By default, the PICBASIC PRO™ Compiler generates programs intended to be run on a PICmicro with a 4MHz crystal or ceramic resonator. All of the time-sensitive instructions assume a 1 microsecond instruction time for their delays. This allows a PAUSE 1000 1000, for example, to wait 1 second and the SERIN and SEROUT command=s baud rates to be accurate.

PICBASIC PRO™ programs may be run at clock frequencies other than 4MHz in a couple of different ways. The first is to simply use an oscillator other than 4MHz and don=t tell PBP. This can be a useful technique if you pay attention to what happens to the time dependent instructions.

If you wish to run the serial bus at 19,200 as described above, you would simply clock the microcontroller with an 8MHz crystal rather than a 4MHz crystal. This, in effect, makes everything run twice as fast, including the SERIN and SEROUT commands. If you tell SERIN or SEROUT to run at 9600 baud, the doubling of the crystal speed will double the actual baud rate to 19,200 baud.

However, keep in mind commands such as PAUSE and SOUND will also run twice as fast. The PAUSE 1000 1000 mentioned above would only wait .5 seconds with an 8MHz crystal before allowing program execution to continue.

The other technique is to use a different oscillator frequency and tell PBP of your intentions. This is done through the use of a DEFINE. DEFINE, as demonstrated in the LCDOUT command in a previous section, is used to tell PBP to use something other than its defaults.

DEFINE OSC 8

Telling PBP the oscillator frequency allows it to compensate and produce the correct timing for COUNT, DEBUG, DTMFOUT, FREQOUT, HPWM, HSERIN, HSEROUT, I2CREAD, I2CWRITE, LCDOUT, OWIN, OWOUT, PAUSE, PAUSEUS, SERIN, SERIN2, SEROUT, SEROUT2, SHIFTIN, SHIFTOUT, SOUND, XIN and XOUT.

Changing the oscillator frequency may also be used to enhance the resolution of the PULSIN, PULSOUT and RCTIME instructions. At 4MHz these instructions operate with a 10 microsecond resolution. If a 20MHz crystal is used, the resolution is increased 5 times to 2 microseconds. There is a tradeoff however. The pulse width is still measured to a 16-bit word variable. With a 2 microsecond resolution, the maximum measurable pulse width would be 131,070 microseconds.

Going the other direction and running with a 32.768KHz oscillator is problematic. It may be desirable to attempt this for reduced power consumption reasons and it will work to some extent. The SERIN and SEROUT commands will be unusable and the Watchdog Timer may cause the program to restart periodically. Experiment to find out if your particular application is possible at this clock speed. It doesn=t hurt to try.

As mentioned earlier, the default configuration settings for a particular device is set in the .INC file with the same name as the device, e.g. 16F84.INC. These settings can be changed at the time the device is physically programmed.

The Watchdog Timer is enabled by PBP. It is used, along with the TMR0 prescaler, to support the NAP and SLEEP instructions. If neither of the instructions are used in a program, the Watchdog Timer may be disabled and the prescaler used for something else.

In general it is not necessary to know how RAM is allocated by PBP in the microcontroller. PBP takes care of all the details so the programmer doesn=t have to. However there are times when this knowledge could be useful.

Variables are stored in the PICmicro=s RAM registers. The first available RAM location is $0C for the PIC16F84 and some of the smaller PICmicros, and $20 for the PIC16C74 and other larger PICmicros. Refer to the Microchip PICmicro data books for the actual location of the start of the RAM registers for a given microcontroller.

Several system variables, using about 24 bytes of RAM, are automatically allocated by the compiler for use by library subroutines. These variables are allocated in the file PBPPIC14.RAM and must be in bank 0 (or bank A on 18Cxxx devices).

User variables are prepended with an underscore (_) while system variables have no underscore so that they do not interfere with each other.

BASIC Stamp variables B0 - B25 and W0 - W12 are not automatically allocated. It is best to create your own variables using the VAR instruction. However if you want these variables to be created for you, simply include the appropriate file, BS1DEFS.BAS or BS2DEFS.BAS, at the beginning of the PICBASIC PRO™ program. These variables allocate space separate and apart from any other variables you may later create. This is different than the BS2 where using the canned variables and user created variables can get you into hot water.

Additional temporary variables may be generated automatically by the compiler to help it sort out equations. A listing of these variables, as well as the entire memory map, may be seen in the generated .ASM or .LST file.

The pseudo-ops, variable types and commands keywords are listed in their appropriate sections. The names of the PICmicro registers are defined in the file PIC14EXT.BAS. If the files BS1DEFS.BAS, BS2DEFS.BAS or MODEDEFS.BAS are included, the definitions inside essentially become reserved words and may not be redefined.

BRANCHL was created to allow branching to labels that may be on the other side of a 2K boundary. If the PICmicro has 2K or less of program space, BRANCH should be used as it takes up less space than BRANCHL. If the microcontroller has more than 2K of code space, and you cannot be certain that BRANCH will always act within the same page, use BRANCHL.

The assembler may issue a warning that a page boundary has been crossed. This is normal and is there to suggest that you check for any BRANCHes that may cross a page boundary.

7.6.  12-Bit Core Considerations 

Because of the architecture of the 12-bit core PICmicro MCUs, programs compiled for them by PBP will, in general, be larger and slower that programs compiled for the other PICmicro MCU families. In many cases, choosing a device from one of these other families will be more appropriate. However, many useful programs can be written and compiled for the 12-bit core devices.

The two main programming limitations that will most likely occur are running out of RAM memory for variables and running past the first 256 word limit for the library routines. These limitations have made it necessary to eliminate some compiler commands and modify the operation of some others.

The compiler for 12-bit core PICmicro MCUs uses between 20 and 22 bytes of RAM for its internal variables, with additional RAM used for any necessary temporary variables. This RAM allocation includes a 4 level software stack so that the BASIC program can still nest GOSUBs up to 4 levels deep. Some PICmicro MCU devices only have 24 or 25 bytes of RAM so there is very little space for user variables on those devices. If the Unable to Fit Variable error message occurs during compilation, choose another PICmicro MCU with more general purpose RAM.

12-bit core PICmicro MCUs can call only into the first half (256 words) of a code page. Since the compiler's library routines are all accessed by calls, they must reside entirely in the first 256 words of the PICmicro MCU code space. Many library routines, such as I2CREAD, are fairly large. It may only take a few routines to overrun the first 256 words of code space. If it is necessary to use more library routines that will fit into the first half of the first code page, it will be necessary to move to a 14- or 16-bit core PICmicro MCU instead of the 12-bit core device.