PIC microcontroller clock settings

What clock?

  First, let's talk about what a clock is. The clock is the heart of the microcontroller. Every time it beats, all the circuits of the microcontroller move synchronously. It's like when we do radio gymnastics, the beats on the radio are 1234 2234 3234... Then all of us students follow this rhythm to perform one action after another. The faster the beat, the faster we move. The slower the beat, the slower we move.

Internal clock or external clock?

  There are many models of PIC microcontrollers that can be set to use an external clock (such as an external 4MHZ quartz crystal), or an internal RC clock. In addition, there are many models that can select clocks of multiple frequencies.

 For example, the internal clock of PICLF1823 can reach up to 32MHZ and down to 31kHz. This actually provides a new way to reduce power consumption. In general, microcontrollers often use sleep mode to reduce power consumption, and the CPU does nothing during sleep. If you want to reduce power consumption and keep the CPU working, lowering the clock frequency is undoubtedly a very good choice.

 How to set the clock?

The clock setting is generally related to only two places: one is the configuration word, and the other is the oscillator control register OSCCON. Below we take PICLF1823 as an example to explain.

   The clock-related bits in the PICLF1823 configuration word settings are FOSC<2:0>: oscillator selection bits, and PLLEN: PLL enable bits.

   OSCCON is related to the internal clock frequency selection. If only an external clock is used, it is generally ignored. The following is a detailed introduction to the OSCCON register in the data sheet.

Example explanation:

Use the internal clock to set the clock frequency to 8MHZ

Development environment: MPLAB X IDE 

Chip model:PICLF1823

#include
__CONFIG(FOSC_INTOSC&WDTE_OFF&PWRTE_ON&MCLRE_OFF&CP_ON&CPD_OFF&BOREN_ON&CLKOUTEN_OFF&IESO_ON&FCMEN_ON);
__CONFIG(PLLEN_OFF&LVP_OFF) ;//Set FOSC<2:0>: oscillator selection bits to INTOSC,
void init_fosc(void)
{
 OSCCON = 0x70;//8MHZ IRCF<3:0> clock frequency selection bits set to 8MHZ 
}
int main(int argc, char** argv) {

    init_fosc();
    while(1);
}

What is PLL used for? Its function is to convert the 8MHZ clock frequency into 32MHZ by X4. Remember, only 8MHZ can be used, other frequencies cannot be used.

Use the internal clock to set the clock frequency to 32MHZ

Development environment: MPLAB X IDE 

Chip model:PICLF1823

#include

__CONFIG(FOSC_INTOSC&WDTE_OFF&PWRTE_ON&MCLRE_OFF&CP_ON&CPD_OFF&BOREN_ON

                   &CLKOUTEN_OFF&IESO_ON&FCMEN_ON);//This should be put on the previous line

__CONFIG(PLLEN_ON&LVP_OFF); //Set FOSC<2:0>: oscillator selection bits to INTOSC, enable PLLEN
void init_fosc(void)
{
 OSCCON = 0xF0; //32MHZ IRCF<3:0> clock frequency selection bits set to 8MHZ, enable 4XPLL
}
int main(int argc, char** argv) {

    init_fosc();
    while(1);
}