clock switching and delays

ktallevi · Joined: 17 Dec 2005 Posts: 58

Using the code below, I am operating a PIC18LF8722 at 8Mhz using the internal osc. I want to switch to an external crystal on timer1 to conserve power without going completely to sleep (so I can still execute certain instructions). I compiled a little test program, but it appears that the delay loops are not compiling correctly for the lower crystal frequency. Relevant code snippets posted below....

*config file*
#FUSES INTRC
#use delay(clock=8000000)

*main program file*
void main() {

/*configure timer and osc. */
setup_timer_1(T1_EXTERNAL | T1_DIV_BY_1 | T1_CLK_OUT);
enable_interrupts(INT_TIMER1);
setup_oscillator(OSC_8MHZ | OSC_INTRC);

/*non relevant code here...*/

/* testing low frequency crystal on timer1 */
setup_oscillator(OSC_TIMER1);
#use delay(clock=38400)
do {
delay_ms(1000); <-- problem appears to be here
output_toggle(BLUE_LED);
} while(1);

}

It is my understanding that the delay_ms should be using the last #use delay(clock=38400) statement during compilation, but its not, the delay is MUCH longer than 1 second in that example. any ideas?

thanks

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Post your compiler version.

ktallevi · Joined: 17 Dec 2005 Posts: 58

sorry, 3.249

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Is that you real #fuses statement ? Because it enables things that
shouldn't be enabled in this test program, such as the Watchdog Timer
and Low Voltage Programming mode.

ktallevi · Joined: 17 Dec 2005 Posts: 58

PCM Programmer,

I got it working thanks for your help, but im having an issue related to this dual osc. configuration. I found a sample piece of code you posted on another thread. It works great, but I added a 2nd serial port (same pins, hardware uart, different stream name), but it doesnt work. It only works if I define "force_sw" for both rs232 statements. Is it not possible to use the same Hardware UART, at the same baud rate, but at different crystal osc speeds??

code pasted below:

Ttelmah · Guest

I'd cheat.....
Setup the 'use rs232', for the default fast clock. Then define yourself two statements:
#define FAST_CLOCK_BAUD 300
#define SLOW_CLOCK_BAUD (FAST_CLOCK_BAUD*256)
#define UART_FOR_SLOW_CLOCK() set_uart_speed(SLOW_CLOCK_BAUD)
#define UART_FOR_FAST_CLOCK() set_uart_speed(FAST_CLOCK_BAUD)

In your code, once you change to the slow speed, call 'UART_FOR_SLOW_CLOCK()' and when you return to fast speed, call 'UART_FOR_FAST_CLOCK()'. This way the code to reprogram the BRG should be generated where it is needed. The problem otherwise, is that the speed change will only occur 'retrospectively', requiring you to send a character, before the switch occurs.
Efectively the system will generate the correct settings, for a baud rate that is the ratio of the two clock rates (hence 256*) higher than is used at the faster clock.

Best Wishes

Ttelmah · Guest

Remember also, that the speed change then needs to be done _before_ the new 'use delay' statement.

Best Wishes