Nuts & Volts - February 2004

Personal Robotics

by Mike Keesling

Understanding, Designing, and Constructing Robots and Robotic Systems

Personal Robotics

All About the Isopod™

F o r

E l e c t r o n i c s

Isopod: Any of numerous crustaceans of the order Isopoda, characterized by a flattened body bearing seven pairs of legs and including the sow bug ( www.dictionary.com).

Well, that's the dictionary definition, but what it means to me, is a small, flat microcontroller system on a board, with many groups of functions, a board suitable for controlling many actuators, and reading many sensors.

In the last two years, the IsoPod has grown in popularity, and even graced the top 100 products of the year in EDN magazine in 2002. It has grown into a family of related products with more functions or smaller dimensions to accommodate the task at hand whatever it is.

When you buy an IsoPod, you are not just buying a microcontroller board; you are actually buying a digital signal processor, voltage regulators, communications transceivers, and a full-featured programming language called IsoMax™. All this comes on a 1.27" by 3.05" board that looks more like a studded breakfast bar than a microcontroller. Based on the 56800 series of digital signal processors and running at 80 MHz, you will find that fully tasking it will be quite a challenge.

NUTS & VOLTS E v e r y t h i n g

The feature set of the IsoPod is impressive, to say the least. To fully understand its power, you have to consider that all of its features are hardware based. What this means, is that you have access to a lot of "set it and forget it" functionality. The advantage to this is that there is little or no processor overhead required to get a lot of stuff done. This frees you

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to write the software that imbues your project with the behavior you are interested in, without having to code the functionality.

As an example, you can program just about any microcontroller to drive a multitude of R/C servos, but how much overhead does this require? With the IsoPod, 26 R/C servos can be driven without intervention on the part of the software, Yes, that is correct, 0% overhead on the processor.

Unfortunately, there is a price for all this functionality. The IsoPod is literally bristling with connections. When you connect to it, you really have to be careful to avoid connecting the wrong signals or permanent damage can result, especially to the analog inputs which will in no way tolerate over-voltage. In addition, there is a weird sort of right angle connector on one end, but this has been phased out in the other members of the IsoPod family.

The PWM module is very complete. With two banks of six PWM pins, PWMA, and PWMB, you can instantly control 12 R/C servos, and a variety of different configurations

of H-bridges for driving a variety of different motors. Each bank has its own frequency setting, so you could have six R/C servos connected to PWMA, and six DC motors off of PWMB.

The PWM pins in each bank can be configured as pairs, complete with dead-time insertion and polarity control, or as individual pins. You are limited to all pairs or all individuals in each bank, but because of the reasons for using one or the other, this would rarely be an issue. One limitation is the minimum frequency they can generate. Without slowing down the processor to half speed, you are limited to 76 Hz. This can cause problems with some R/C servos, but so far, my experience has been okay. In any case, if it is a problem, there are the timer registers that can also generate PWM, and can easily go as slow as necessary to drive R/C servos.

The timer module is hands down the most versatile part of the IsoPod. Consisting of four banks of timers (TA-TD), it can generate PWM, read quadrature encoders, decode step/direction signals, count pulses, or measure pulse lengths. For instance, I have an application that has the timer unit configured to read six quadrature encoders into 32 bit counters, and perform high-resolu-tion pulse measuring on each encoder, to provide ultra-high resolution velocity information. I have yet to scrape the surface of its functionality. Covering an address space of 128 bytes, it is quite a bit to get acquainted with.