Servo - December 2010

Rate the eM8 Part 2

By Fred Eady

You can’t build robot smarts if you can’t speak the language. Or, in this case, speak the languages. We’re about to continue our port of the eM8 assembler application to CCS C. Although this is a tutorial of sorts, we’re also forming a picture of the eM8 hardware as we translate the assembler source code. When we ran out of paper last time, we had just finished porting the PIC16F882 definitions and SRAM allocations. At this point, we’re ready to port some eM8 executable code.

Start

Assembler-based embedded programs don’t have to house their code inside the braces of the C main function. However, from a porting standpoint, the assembler code behind the START label will be rounded up and placed within a pair of CCS C braces. We don’t have to port the RESET code, but I thought it might be a good idea to show you how the assembler application kicks off following a microcontroller reset:

;——————————————————————————————————————
; Start Of Program after Reset
;——————————————————————————————————————
RESET ORG 0x000

GOTO START
; processor reset
; vector
; go to beginning of
; program

The interrupt handler code lies between the RESET label and the START label in the original eM8 source. We’ll deal with the interrupt handler port in due time. For now, let’s dig into the oscillator setup:

START
;*******  Set Oscillator Factoty Correction
 BANKSEL OSCTUNE ; select bank 1
 ; using mpasm macro
 MOVLW 0x00 ; set oscillator to
 ; factory calibrated freq
 OSCTUNE
 OSCCON
 OSCCON,IRCF0
 ; set osc to 8M operation
 BANKSEL STATUS ; select bank 0 using mpasm
 ; macro
MOVWF
BANKSEL
BSF
clrf
clrf
UF
CF

We already know that the PIC16F882 is clocked from its internal oscillator. So, according to the PIC16F882 datasheet, the OSCTUNE register will reset with a value of

38 SERVO 12.2010

0x00 which does not apply any frequency correction to the default clock frequency of 4 MHz. At this point, there is no reason to adjust the instruction clock frequency. However, we do need to set up the internal oscillator to run at 8 MHz. As for setting the internal oscillator base frequency, CCS C provides a couple of ways for us to specify the instruction execution rate. The first method is found in the em8.h include file that is created by the CCS C PIC Wizard:

#use delay(clock=8000000)

The CCS C #use delay directive sets the necessary bits in the associated SFRs (Special Function Registers) to select an internal oscillator speed of 8 MHz. As you can see in the eM8 assembler source, bit IRCF0 in SFR OSCCON is set to complete the bit pattern necessary to initiate 8 MHz internal oscillator operation. Here’s method number two which happens to be a CCS C built-in function that resides inside of the C main function:

setup_oscillator(OSC_8MHZ);

It doesn’t hurt to specify the internal oscillator frequency more than once. However, the most recently encountered frequency statement sets the operating clock speed.

The eM8 application also clears the flags in this assembler code snippet and so will we. The original eM8 assembler source allocated an eight-bit SRAM location for each set of flags. We did the same but in a CCS C kind of way. The results of clearing the UF and CF flag structures are shown in Screenshot 1. The CCS C code that cleared the bits in Screenshot 1 looks like this: