Another newbie question

[Charles Linquist]

I have to apologize for my lack of understanding. I think all I need is
a little "shove" to get me started. I'm very familiar with PicBasic Pro, and
I think that my knowledge of that language is almost a hindrance to my 'C' education.

I tried to write a very small program that would toggle a pin
every millisecond in an ISR.

A couple of questions:

Do I have to clear TMR0 (RTCC) before I enable interrupts, or is
this done for me automatically?

Do I have to set the TRIS register to force portb_0 and portb_1 as outputs?

Why won't the following program toggle portsb.0 and b.1?





#include "C:\Program Files\PICC\Charles Code\Test2.h"
#ZERO_RAM
#int_RTCC

int8 a,b,d,e;
char c;



RTCC_isr()
{
output_toggle(PIN_B0);
}



void main()
{

port_b_pullups(TRUE);
setup_adc_ports(NO_ANALOGS);
setup_adc(ADC_OFF);
setup_psp(PSP_DISABLED);
setup_spi(FALSE);
setup_wdt(WDT_OFF);
setup_timer_0(RTCC_INTERNAL|RTCC_DIV_4|RTCC_8_bit);
setup_timer_1(T1_DISABLED);
setup_timer_2(T2_DISABLED,0,1);
setup_timer_3(T3_DISABLED|T3_DIV_BY_1);
enable_interrupts(INT_RTCC);
enable_interrupts(GLOBAL);

a = 0;

while (a = 0);
{ output_toggle(PIN_B1); }


return;

[newguy]

Just a couple things....

Usually where you start the declaration of the interrupt and the interrupt code itself comes right after. You have a bunch of variable declarations in the way.

The critical error is in the while loop.

while (a = 0) {}

will never run. The way any test (a while, if, etc. loop) runs is if the test condition evaluates to true (1), the loop runs. By saying "while (a = 0)", you are telling the compiler to set a equal to 0, then test a to see if it is 1, which it isn't. What you need is "while (a == 0)", which means "see if a is zero" - if it is, the test evaluates to true (1), and the while loop executes.

Long story short, if you're testing something, you need two equals (==). If you're assigning a variable a value, you need one (=). Little mistake, big difference in how it runs.

Hope this helps.

[ckielstra]

Some more things....

When posting code please use the 'code' button, this will ensure the formatting of your code is preserved in this forum.

Please post complete code examples that we can compile. In your code the #fuses line and the include for your processor are missing. And what is test2.h containing???

[Charles Linquist]

Thanks so much for your explanation and help. I'm trying to slowly convert my 4000+ lines of a PicBasic Pro program to C. My 8720 is very nearly full. It will be interesting to see if C is more code efficient in such a large program.

I have my work cut out for me.

Thanks again,
Charles Linquist

[Charles Linquist]

Your code works. But I see two strange things:

1. I "told" the wizard that I wanted to use a "Resistor/Capacitor oscillator"
What I wanted was to use the INTERNAL oscillator of the '452. Apparently,
the chip is set up for an external R/C network, because nothing happened
until I fed an external oscillator into the CLK in pin.

Is there a way to use the Wizard and use the internal oscillator of the newer parts?


2. On RB0, I noticed that the rise time was nice and fast, but the fall time
was extremely long (> 500uSec). It seems like the chip had no drive in the negative direction. I set up RB0 as an output in the Wizard.

Is this typical, or am I doing something wrong?

[Ttelmah]

The answer here is to read the manufacturers data sheet for the chip....
The oscillator modes for the 18F452, are:
LP
HS
HS+PLL
RC
RCIO
EC
ECIO

These correspond to an external low frequency crystal, external high frequency crystal, external crystal with PLL, external resistor/capacitot, external resistor/capacitor with the second oscillator pin as IO, external clock, and external closk with the second pin as IO.
You will see there is no mention of an internal oscillator option.
The 452, does not have an internal oscillator, hence the compiler has no option for this...
Generally, most of the chips supporting the 4*PLL, do not have internal oscillators.

Best Wishes