MSP430
Ultra-Low Power | High Integration | Easy-to-Use
“How To” Series: Clock System
2011
Agenda
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Purpose
– Provide a brief but informative technical overview in “HOW-TO” use
the MSP430 clock system
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Objectives
– Clock system basics & ULP
– Clocking & MSP430 generations
– Design considerations
Why is the clock system
important?
MSP430 Ultra-Low Power
• Ultra-Low Power Active Mode
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• Up to 7 Low Power Modes (LPMx)
• Instant Wakeup
• Autonomous peripheral operation in
low power modes
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The clock system is
fundamental to enabling
the ULP feature set of the
MSP430
Careabouts:
CPU Clock
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Peripheral Clock
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High frequency
Low power
Stable & Flexible
Fast wake-up
High frequency
Low power
Stable & Flexible
Fast wake-up
Independent from CPU
Standby Clock
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–
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Ultra-low power
Low frequency
Accurate (RTC)
Reliable (WDT)
MSP430 Clock System Basics
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All ULP MSP430 devices
have a fundamental
clocking architecture
32768
low-power peripherals
On demand
DCO

~1uA
Always-on
ACLK
– On-chip, high frequency,
instant-on oscillator (DCO)
– ULP, low frequency, 32.768kHz
crystal oscillator (LFXT1)
~250uA
MSP430
MCLK
CPU and peripherals
Enables ULP embedded
operation
DCO for fast, low power active
CPU processing
LFXT1 Ultra-low power standby
clock for timer wakeup
DCO Instant-on ablity to service
interrupts
Breaking down the terminology
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LFXT1
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Extremely ULP on-chip oscillator, fixed to
~10kHz
Useful for standby clocking where power
is more critical than accuracy
REFO
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On-chip ULP 32.768kHz oscillator for
crystal-less operation at more accurate
frequencies that via the VLO
DCO
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Feature in some devices (4xx, 5xx, 6xx) that
will automatically adjust the DCO frequency
based on a provided reference clock
MODOSC
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Digitally Controlled Oscillator built into every
MSP430 device
Capable of 100’s kHz to 10’s MHz & is user
programmable
FLL
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VLO
–
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This is the ULP low frequency crystal
oscillator in every MSP430 device
Typical usage is with a 32.768kHz watch
crystal for RTC
In some devices (2xx, 5xx, 6xx) it is also
possible to source a digital clock directly
to the LFXT1 input
Most devices also support a high
frequency mode (see XT1) that can
interface with MHz-range crystals &
resonator
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5MHz on-chip oscillator for automatic use with
select peripherals
XT1 & XT2
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Oscillator circuit for crystals & resonators
(HF)XT1 on almost all MSP430s (except the
F20xx and some G2xx devices
XT2 present only on larger pin count
MSP430s (64+)
High frequency support only
Clocking & MSP430 generations
Clock/
Feature
Frequency
Range
Relative
Precision
Application/Devices Supported
High-Frequency
DCO
+ROSC
100kHz to
25MHz+
+FLL
Low
(2xx:
Medium)
ALL MSP430s: High frequency source for CPU & peripherals
2xx only: Integrates a high stability DCO over temp & Vcc w/
calibrated constants stored in flash for common freqs
Better
1xx, some 2xx: Can help stabilize DCO drift over temperature
using external precision resistor
Best
4xx,5xx,6xx: Auto-tunes DCO based on reference clock such
as 32kHz xtal
HFXT1/
XT2
100’s kHz to
10’s MHz
High
Most MSP430s: use when high accuracy/frequency is
required. Higher current than DCO.
Typically, smaller devices do not support XT2
’20xx & some G2xx devices do not support HF mode at all
MODOSC
5MHz
Medium
5xx, 6xx: Integrate osc for auto-on use with FLASH, ADC &
other modules
Low-Frequency
LFXT1
32kHz
High
ALL MSP430s: standard use in LPM3/3.5 & RTC applications
VLO
~10kHz
Low
2xx, 5xx, 6xx: Integrated low freq osc consuming lowest
current of any clock option
REFO
32kHz
Medium/
High
5xx, 6xx: Integrated 32kHz osc for crystal-less applications
providing reasonable accuracy
Design considerations
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MSP430 has 3 clock trees
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Design for robust 32kHz crystal operation if needed
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As with any ULP oscillator, proper crystal selection, loading & PCB layout are key factors in
achieving a reliable 32KHz XTAL design
Check oscillation allowance BEFORE production to address any weakness
Know the differences in device family features
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ACLK: typical use as low frequency, low power LPMx peripheral clock
MCLK: active CPU clock
SMCLK: high or low frequency peripheral clock
Basics are the same, however a lot of variation in feature set exists
Understanding what is possible is key to realizing ULP operation
Understand device/family-specific behavior:
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Active WDT: sourced from the DCO at power up/reset: handle the WDT first in your software!
Clock failsafe: crystal fault detection means safe & reliable operation but may result in incorrect
frequencies. Take care to configure clocks and properly handle osc fault conditions.
Clock requests: some clocks may or may not automatically remain active when entering LPMx
modes, resulting in unexpected halting of peripherals or higher than expected LPMx current
consumption
References
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MSP430F5xx and MSP430F6xx Core Libraries
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MSP430 32-kHz Crystal Oscillators
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Provides functions that implement the most common operations using the PMM, UCS, PMAP and Flash modules,
such as changing the core voltage to operate at higher frequencies, crystal/clock initialization, mapping port I/O,
and write/erase flash operations.
Describes selection of the right crystal, correct load circuit, and proper board layout key to stable crystal oscillator
performance. Information regarding recommended PCB desing and possible oscillator tests to ensure stable oscillator
operation in mass production are included.
MSP430 LFXT1 Oscillator Accuracy
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Details the factors that influence achievable accuracy of the low frequency oscillator, specifically for real-time clock
(RTC) applications in 1xx & 4xx devices.
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Microcrystal Recommendations for 32.768kHz crystals
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Epson Toyocom Recommendations for 32.768kHz crystals
Your specific MSP430 device’s datasheet & Family User’s
Guide
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Thank you!