ARM embedded processors – selecting the right one

ARM embedded processors are ubiquitous. With the vast number of ARM-based processors available, selecting the right one for a senior project can seem daunting. This chapter will help you select the right TI ARM processor.

At a high level, ARM embedded processors can be split into three tiers of performance: the Cortex-A, -R and –M series.

ARM Cortex-A microprocessors

ARM Cortex-R microcontrollers

ARM Cortex-M microcontrollers

TI's ARM-based platforms (as of August 2012) are outlined in Figure 1.

Figure 1

Stellaris microcontrollers. Offering performance up to 80 MHz and a high degree of connectivity and analog integration to the microcontroller markets, Stellaris MCUs are ideal for applications that require memory, analog components and communications interfaces to be on a single chip within a compact package.

Concerto™ microcontrollers. Concerto F28M35x microcontrollers combine TI’s performance C28x™ core and control peripherals with an ARM Cortex-M3 core and connectivity peripherals to deliver a clearly partitioned architecture that supports real-time control and advanced connectivity in a single MCU.

Hercules™ microcontrollers. For high-reliability applications such as transportation, the TMS570 provides DSP-like performance with safety-critical features implemented in hardware to ensure continuous and uninterrupted operation. The TMS570 has a dual-core architecture, with the second core running in lockstep for redundancy and self-checking.

Sitara™ microprocessors. Offering performance up to 1.5 GHz, Sitara ARM MPUs are ideal for applications needing a high-level operating system, wired and wireless network connectivity, concurrent applications, and support for rich graphics with an advanced user interface.

OMAP™ multicore microprocessors. The OMAP platform for digital media processing brings together an ARM core with advanced video acceleration, plus a high-performance C6000™ DSP for applications that need to support real-time video, image and audio data processing. With the ability to provide computationally intensive real-time signal-processing performance, developers can implement complex applications such as video analytics, speech recognition, baseband channel management and power monitoring with a single chip.

Deep dive into Cortex-M – Stellaris ARM Cortex-M4F

Stellaris microcontrollers (MCUs) are 32-bit, RISC-based mixed-signal processors designed specifically for ease of use and connectivity. Common applications include end-equipment in the computing, industrial and smart grid markets.

To get a better idea of what Stellaris MCUs are and how they can be used to solve an application problem, let’s take a look at a typical block diagram for the device. Figure 2 shows the block diagram for the LM4F232H5, one of the LM4F devices.

Figure 2

The block diagram provides guidance on the key features of a particular device. Let’s take a closer look at some of the more prominent features. (Most of this section is transcribed from the LM4F232H5QC data sheet, available at http://www.ti.com/lit/gpn/lm4f232h5qc.)

Each of these modules is brought out through pins on a package. In the case of the LM4F232H5QC, the pin diagram is shown in Figure 3. This pin diagram can also be found in the LM4F232H5QC data sheet under the bookmark “Pin Diagram.”

Figure 3

This view is helpful because it identifies the necessary connections to the LM4F232, namely the power supply (VDDx, GNDx), the programming/debugging tool connections (TCK, TMS, TDI, TDO) and some of the key peripheral connections (HIB, USB). However, many of the signals, such as the timer capture/compare pins and analog inputs, are not shown. That granularity is provided in the signal tables in the data sheet. An excerpt from the LM4F232H5QC signal tables is shown in Table 1.

Table 1. Exerpt from the LM4F232H5QC data sheet showing showing part of the signal table.

Using these tables, you can start to map the desired functionality to your design. Figure 4 is a schematic drawing depicting the baseline connections for an LM4F120 design. The LM4F120 and the LM4F232 are actually members of the same device family, so the schematic applies to the part under discussion.

Figure 4

Since the LM4F power rail is specified as 3.3 V ± 5 percent, a typical 3.3-V supply will work well. Note the decoupling capacitors on the VDD pins. Signals like WAKE and RESET are connected to user switches that are configured as pullups. A 16-MHz oscillator and 32-kHz oscillator are connected to the MOSC and HIBOSC pins, respectively.

Figure 5 goes one step further to demonstrate an example connection to the debugging, USB and user interface signals.

Figure 5

Note the connections to the USB connector and the JTAG signals. Since this is a schematic from the Stellairs LaunchPad evaluation kit, this also includes the connections to a virtual COM port bus that uses UART on Port A pins 0 and 1 to communicate back to the debugging emulator. The schematic also includes the breakout to the pin headers. These other pin connections can be used to connect all other signals.

Now that we have explained the basics of how to include an LM4F device in your system, let’s review the documents you can use to take your design to the next level.

Data sheet deep dive

Beyond the pin diagram and signal tables, let’s highlight some other areas you should focus on when you first look at the data sheet.

Boundary and recommended operating conditions

Table 2. Table from data sheet showing reccomended operating conditions.

The maximum ratings indicate the voltage and current conditions that should not be exceeded, under any condition, without risking damage to the device (Table 3). The recommended operating conditions document the conditions under which the part is guaranteed to operate (Table 4). The minimum and maximum electrical specifications in the data sheet are specified under these conditions.

Table 3. Table in data sheet showing maximum ratings.

Table 4. Recommended operating conditions table from a data sheet.

Power consumption numbers (Table 5):

Table 5. Power consumption data table from a data sheet.

Figure 6

To select your part, please refer to the series of questions and answers near the end of Section 2.2, "Overview of the MSP430 microcontroller from Texas Instruments," that begins with the question, "What problem am I trying to solve?" You can walk through this process for Stellaris processors just as you can for MSP430 microcontrollers.

There are a variety of LM4F devices that will fit a variety of different needs. Sometimes the easiest way to see whether a device is the right one for your application is to get your hands on an evaluation module. In the case of the LM4F232 family, the Stellaris LaunchPad development tool is the recommended kit.

The Stellaris LaunchPad evaluation kit costs less than $10 and provides a variety of booster packs to extend its functionality (Figure 7). See www.ti.com/stellaris to start your design today.

Figure 7

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Last edited by Gene Frantz on Mar 7, 2013 3:51 pm -0600.