Issue 147 October 2002
12, 16, 18 Hike!
Dashing for Flash

Start • PIC18F252 • Instruction Set • Reset • Serial Port • In-Circuit • Sources & PDF

In the past few issues while covering my SmartMedia project, I’ve been talking about Microchip’s PIC18F252. I didn’t spend much time discussing the processor then, but it’s an apropos topic now that Circuit Cellar is kicking off the Mad Dash for Flash Cash Microchip Design Contest 2002. In addition, to give you a leg up in the contest, I’ll cover the rest of the Microchip product basics in this article.

RISC-based MPUs began like most with ROM parts for production and EPROM-based parts for development. Although EPROM (windowed) parts are erasable via UV, one-time programmable (OTP) parts don’t have a window and cannot be erased even though they’re conveniently user-programmable. For many years, development was accomplished with expensive emulators or blow-and-go, using slow-turnaround (erase time) EPROM-based parts. Some frustrated engineer asked about using the new EEPROM/ flash memory technology in a micro, allowing it to be quickly erased electrically. Although the packaging would not require the expense of a ceramic-windowed part, using the part only for development would keep volumes low and costs high. Because cost was of great concern, it was deemed foolhardy to consider a flash memory part for production use. After all, when the code works, there’s no need for further tweaking and higher costs for erasable parts.

Upgrading a product with external program space consists of replacing the code-bearing ROM/EPROM devices. Thanks to the companies that used socketed parts, this replacement was easy even though it was expensive. When a product used a microprocessor with internal code, the costs were higher, even if it was socketed. On the production line, it took months to use up the old revision stock. This, of course, would lead to unhappy customers further down the road. In terms of inventory, there were advantages to using a reprogrammable part in production, such as the lack of waste and immediate revision updates without the loss of stock.

Early programmable parts required special voltages for the programming process and high programming currents prevailed. Although using internal voltage-boost converters hid some of this, flash memory technology has now advanced to the point where devices are fully reprogrammable in-circuit, even using normal operating voltages. Today, a product can be updated without having to remove the processor from the circuit. In fact, many products that are currently on the market can be updated through a phone/cable/network connection.

DRIVING DOWN COST

OK, so it doesn’t start out that way. Investing in technology costs money; however, when the long-term benefits outweigh the costs, the end product’s cost can go down. For example, take the PIC16C6x/7x, which is one of Microchip’s most useful parts. It was redesigned using flash memory technology, and now it sells for about half of the original OTP part and one-third the cost of the EEPROM version. Essentially, you’re getting a better part for a cheaper price.

From day one, Microchip’s top priority was to keep future products code-compatible. Their "seamless migration path" objective is not an easy one to obtain. Even though the word "seamless" probably originated in Microchip’s marketing department, the engineering department has obviously attempted to hold onto this mantra with both hands.

LINE ’EM UP

The product lineup began with 12-bit instruction products using only 33 instructions (i.e., the 16C5x-series parts). The industry’s first 8-pin micro also used this 12-bit RISC instruction set, which is referred to as the 12-series. (Remember asking yourself, "What good is a micro with only six I/Os?")

New devices bragged an increase in instructions to 35 with an increase in instruction width to 14 bits and the addition of interrupts. Reprogrammable parts began to hit production (the PIC16C84) and simplify development. Success triggered a natural progression toward a 16-bit instruction set that used 58 instructions. As a result, the 17-series was born.

C compiler-optimized architectural enhancements gave rise to the 18-series micros and an increase of the instruction set to 77. Meanwhile, flash memory technology has been applied to many devices in the 12-, 16-, and now 18-series of micros.