PIC18LF46K22 clk problem !

jsafaie · Joined: 21 Dec 2012 Posts: 5

Hi,
I designed a low voltage system based on 18LF46K22 micro which is working with 2v.
I am using 14.7456 MHz crystal to drive it, and run a very simple program just to test it as below:

#INCLUDE <18LF46K22.H>
#FUSES HSH , NOPROTECT , NOWDT
#USE delay ( clock=14745600 )
void main( void ) {
while ( true ) {
output_bit(PIN_B2,0);
output_bit(PIN_B2,1);
}
}

The result is a square wave with nearly 8 micro sec. "0" and 16 micro sec. "1".
But normally it should work much faster than this, isn't it ? Question

asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

Ttelmah · Joined: 11 Mar 2010 Posts: 19504

The two output instructions, using fixed bit values, and levels, each take two machine cycles. The loop also takes two. So to get 8uSec, and 16uSec, for a total of six machine cycles, implies you are clocking at 1MHz (250K instructions per second, 6 instructions).
By default, the chip will wake up with FSCM enabled, so will 'fall back' to the internal oscillator if it can't start the main oscillator, or something is wrong.
The default speed of this is 1MHz.
First thing I note, again by default the PLL will be enabled. You don't want this, so need NOPLLEN. HSH, is designed for crystals over 16MHz, so you really want HSM. You should have PRIMARY_ON selected.

So:
#FUSES HSM , NOPROTECT , NOWDT, NOPLLEN, PRIMARY_ON, PUT, NOIESO, NOFCMEN

Now, the last two fuses, now mean it won't start at all, if the crystal doesn't work, makes testing easier!. If it doesn't start, then there is a problem with the crystal circuit.

Best Wishes

jsafaie · Joined: 21 Dec 2012 Posts: 5

Thanks a lot guys, it is solved by your proposition.
#FUSES HSM , NOPROTECT , NOWDT, NOPLLEN, PRIMARY_ON, PUT, NOIESO, NOFCMEN