Need help with crystals

[moihussain 0]

I am new to microcontroller projects and am planning to use a msp430F149 in my project. I have a 32.5KHz crystal with Cl=12.5pF. I need to use this crystal to generate a baud rate of 9600 for serial communication. I understand that I will need to connect this crystal through the XIN and XOUT pins, but am not aware of the requirement of external capacitors. Do I need any? If yes, how must I calculate it? Please enlighten me on this.

[fatihinanc 14]

Due to the integrated capaticance, don't require use an external capacitor.

You only need to a 32.768kHZ crystal.

As shown in the picture on below;

 

For the UART. You simply reference to code examples from TI.

http://www.ti.com/lit/zip/slac015

[moihussain 0]

Thank you.

I would also like to know if I may need a capacitor arrangement for an 8MHz crystal that I will be connecting through XT2IN and XT2OUT pins from which I would source the MCLK, the clock that drives the basic operations of the msp. Should I use external capacitors? If yes, how do I calculate it?

[fatihinanc 14]

You can find these informations in datasheet(Page 33). Link : http://www.ti.com/lit/gpn/msp430f149

 

For the XT2 osc, you can use 10 - 12pF capacitor for 32.768 kHz and 20 - 22pF for 8 MHz crystal. I don't know any calculation formula but you can review other schematics esipecialy F149 microcontroller.

 

One of them : http://www.olimex.com/dev/msp-h149.html

[SugarAddict 227]

I don't know any calculation formula but you can review other schematics esipecialy F149 microcontroller.

 

Crystal Required Capacitance * 2 - 2 = Capacitor Value

 

SLAA322 shows this:

Crystal requires 12 pF load.

Parasitic capacitance per pin is 2 pF.

CL1 = (2 x CLoad)

[larsie 121]

The MSP430 has internal capacitors you can program though, so you don't need external capacitors. Maybe you know, but I just wanted to add that.

 

The LFXT1 oscillator supports ultralow-current consumption using a

32,768-Hz watch crystal in LFmode(XTS = 0).Awatch crystal connects to XIN

and XOUT without any other external components. The software-selectable

XCAPx bits configure the internally provided load capacitance for the LFXT1

crystal in LF mode. This capacitance can be selected as 1pF, 6pF, 10pF or

12.5pF typical. Additional external capacitors can be added if necessary.