PIC16-bit MCU CAN (2) Thoroughly understand the clock-EEWORLD

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I have always been reading reference materials and books when writing programs. This time, I don't have any information about the 16-bit single-chip dsPIC33EP32GP502. So I took the opportunity to study it in depth! First, let's look at the choice of oscillator.

First look at the clock block diagram: PIC16-bit MCU CAN (2) Thoroughly understand the clock - zmurder - Qidian's blog

We are using an external crystal oscillator, so the configuration is HS, FCY is the CPU clock, FP is the peripheral clock, FCY=FP (except for doze mode). FCY=Fosc/2. That is, the CPU clock is 1/2 of the crystal frequency. There is also frequency division below. DOZE<2:0>: 011 = FCY is divided by 8 (default) Processor clock division ratio selection bit.

Therefore, the actual FCY = 0.5MHz (using an 8M external crystal oscillator and dividing it by eight 8/2/8 = 0.5MHz).

Take a look at the description of the oscillator selection configuration bits:

What we need to see here is the second column of the FOSCSEL configuration bit, which is the register address. What needs to be set is IESO<7> and FNOSC<2:0>

Take a look at the description in the header file p33EP32GP502.h:

/* Register FOSCSEL (0x57f8) */

extern __attribute__((space(prog))) int _FOSCSEL;

#define _FOSCSEL(x) __attribute__((section("__FOSCSEL.sec"),space(prog))) int _FOSCSEL = (x);

** Only one invocation of FOSCSEL should appear in a project,

** at the top of a C source file (outside of any function).

** The following constants can be used to set FOSCSEL.

** Multiple options may be combined, as shown:

** _FOSCSEL( OPT1_ON & OPT2_OFF & OPT3_PLL )

** Oscillator Source Selection:

** FNOSC_FRC Internal Fast RC (FRC)

** FNOSC_FRCPLL Fast RC Oscillator with divide-by-N with PLL module (FRCPLL)

** FNOSC_PRI Primary Oscillator (XT, HS, EC)

** FNOSC_PRIPLL Primary Oscillator with PLL module (XT + PLL, HS + PLL, EC + PLL)

** FNOSC_LPRC Low-Power RC Oscillator (LPRC)

** FNOSC_FRCDIVN Internal Fast RC (FRC) Oscillator with postscaler

** Two-speed Oscillator Start-up Enable bit:

** IESO_OFF Start up with user-selected oscillator source

** IESO_ON Start up device with FRC, then switch to user-selected oscillator source

#define FNOSC_FRC 0xFFF8

#define FNOSC_FRCPLL 0xFFF9

#define FNOSC_PRI 0xFFFA

#define FNOSC_PRIPLL 0xFFFB

#define FNOSC_LPRC 0xFFFD

#define FNOSC_FRCDIVN 0xFFFF

#define IESO_OFF 0xFF7F

#define IESO_ON 0xFFFF

From the header file

1: Register FOSCSEL (0x57f8), that is, the register address is 0x57f8, which is consistent with the table.

2: _FOSCSEL(FNOSC_PRI&IESO_OFF); We declare our configuration bits as 0XFFFA&0XFF7F=0XFF7A at the beginning of the program.

That is, FOSCSEL = 0XFF7A. According to the register description, our configuration is an external HS oscillator and uses the user-selected oscillator source to start the device.

3: The configuration bit only indicates the oscillator to be used, and does not set the division ratio DOZE. Therefore, DOZE is still the default division ratio of 8.

Summary: There are several aspects to configuring the clock:

1: Configure the corresponding configuration bits to select the clock source as internal clock or external crystal oscillator, etc.

2: Configure the corresponding register to select the division ratio.

Keywords：PIC Reference address：PIC16-bit MCU CAN (2) Thoroughly understand the clock

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