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Design with ARM Cortex M based SoCs: The basics

Posted: 28 Apr 2014     Print Version  Bookmark and Share

Keywords:system-on-chips  ARM  Cortex M  SoC  PSoC 

Direct Memory Access Controller (DMA): To improve data access performance, DMA is also included in the list of peripherals. DMA frees the CPU from data-related tasks and hence improves overall data throughput of the SoC. DMA controllers can be configured for single/burst data transfer requests, interrupt signal on completion of data transfer, etc. Transfers of data can take place between:
 • Memory to peripheral
 • Memory to memory
 • Peripheral to memory
 • Peripheral to peripheral

Programmable Digital Blocks: For applications that require digital capabilities that cannot be built using the components that exist in the SoC, some SoC vendors provide the option to build your own digital application using configurable PLDs and ALU to offload tasks from the CPU. With this functionality, the developer can make his/her own custom digital application suiting his requirement. Programmable digital blocks are known as Universal Digital Blocks (UDBs) in the Cypress' PSoC architecture. Each UDB is made up of two PLDs, status and control logic, and a datapath module. The datapath module is capable of performing simple arithmetic functions. Developers can describe their digital block in Verilog and have the PSoC Creator tool compile, synthesise, place-and-route, and automatically build it as part of the system.

Programmable Analogue Blocks: Vendors provide analogue programmability using Switched Capacitor blocks. These blocks follow mimic resistance using a capacitor. Different op-amp configurations like inverting amplifier, non-inverting amplifier, filter, integrator, and differentiator can be created using a single op-amp by varying the input and feedback configurations using switched capacitances.

System Resources: The system resources block is the controller block which deals with:
 • Clocking sub-system
 • Power and Operational modes
 • Power supply and monitoring sub-system
 • Watchdog timer, etc.

These blocks are connected to each other by a data bus which can be vendor proprietary or using the industry standard ARM's Advance Microcontroller Bus Architecture which includes the Advanced High Performance bus (AHB), Advanced Peripheral Bus (APB), etc. This bus architecture promotes modular system design, provides higher performance, and reduces power dissipation in on-chip communication.

The peripherals mentioned above are highly configurable and their properties can be modified easily to suit the needs of the application. This can be done using firmware by writing to peripheral registers or using a vendor's proprietary software GUI to modify the configuration settings of the peripherals. For example, PSoC Creator Software Suite allows developers to configure these components while designing an application using PSoC.

Figure 2 shows a snapshot of a design built using PSoC Creator Software.

These GUIs also provide a schematic approach to circuit design. The circuit can be designed by dragging and dropping components from a component list onto the schematic. After this, they can be connected using virtual wires from the GUI. On the hardware level, this is managed by the interconnect system designed in the SoC. This interconnect system is highly configurable and can route any signal to anywhere on the silicon. As can be seen from the diagram above, this is an actual design to showcase low power modes but it looks like a normal paper-based design following a block-level approach.

Designing a mixed signal system: traditional vs SoC approaches
Let's begin by exploring how SoCs have changed the way embedded systems are designed by taking an example and comparing it with traditional design approaches. Consider a basic gas sensing application which detects the concentration of CO in the environment. Figure 3 shows a functional block diagram for this application.

Figure 3: Functional block diagram of gas sensing system.

In this design:
 • A hardware gas sensor is required to detect the concentration of CO gas
 • An op-amp is needed to amplify and convert the current signal obtained from the sensor output to a voltage
 • An ADC converts the voltage signal to digital value
 • A pulse width modulator (PWM) generates an alarm tone from the buzzer when the gas concentration crosses the defined threshold limit determined by the processor

Traditional system development approach
As shown in figure 4, generally, to design any system the developer takes the following steps:
1. Schematic design
2. Complete the schematic design and verify functionality theoretically before proceeding further.
3. Component selection

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