Category Archives: Embedded Systems

Designing a System? Consider NRE vs. COGS

Electrical Engineering, Embedded Systems, Hardware, Software

There are always tradeoffs between Non Recurring Engineering (NRE, or product development costs) and Costs of Goods Sold (COGS, or the cost of the final product). The balance between the two depends on the project. Simple products will require lower COGS. However, never forget about NRE costs. It is possible for you to increase COGS slightly and save a lot of money on NRE. One example of this is an embedded project requiring UART. Assuming your microcontroller or design has no UART capability, you can go either the software or the hardware route. Most will go the software route. Why? obviously because software is cheap (the most erroneous assumption by some).

When you look at software, development is very expensive. Sure, you can bit bang UART and it will work, but to get it to work reliably will require code design, development and debugging. For just a little added cost to COGS (the price of the final product), you can save yourself major headaches, not to mention that you won’t be burdening your microcontroller with handling UART itself.

If you don’t believe it, consider that IBM’s own analysis of hundreds of software projects finds that software development does not scale linearly with complexity. Simpler software costs less.

Nonetheless, there are cases where hardware won’t do. This is especially true when upgrades will be required. UART is standard and won’t require upgrades, but a proprietary algorithm that’s in flux will change.

In defense of sub-1GHz transceivers for Sensor Networks and Wireless Devices

Electrical Engineering, Embedded Systems, Wireless Sensor Networks

When you think of Sensor Networks, 2.4GHz transceivers likely come to mind. The CC2420 is one of the most popular transceivers for sensor networks and other applications and it uses 2.4GHz as per Zigbee. Of course, Zigbee does allow Sub-1GHz frequencies, but I’ve yet to see many of those made. Undoubtedly this is due (among other reasons) to the smaller antenna which results in a smaller PCB. However, there are several issues with these transceivers, among them:

  • Operating sensor networks and other devices at 2.4GHz can be a coexistance nightmare. Consider that most deployments will be done in urban environments where the 2.4GHz band is saturated with Microwave Ovens, Baby Monitors, Bluetooth, WiFi, etc. A node needing to survive in this environment will experience frequent collisions. There have been several papers analyzing the performance of devices in these conditions and needless to say the results were far from acceptable.
  • Decreased range is to be expected from higher frequencies as per the Friis transmission equation.

2.4GHz devices do have some advantages such as popularity, small size, and ease of interoperability. However, next time you consider a transceiver, don’t disregard the sub-1GHz options which might be better.

Some useful transceivers:
TI CC1101 – sub-1GHz transceiver with multiple bands, 10dBm output power, flexible modulation formats and 500kbps
TI CC2520 – 2.4GHz Zigbee compliant transceiver

There are many other manufacturers of RF Transceivers (especially Zigbee/ 802.15.4) such as Microchip, but I listed TI transceivers because they seem to be quite predominant in the market and I’ve personally used them in several projects. The CC2520 is an update to the CC2420.

The Cortex-M4 is in the pipeline

ARM, Embedded Systems

Some of you might have heard about the new Cortex-M4 ARM Processor.

It’s a superset of the Cortex-M3, meaning it can run Cortex-M3 instructions while adding new ones. The processor is primarily targeted at DSP applications (which seems to be everywhere these days) but I suspect will be good for almost any application. Texas Instruments, NXP and Freescale have licensed the core, which promises an ample supply of choices for your next project. Another huge advantage of the Cortex-M4 is the addition of SIMD. If your data is 8 or 16-bit, you’ll be able to perform multiple operations in one clock cycle, giving a huge boost to many algorithms. The following table shows the difference in cycles for the several of the instructions:

Instruction Description Cortex-M3 Cortex-M4
MLA Multiply accumulate 2 1
MLS Multiply subtract 2 1
SMULL Long signed multiply 3 to 5 1
UMULL Long unsigned multiply 3 to 5 1
SMLAL Long signed accumulate 4 to 7 1
UMLAL Long unsigned accumulate 4 to 7 1

Many algorithms will benefit from the single cycle instructions and even more from the SIMD instructions. There will probably be a lot of demand for those of us who still use Assembly.

For more information:
Cortex-M3 Technical Reference Manual

Cortex-M4 Technical Reference Manual

Embedded Systems – Electrical engineering all in one

Electrical Engineering, Embedded Systems

Electrical Engineering is usually divided into many specialties –  Power, Control, Microelectronics, Signal processing, Telecommunications, etc. and it’s rare for engineers to move away from their specialty. I’ve found that Embedded Engineers are sometimes required to work in many of these areas at once – to be able to get an application off the ground, you need to be able to understand many of the issues surrounding the system. Imagine how a microcontroller would perform if its supply rails were full of noise due to a bad power supply design or trying to perform an FFT on an 8-bit microcontroller? This clearly calls for knowledge in many disciplines.