’s memory caches the instructions such as add multiply load-store compare jump and many more flow this way and the actual data being processed flows this way and then over here are the blocks that actually execute the arithmetic branching and storing of data let’s move on to a few additional details about this processor all smartphones use a reduced instruction set computer or risk architecture and almost all of these architectures are licensed from a company called arm

Which stands for advanced risk machines some companies like apple license the instruction set architecture or ISA from the arm and use this ISA to design their processor cores in-house whereas other companies like Qualcomm license complete blueprints which are technically called intellectual property cores from arm Qualcomm then integrates the arm IP cores into its socs sometimes keeping the design of the core as is but more often modifying or customizing the arm core to better suit Qualcomm’s design needs and in the process, Qualcomm rebrands the name of the CPU however all this information still begs the question of how do these socs get designed and manufactured instead of talking about the entire design process which you can find in the creator’s comments we’re going to discuss two design principles of the SOC.

The first principle is called hardware acceleration and this principle deals with the fact that instead of having a single very powerful general-purpose CPU with a lot of cores this chip has a variety of special-purpose blocks dedicated to performing specific functions these special function blocks or hardware accelerators compute their tasks faster while using significantly less power than if the same operation were performed by the general-purpose CPU.

For example, as we saw with the processes involved in taking a picture there is an entire block of the soc that’s dedicated to processing the image and then a separate block whose job is to encode decode or compress or decompress the image and if you’re Reading this Post or watching our youtube Videos on your smartphone it’s actively decompressing this Article or video as you watch/reading it, these sections are useful for taking pictures but critical for recording 4k video and saving battery life hardware accelerators are utilized for pretty much all computationally intensive tasks performed by your smartphone.

The next design principle is that because SOC are used in smartphones that operate on a battery there’s a huge focus on having the soc consume as little power as possible.

One example of low power consumption design is in the CPU, instead of having all high-performance cores chip designers often follow a big little design structure where there are two or four big cores that are high-performance cores but consume more power and four little cores that are lower performance but are energy efficient and your smartphone prioritizes using these lower performance energy-efficient cores when possible furthermore the design of the transistor which is the smallest and fundamental building block of the soc has had its design evolve to both be smaller but more importantly to consume less power as of 2021 the latest technology of transistors is called a gate-all-around field-effect transistor.

let’s finally move on to how these chips are manufactured to start the lion’s share of all socs get manufactured by a single company called TSMC or Taiwan semiconductor manufacturing company and a smaller yet increasing proportion of socs are manufactured by Samsung these microchips are manufactured on 300-millimeter wide silicon wafers in factories called fabs.

In order to manufacture an array of chips the silicon wafer has to go through a series of 120 to 160 processes or steps performed by dozens of different machines ,this is the most common sequence of steps for manufacturing microchips as you see ( in Image ) there are only a dozen or so steps but each of these processes are performed dozens of times in order to build an incredibly complex layout of transistors but this Post is getting too long so we’ll have to save the details of each step for a future Post, for now here’s what a nanoscopic view of the soc looks like on the bottom is a silicon wafer and the billions of transistors are implanted and built into and on top of the silicon on top of the layer of transistors are layers of local interconnects that connect transistors to one another and then on top of that are the global interconnects that connect sections of transistors this is just a smallfraction of the entire soc layout let’s zoom out in order to get a sense of the size of these components, i’m sure you’ve heard the transistors are incredibly small and well here’s a single grain of fine table salt mind blowing right it makes you think the potential of humanity to reach incredible levels of science and engineeringis limitless.

That pretty much sums it up for this quick overview of the soc we know we’ve covered a lot but we only just scratched the surface of

How does the semiconductor chip work?

and

how they’re designed and manufactured as usual?

 

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How does the smartphone CPU Work? Part 1