THE MAGAZINE FOR COMPUTER APPLICATIONS
Circuit Cellar Online offers articles illustrating creative solutions
and unique applications through complete projects, practical
tutorials, and useful design techniques.

THROUGH THE LOOKING GLASS

Taking a Look at the PIC18Cxxx Series
by David Brobst

Start ý PIC18Cxxx Chips ý Memory ý Data Memory ý Advanced Indirect Addressing ý Deep and Accessible Stack ý Interrupts ý Power-On Features ý Clock Speed ý 10-Bit A/D ý Hardware Multiplier ý Timers ý CCP/PWM ý USART ý I²C Master ý Table Read/Write ý Current Status ý Sources and PDF

TABLE READ/WRITE

For those of you who are not familiar with the PIC17Cxxx series of controllers, perhaps the most intriguing new feature on the PIC18Cxxx family is the table read/write capability. This allows the PIC18Cxxx to directly read from and write to its own program memory. Almost surely, the most important aspect of the read capability is that it allows the PIC18Cxxx controllers to perform program memory integrity checks versus a checksum. This allows the PIC18Cxxx controllers to periodically monitor their program memory and ensure that the code is working properly and has not been changed.

For mission-critical systems, this is an absolute must. Additionally, the read capability can be used to read the configuration fuses and ID locations directly. These could then be reported directly to a querying unit for inventory purposes and system verification. Listing 3 shows the table read command being used to read some of the configuration bytes.

More intriguing is the write capability. Basically, the PIC18Cxxx chip can modify its program space with executable code. Note that with the initial PIC18Cxxx offerings, the program space is still EPROM-based and inherently one-time-programmable. In the PIC18Cxxx, any non-programmed byte is all ones (FF hex). A one can always be changed to a zero, but a zero can revert to a one only through UV exposure. So if program space already has executable code, writing over the program space with the table-write command would simply destroy the code at best and make some new unintended instruction at worst.

With this limitation in mind, there is still a myriad of uses for the table write command that spring to mind. Because the EPROM memory is OTP, any table write is going to store information in a nonvolatile manner. Thus, the table write command could be used for maintenance logs, serial numbers, time stamps, and so on. In addition, if the reset code was appropriately designed, the table write command could be used to patch existing code or provide field upgrades.

This all sounds great in theory, however, implementation is a bit more complicated. The first major problem with the table write command is that it requires 12 V on the /MCLR line. This is referred to as the programming voltage (V_PP). The tricky part for the table write command is providing the 12 V_PP on *MCLR when needed and then the typical reset voltage during other times. This is a minor problem and easily overcome with a small amount of inexpensive circuitry. Figure 4 shows a circuit that can be used to accomplish this.

Although these initial voltage and one-time writes are limiting at the moment, it is easy to see where this functionality can lead. On the drawing board for the PIC18Cxxx family are flash memory versions of the part. When these become available, the voltage and memory reuse problems should disappear. Listing 4 shows some code that implements a table write to program memory. A pushbutton ends the programming cycle and wakes up the controller.

PREVIOUS • NEXT