Digital System

Created: April 2020Last Updated: 10/01/2020

Introduction

Digital system simply store, processes, and interface with digital data. It is central to both general purpose system (e.g. mobile phone) and embedded system (e.g. Chromecast streaming stick). Many consumer electronics (e.g. phone, tablet, TV, smart speaker) has almost identical digital system with slight variation in processor architecture, memory storage and bandwidth, and I/O peripheral devices.

Background

A prime example of a digital system is a computer, and it's basic building blocks are, CPU, memories, and input and output (I/Os) peripherals such as key board and display. In this article, we will examine the digital building blocks of modern electronics.

Goal

The goal of the of this article is to provide a common system architecture with detailed analysis and design thinking process on design choice and selection for each block in order to meet the overall system requirement.

Overview

An typical modern system on chip supports but not limited to following features, multimedia (video, audio, and imaging processing ), memory controller, graphic engine, audio processing, system interface (i.e generic communication interfaces), and peripheral I/O ports (e.g. Display, Audio, USB , etc.)

Following example, we showed a typical design done with ARM cores found in devices like TV, Set top boxes, smart speakers, etc.

System Block diagram

Digital System Design

Block Descriptions

App cores: it stands for application processors and is the main general purpose cores that the operating is running on.

GPU: it is needed for graphic rendering for display use cases.

Internal memory: each SoC has built in memories such as Boot ROM and caches

    • Boot Read Only Memory (ROM) contains a piece of code that is executed first during device power on reset (i. cold boot from a power on event); it's main responsibilities are Initialization of the SoC and searching for system images to boot on.

    • Caches memories is a smaller and fast memories built-into the application cores; it's purpose is to store a small but frequently used data from the main memory in order to reduce main memory accessing time.

    • Dynamic RAM (DRAM): it's volatile memory that can only retains its data when device is powered on. Dyanmic RAM is type of RAM that is widely used in industry due to it's low cost per storage capacity compared to other memory technology such as SRAM.

    • FLASH: it's is a non-volatile memory that retains its data regardless what the power state is. Flash memory is widely used as storage and for transferring data to and from SoC. There are two types of flash memories, NAND or NOR.

      • Note: to learn more about memories please navigate to memories page under fundamentals section

    • Under the I/O peripheral interfaces: we see digital interfaces for 4 common types of multi-media devices, Ethernet, Display, HDMI, and Audio.

      • Note: some interfaces are more complex that others, Audio is the simplest while the HDMI, Display, and Ethernet requires special control and driver circuits (as commonly refer to as phy) to generate strict voltage and timing digital signals for transmission and reception.

      • Note: The terms mac and phy generally pops up often in high speed digital design. In short, mac and phy are the lowest two communication theory layer link layer(mac) and physical layer (phy). For details please see here.

Detailed Design

First step to any design is gather and understand product system requirements

A list of product system requirements for a smart TV is enumerated below

  1. Must have sufficient memories for OS and on device

  2. Must have sufficient memory bandwidth for 4K streaming

  3. Must support LAN based connectivity for 4K streaming

  4. Must support stereo audio

  5. Support HDMI Out in/out

  6. Support Optical audio

Second step is to translate product requirements into associated engineering requirements:

  1. Must have sufficient memories for OS and on device => 2GB memory

  2. Must have sufficient memory bandwidth for 4K streaming => DDR4 memory

Note: DDR4 is not required for 4K streaming. This is just an illustration. One instance of requiring DDR4 is low latency gaming application in which large memory bandwidth is needed to share between CPU processing as well as GPU graphic rendering.

  1. Must support LAN based connectivity for 4K streaming => a minimum of 100M Ethernet

Note: Actual streaming bandwidth for 4K is roughly 20 Mbps but having so 10M Ethernet would not be sufficient.

  1. Must support stereo audio => LEFT and Right Audio speakers

  2. Support HDMI Out in/out => HDMI in/out controllers and interconnect on SoC and external ports.

  3. Support Optical audio => SPDIF controller and interconnect on SoC and optical port.

Third step is translate engineering requirements in to System Architecture.

