Clock System and Low Power Modes of MSP430

This is a brief introduction into the clock system and low power modes  supported by MSP430x2xxx series microcontrollers (like MSP430G2553).MSP430 microcontroller is designed for energy constrained applications which should operate reliably over long period of time with as little power as possible like Wireless sensor networks,body worn medical devices,measuring instruments etc.In order to address the conflicting requirements of fast processing and low power consumption,Designers of MSP430 created multiple clock sources and signals which can be configured to suit conflicting requirements of various applications.

A simplified block diagram of the MSP430x2xxx series is given below.

Simpified block digram of msp430x2xxx basic clock system


Clock Sources (oscillators) of MSP430

Basic Clock System provides the required clocks to the MSP430x2xxx devices.Most MSP430 microcontrollers have 4 clock sources  to produce fast and slow clocks.The main clock sources (oscillators) of MSP430x2x family are 

LFXT1CLK - It is a crystal powered low/high frequency oscillator that can work with either a 32KHz standard watch crystal,standard crystals that operate in the MHz range or external clock sources (400KHz -16MHz range).Please note that,not all clock features are supported on all msp430x2xxx devices.For eg: LFXT1CLK of MSP430G2553 supports only the low frequency mode.You can only connect a 32KHz watch crystal to the oscillator inputs,higher frequency crystals are not supported.

XT2CLK -This is an optional high frequency oscillator that can be used with standard crystals,resonator or external clock sources in the 400KHz to 16MHz range.Please note that XT2 is absent in MSP430Gxxx series like MSP430G2553.

DCOCLK - DCO is an integrated Digitally Controlled Oscillator which do not use any external crystals.The DCO can generate a wide variety of frequencies ranging from  few 100 kilo hertz to 10's of MHz.The oscillator has a first startup time and is the default clock of the MSP430 after reset.One problem with DCO is that clock frequency is temperature dependent.

VLOCCLK - this is  a very low power/low frequency internal oscillator than can produce a frequency of 12 KHz.

Clock Signals of MSP430

MSP430 produces 3 different clock signals using a combination of the above mentioned oscillators.These clock signals can be routed to various peripherals like CPU,Timers,ADC etc depending upon the requirements.The 3 clock signals are

MCLK - Known as Main Clock.It is a fast,high frequency clock usually used by the CPU or other fast components like ADC10,ADC12,Watch Dog Timer (WDT).MCLK can be sourced either from LFT1CLK,DCO,VLO or XT2 oscillator (if it is present).You can divide the clock signal by 1/2/4/8 using the internal divider.

SMCLK - Known as Sub Main clock.It can be sourced either from LFT1CLK,DCO,VLO or XT2 oscillator (if it is present).You can divide the clock signal by 1/2/4/8 using the internal divider.

ACLK - Known as Auxiliary Clock is a slow, low frequency clock(12 KHz or 32KHz).It can only be  sourced from LFT1CLK or VLO.You can divide the clock signal by 1/2/4/8 using the internal divider. ACLK is active in all Low power modes except LPM4.


Low Power Modes of MSP430

To conserve power, MSP430 spends most of its time in sleep and periodically wakes up to perform some operation and again goes back to sleep.MSP430 has 5 low power modes named as LPM0 to LPM4 .The table below shows which clocks are active and which are disabled in various low power modes.

Low power modes of MSP430
LPM0 - Both CPU and MCLK are disabled.SMCLK and ACLK remain active.Peripherals which use SMCLK and ACLK also remain active.

LPM1 - Both CPU and MCLK are disabled.DCO(digitally controlled oscillator) is disabled.SMCLK is not available if it is sourced from DCO.ACLK remains active.

LPM2 - CPU,MCLK,SMCLK and DCO are disabled,ACLK remains active

LPM2 - CPU,MCLK,SMCLK,DCO and DC generator are disabled,ACLK remain active.

LPM4 - All the clocks including ACLK are disabled in this mode.The controller can only be waken from this mode by an external interrupt.This mode consumes the least power compared to all other modes.


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