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Explain clock circuit and registers used to control it for MSP430.
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The various sources of clock for the MSP 430 processor are:

  1. LFXT1CLK: It is a low / high frequency oscillator which can be used either with low frequencies like 32768Hz to high frequencies upto 8MHz.
  2. XT2CLK: It is a high frequency oscillator and can be used with high frequency crystals of upto 8MHz.
  3. DCOCLK: It is an internal oscillator controlled digitally with the characteristics as an RC oscillator.

The various clocks available are:

  1. ACLK (Auxiliary Clock): It is the LFXT1CLK source clock with a prescaler of 1,2,4 or 8. It is used by peripherals.
  2. MCLK (Master Clock): It has a selectable clock source from either LFXT1CLK, XT2CLK or DCOCLK. It also has a prescaler of 1,2,4 or 8. It is used by the main processin unit.
  3. SMCLK (Sub-main clock): It also has a selectable clock source from either LFXT1CLK, XT2CLK or DCOCLK. It also has a prescaler of 1,2,4 or 8. It is used by the peripheral modules.

The various registers required to control the function of the clock module are as listed below:

1.BCSCTL1

| XT2Off | XTS | DIVA.1 | DIVA.0 | XT5V | RSEL.2 | RSEL.1 | RSEL.0 | | --- | --- | --- | --- | --- | --- | --- | --- | This is the basic clock system control 1 register. The various bits have following significance:

RESL.2, RSEL.1, RSEL.0: These bits are used to select the DCO nominal frequency.

XT5V should always be reset

DIVA.1 and DIVA.0: These bits are used to decide the prescaler value or the dividing factor for the frequency for ACLK. The factors are 1, 2, 4 and 8 for the values of DIVA.1 and DIVA.0 as 00, 01, 10 and 11.

XTS: This bit is used to indicate whether the LFXT1 is supposed to work on low frequency or high frequency.

XT2Off: This bit is used to switch 'ON' or 'OFF' the XT2CLK clock source.

2.BCSCTL2

| SELM.1 | SELM.0 | DIVM.1 | DIVM.0 | SELS | DIVS.1 | DIVS.0 | DCOR | | --- | --- | --- | --- | --- | --- | --- | --- | This is the basic clock system control 2 register. The various bits have following significance:

DCOR: It is used to decide the internal pull up resistor to be activated or deactivated for the clock source.

DIVS.1 and DIVS.0: These bits are used to decide the prescaler value or the dividing factor for the frequency for SMCLK. The factors are 1, 2, 4 and 8 for the values of DIVS.1 and DIVS.0 as 00, 01, 10 and 11.

SELS (Select source for SMCLK): It is used to decide the source for the SMCLK to be DCOCLK or XT2CLK.

DIVM.1 and DIVM.0: These bits are used to decide the prescaler value or the dividing factor for the frequency for MCLK. The factors are 1, 2, 4 and 8 for the values of DIVM.1 and DIVM.0 as 00, 01, 10 and 11.

SELM.1 and SELM.0: These bits are used to decide the source for MCLK to be DCOCLK, XT2CLK or LFXT1CLK.

3.DCOCTL

| DCO.2 | DCO.1 | DCO.0 | MOD.4 | MOD.3 | MOD.2 | MOD.1 | MOD.0 | | --- | --- | --- | --- | --- | --- | --- | --- | This is the DCO control register. The various bits have following significance:

DCO.2, DCO.1, DCO.0: These bits are used to select one of the eight discrete DCO frequencies is selected.

MOD.4, MOD.3, MOD.2, MOD.1, MOD.0: These bits are used to define how often frequency within the period of 32 DCOCLK cycles are used. The DCO frequency will be used for 32 - MOD frequency, where MOD is the value given by the 5 bits MOD.4 to MOD.0

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