An example of System Architecture of a Smart 4K TV is shown below.

Digital System Design Streaming Dongle Example

As we can see from above block diagram, all most all components on this circuit board are digital system (i.e process, store and communicate digital data).

    • Note: System on Chip (SoC) above almost always contains mixed signal hardware such as ADC and DAC. So it's not entirely 100% digital.

Why choosing DDR4 2GB memory?

The 2GB memory is chosen as the minimum memory to run the operating system with its running applications. DDR4 provides a faster memory clock speed (max 1600 MHz) vs. DDR3 (max of 1300 MHz). Fast memory bandwidth generally increases system response such as faster UI navigation, faster app launch time, etc. However, one needs to evaluate the memory bandwidth required of the system based on product requirement such as TV gaming (high memory requirement), TV streaming, etc.

Why what is an eMMC storage?

Embedded multi-media controller (eMMC) memory contains both the flash memory controller and NAND flash memory in one silicon package. This allows memory to be access using standard multiple media interface 8 bit bus. The pros of using eMMC is low cost and common interface.

Isn't transformer used for AC to AC power converter?

Yes, differential signals are AC signals, so it can pass through a transformer. However, the primary reasons for using it is signal conditions (i.e filter out interference noise) and isolation (i.e keeping the any high voltage spikes from spreading among interconnected network devices. Example Gigbit transformer from Pulse Electronics

What is Ethernet phy transceiver? And why is it needed?

Ethernet phy transceiver is a bridge chip that converts input 4 differential pairs data from RJ45 connector to a standard RGMMI interface for Gbit ethernet feature. It is needed because sometimes SoC does not have transceiver hardware (physical layer) IP block to transmit and receive electrical data properly on the wire due to cost or die size limitation. Often we see design have separate phy chip to bridge physical signal to a standardized interface, RGMMI.

What is LVDS?

LVDS standands for low voltage differential signaling. It's a type of electrical data transmission using low voltage swings and differential pairs to send high speed data over long distance. Lower voltage signal swing can used to transmit at higher rate; moreover, differential signaling improves noise immunity against any noise that could be picked up by long cable. LVDS display interface Display utilizes these two features to achieve high bit rate. This interface has 8 data pairs and 2 clock pairs.

  • Note: to learn more about signal integrity please navigate to signal integrity page under fundamentals section

Why choosing HDMI 2.0 vs HDMI 1.4?

One main feature of HDMI 2.0 standard is that it can support up to 4K resolution and 60 fps, and the product requirement is two built a 4K device. Hence we need to select SoC with right type of HDMI receiver.

What is SPDIF audio?

It's a common digital audio interface that is used to transmit interconnect audio equipment. Optical out is a common tranmission medium using fiber optics, hence we need electrical to optical signal coversion. Toshhiba TORX 1950 series makes general purposed optical transmitter or receiver module.

What is I2S?

I2S stands for inter IC sound digital interface that is widely used for streaming digital audio data to among audio devices. We have selected a I2S enabled class-D amplifier+speaker driver to direct covert digital audio stream to acoustic sound.

Summary and Conclusion

In this article we first reviewed text book definition of a digital system and then we went through a practical example of digital system block diagram of a modern 4K Smart TV and it's component selection process. We learned that a system on chip contains main different processors not just general purposed cores but also special and highly optimized cores such as graphic and audio engine.

We have also see that most of consumer electronics design are digital. Even wireless system and sensor system are highly mixed signaled or packaged in modules where hardware designer can treat it just as if it is digital IC with digital host interfaces.

The digitization and highly integrated complex solutions simplifies hardware system engineer's job, However, it is at an expense of relinquishing design responsibilities and control to SoC engineering team. As a result of this trend, a system design engineer, more focus need to spend on specifying the system requirements, select the corresponding parts, and make right the trade-off in order to deliver a product that meets product performance and features and end user's budget.