MSP430F5340: Cystal Oscillator selection

Hi Rineet,

On this family of MSP430 MCUs you can have two different crystals to provide both low frequency (low power) and high frequency.

Both are not required, as you mentioned seeing on the EVM schematic.

If you look at the block diagram of the MCUs clock system, you can see the XIN/XOUT pins which connect to a 32Khz crystal and is used to provide the XT1 clock which can in turn provide the clock source for the system's ACLK, SMCLK and MCLK or as a reference clock for the DCO.  If you don't want to use this XT1 clock source you can use the internal VLO or REFO clock sources instead and save the cost of the external crystal. 

Why would you use an external 32KHz crystal?  The advantage is lower power, compared to the REFO internal clock source, and better accuracy and stability which is important for asynchronous communications, such as when using the UART for example.

The XT2IN/XT2OUT pins is where you can connect a high frequency clock, typically 1 to 4MHz.  This clock source can drive the ACLK, SMCLK and MCLK too.  

Why use a crystal on the XT2 pins?  Most of the time you don't need to. The most common configuration is to use a 32KHz crystal for the XT1 clock or the REFO clock and enable these as the FLLREFCLK for the DCO, which generates the CPU and peripheral clocks.  It is possible to tune the DCO to a specific frequency (within the limits of the DCO).

But, if you have situation where you must run the CPU or peripheral at a very specific frequency, such as 1.842MHz for certain communication bit rates, then you can use an appropriate crystal.  Frankly though, the MSP430 clock system is flexible enough to achieve many common frequencies so using a crystal on the XT2 pins would be optional.

For information about the crystal/clock frequency ranges, refer to the MSP430F5340 datasheet, around page 44.

The XT1 max is 50KHz and the XT2 max is 32MHz.

Tell me more about your application or design and I can help you determine the best configuration.

Refer

Hi Dennis,

Thank you very much for the provided details. As you asked about application and design, Below is the block diagram of the design. This is a self powered medical device. which is an interface between ultrasound transducer and Mobile device. MSP430 controller is used to monitor the battery status on SMBUS lines and it will communicate to charger IC, Micro-Processor and CC/Mode controller on I2C lines. Micro controller is also used to drive the LED display for battery indication.

Please let me know for this particular application is it OK if I put a 32KHz crystal oscillator on XIN and XOUT pins. I also want the low power consumption mode for controller to extend the battery life.

Hi Rineet,

Ok, yes, this is helpful to understand the application.

I took the liberty to cut a few sections from the MSP430F5340 datasheet and highlight the specific parameters I want to share with you.  I want to compare the pros and cons of using the XT1 with an exernal crystal and the internal REFO clock source.

***If you have an opportunity, I would strongly suggest after reading this that you visit the data sheet for this device and see these specific pages I mention in their complete form so you won't be overlooking anything.

That said, using the 32kHz crystal on the XT1 inputs does require two compensation capacitors and you will need to go through a simple process to select the appropriate drive strength based on the crystal you choose for your design as pointed out in foot notes 4 and 6 below.

Shown here is the various XT1 oscillator currents required to drive the external crystal.

I also want to point out there is also a significant startup time you may need to consider but typically is not a problem for most applications.  For example, if at power up your host MCU needs to communicate with the MSP in the first 100msec and the XT1 clock is used as a buad rate clock for the UART, then should be something we need to discuss.  But I see you are using I2C, which is a synchronous interface and I'm guessing the host MCU is the master.  So no concern here.

In reality, the XT1 clock source is going to drive something in the MCU, watchdog timer, RTC,... so the current in the MCU will be slightly higher as shown when the MCU is in its low power mode LPM3.  So this is what you should use as a comparison to the REFO clock.

So now let's look at the internal REFO oscillator.  As you can see the current is only a little higher than the XT1 shown above and the start up is faster.  In fact, the REFO is enabled by default at POR to provide a reference to the DCO FLL and clock system if needed and therefore needs to be up and running quickly   You don't need to add any capacitors and won't need to figure out what drive strength to use.  So far the REFO is looking like a good candidate.

So at the end of the day which one is better for your application?  It really just depends.  If your host MCU and all the other circuitry in the system is consuming 100uA in low power modes, then whether the MSP430 is consuming 2 vs 3uA, does it really matter between the two?  To save the expense of the crystal and two capacitors, it appears the REFO is the way to go.

Let me know your thoughts.