MSP430FG4619 RTC problem & external 32khz crystal question

The 4x family RTC uses teh basic timer as source for the RTC clock. In calendar mode, the basic timer is configured for a 1:32768 prescaler and ACLK source. In counter mode, however, you can pick SMCLK or ACLK as clock source. There will always be a /128 predivider, but you can pick whether you use ACLK or ACLK/256 or SMLCK or SMCLK/256 as clock source, resulting in ACLK/128, ACLK/32768, SMCLK/128 or SMCLK/32768 as clock for the RTC counter.

In our private conversation, I didn't know you're using a 4x family device. The 5x devices have their own timer/prescaler inside the RTC module, and there is no basic tiemr at all (whcih is mainly there for the LCD controller in the 4x family)

Hi Michael,

Thank you for your reply. So I do need to connect the external 32khz crystal to the board in order to source ACLK if I need to use RTC in calendar mode right? How should I do that? Can I just solder it to the target board? Is there any other component such as capacitor needed?

Thank you for your help!

Regards,

Vance

ForzaJuve said:

So I do need to connect the external 32khz crystal to the board in order to source ACLK if I need to use RTC in calendar mode right?

Right. The FLL+ module only has Xt1 as source for ACLK. However, if you add the crytal, you can also use it for stabilizing the DCO to (almsot) any frequency, as XT1 is also used as reference signal for the FLL. So two uses for one component :)

ForzaJuve said:

Can I just solder it to the target board

Yes. However, if you do so, you should sewer all other connection to this pin. If the board has breakout connectors attaached to the XT1 pins, then tehy need to be disconnected (parasitic capacitance/antenna). Usually, tehre are 0Ohm resistors or jumpers you can remove.

ForzaJuve said:

Is there any other component such as capacitor needed

Normally not. The XT1 oscillator in LF mode contains itnernal capacitors for some common loads. These are controlled by the CCAPxPF bits in FLL_CTL0. If the available options do not match, leave it on the default (1pF, the pin capacitance) adn add two external capacitors twice as large as the required load capacitance according to crystal datasheet (-1pF for the pin capacitance, to be exact). Usually, there are two pads for these already on the board besides the crystal mount, as you'll always need them in HF mode.

Hi Michael,

Thank you so much for such clear answers!

The board is MSP-TS430PZ100, there is one location that indicates LFXTL, so this is the place I should solder the external 32khz crystal to right?

And you told me there might be 0Ohm resistors or jumpers I can remove, I don't know how to do it, and I cannot find any instruction on how to connect external crystal to my target board either, sorry I don't have any experience about removing resistors on target board, I am so afraid I would damage the board.

One more question, does the crystal have polarities?

Thank you very much for your help!

Regards,

Vance

On the TS430PZ100 board, the XIN and XOUT signals are directly connected to the Q1 crystal mount and the two pads for the capacitors. The two signals are not connected to the breakout header. On the TMS430PZ100A/B/C board, there are two resistors between the crystal mount and the breakout header. Teh C version even has different pads for LF and HF crystals. However, the A/B/C boards d o not have pads for the optional 5MOhm resistor that is soemtimes needed to startup the crystal on low supply voltages. (VCC <2V)

ForzaJuve said:

One more question, does the crystal have polarities

Normally, the crystals are symmetrical. Because the electrical oscillation is converted into a mechanical oscillation and back. Assymmetrical mechanics wouldn't be good for an oscillation, and so the electrical side is symmetrical too. :)
Crystals made for e.g. lighter-ignition (same stuff, different use) are not necessarily symmetrical.

Jens-Michael Gross said:

On the TS430PZ100 board, the XIN and XOUT signals are directly connected to the Q1 crystal mount and the two pads for the capacitors. The two signals are not connected to the breakout header.

So all I have to do is solder the crystal to Q1 position right? And what about the two pads for capacitors? Do I need to solder capacitors as well or I just leave it there?

Jens-Michael Gross said:

 The two signals are not connected to the breakout header.

Sorry I am a little confused, breakout header is the hole on the board or pin on MCU?

Thank you very much for you help!

Regards,

Vance