Nuts and Volts - August 2008

PICAXE PRIMER

SHARPENING YOUR TOOLS OF CREATIVITY

■ BY RON HACKETT

GETTING STARTED WITH THE

PICAXE-28X1 MICROCONTROLLER

IN ESSENCE, THE PICAXE-28X1 IS LIKE A 28X ON STEROIDS. It’s pin-for-pin compatible with the 28X and all the 28X commands will function correctly on a 28X1. In other words, if you have an existing 28X circuit, it will function exactly the same with a 28X1 installed. This is a good thing, because Revolution Education considers the 28X to be obsolete and has discontinued it. Amazingly, the new 28X1 is about the same price as the old 28X.

The PICAXE-28X1 has so many improvements and powerful new features that it’s hard to know where to begin. In order to avoid the risk of information overload, I have decided on a two-phase approach. In this installment of the Primer, I’ll present a brief overview of most of the new features and then go into more detail on a couple of them that we will be using right away. In future Primer installments, we’ll explore each new feature in depth as we have a need to use it in a project.

of all the current PICAXE processors; a quick glance will give you some idea of the scope of the 28X1’s improvements in this area. Most of the information in Figure 1 is self-explanatory but a couple of points are worth noting.

First of all, the general-purpose byte variables (b0 to b13 in all the previous PICAXE chips) have been doubled to b0 to b27 in the 28X1. If you have ever run out of variables in a program, you will appreciate how significant this improvement is. Also, the 28X1 includes an entirely new 128-byte storage area (the scratchpad) that can be used for additional storage; it’s especially useful for tables and arrays as we will see in a future installment of the Primer.

Finally, although it’s not included in Figure 1, it’s worth mentioning that the maximum system speed has been

increased from 8 MHz in the earlier processors to 20 MHz in the 28X1. System speed options have also become much more flexible — we’ll get to that in detail later in this installment when we take a look at the new resonator features of the 28X1.

SOFTWARE IMPROVEMENTS

HARDWARE IMPROVEMENTS

Let’s begin by taking a look at the memory and storage capacity of the 28X1. Figure 1 presents a comparison of the relevant features

08M/14M/20M 18X 28X1/40X1 BASIC Program Memory 80 Lines 600 Lines 1000 Lines General Purpose Variables 14 14 28 Storage Variables (peek/poke) 48 96 95 Scratchpad Variables (get/put) - - 128

Special Function Variables 3/4/4 4 7

EEPROM Data Storage 256 - program 256 256

Figure 2 presents the pin-out of the 28X1; in it, some of the major new software functions are highlighted in a larger bold font. We will consider the pins in numerical order, so the first new feature that jumps out is “ULPWU” (pin 2), which is an acronym for the new “Ultra Low-Power Wake-Up” feature. We will explore the ULPWU in some detail in a future installment of the Primer when we begin working on a 28X1 battery-powered data-logging system, but for now all we need to know about it is that ULPWU provides a means for greatly reducing power consumption.