MSP430FE42xA Crystal slow down

S Fighter said:

But, when I plug in the device for measurement, the device slows down significantly:

Could be a time distortion field generated by the flux generator in the measurement unit. No? Well, then how about giving some more informations?
Where do you the device plug in for measurement? What do you measure and how? What is the setup of the device? I guess form the thread topic that you're using a crystal somewhere and somehow. But where and how? What crystal?

To get a specific answer you'll need to ask a specific question. A general question can only get general answers.

Hi Jens,

I'm sorry that I wasn't more specific. MSP430FE42xA is a Power Meter IC. It has an embedded chip - ESP430CE (analog front end), which calculates the power values when given voltage and current input channels. It has a 16 bit ADC and the ESP does all the calculations without disturbing the MSP430 core. The calculated power values are stored onto a register, which can be read my the MSP430.

I have followed the circuit given in this Application Note: http://focus.ti.com/mcu/docs/litabsmultiplefilelist.tsp?sectionId=96&tabId=1502&literatureNumber=slaa203c&docCategoryId=1&familyId=914

This MSP430 requires a 32.768kHz crystal. I have used a crystal with 6pF load capacitance.

So, when I plug in the device to the AC outlet for the measurement, I observe the slow down in the crystal that I explained  in my previous post. I still see measurement values being generated in the registers, but they are generated at a much slower rate too.

Possible causes to the problem:

- The crystal in my board is quite close to the 120VAC line coming for measurement.This might cause interference and slow down the crystal. The V+, V- pins are next to the crystal pins in the IC, so its difficult to maintain a distance between them.

- The supply Voltage is not steady. I did measure the supply voltage (output from a 3.3V LDO Regulator) using a Multimeter, and it was quite stable. I also have 0.1uF and 10uF 0603 Caps across both AVSS, AVDD and DVSS, DVDD.

It might be also that the crystal slow down is not caused because of this MSP430 alone. Any other causes of slow down will also help me investigate further.

Also, any method which will help me debug this better will also help. Please let me know if you can think of anything.

Thanks,
SF 

Hi,

Thanks a lot for your input.

I scoped out all the signals last night and observed some interesting things.

- The Power Supply is at a steady 3.3V. Our GND is connected to the Neutral. I also connected the MSP to an external Supply, and the crystal was still 'slowing down'.

- I also removed all the measurement and UART code. Just blinked the LED. The slow down was still there. So, I eliminated a slow down in the execution of MSP.

- I removed the crystal entirely. The MSP worked now in the 'slow mode'. So, the crystal was not slowing down. But, because of some interference, the MSP was not able to lock onto the frequency.

- When I scoped out the signal from the crystal, I saw it gave a good 32kHz sinusoidal signal from about -150mV to +150mV. However, when the board was plugged in, there was a DC shift in the signal of about 150 mV. I believe the MSP was not able to lock onto this signal.

- Also, I did not have put any load capacitance as the design didn't suggest any. I thought the parasitics would provide enough capacitance.

I will try out the things that you suggested, and report back, hopefully, with a working device.

Thanks again,
SF 

SF,

There's a software fix for your load-capacitance shortage.  Set the XCAPxPF field in FLL_CTL0 for 6pF (setting = 01).

Actually if you're lucky that fix could be all you need!

Jeff

Hi Jeff,

I am using the software to set FLL_CTL0 to 6pF. Unfortunately, the device still malfunctions.

Ideally, I should not be putting any load capacitances outside the chip if I'm setting it in software? Please let me know.

Thanks,
SF

You're correct.  If you are using software to configure the MSP430 for 6pF loading, you don't need any external capacitors.  It's a very nice solution.

Unfortunately, it looks like you are going to have to look at the layout, grounding, and guarding.  JMG had great suggestions above, and I would also recommend this classic from TI:

http://focus.ti.com/lit/an/slaa322b/slaa322b.pdf

By the way, the UART failure and the "slow down" are a direct result of the XT1 failure.  Once you solve the XT1 failure, the other problems will disappear.  The FLL is slowing down the DCO because XT1 isn't working properly.  (Your LED blinking and your UART baud rates are apparently based on the DCO rate.)

Good luck!

Jeff

Jeff Tenney said:

If you are using software to configure the MSP430 for 6pF loading, you don't need any external capacitors. 

I fear that the layout is forming a capacitor with way more than the required 6pF.
Also, it seems to work as antenna and charges due to the near AC line.

Jeff Tenney said:

UART failure and the "slow down" are a direct result of the XT1 failure.

I forgot about the FLL. Indeed. It's likely not the crystal slowing down, it's the FLL which slows down the DCO because there are 'too many' DCO pulses coming in between the last crystal pulse and the (perhaps never coming) next one. So the FLL will step down the DCO until it reaches the lowest setting. This of course slows down everything.

As a test, the timer interrupt should check for the OFIFG flag or maybe one of the other fault flags (DCOF, LFOF or whatever they are named) and set an LED if one comes up.
One check would be to disable the FLL once the device is up and the nominal DCO frequency is achieved. Check the datasheet how to do it. Once the DCO has settled, the FLL is only necessary to stabilize over temperature changes. If you disbale the FLL once the device is running, plugging it in for measurement shouldn't affect the speed anymore. Of course this is not a solution, just a test.

hi.

i am using MSP430fe427 and i have same problem. my crystal has a problem. it is not working properly. i tried it this code:

 

#include "msp430fe427.h"

volatile unsigned int i;

void main(void)

{

  WDTCTL = WDTPW + WDTHOLD;                 // Stop WDT

  FLL_CTL0 |= XCAP18PF;                     // Configure load caps

  IE2 |= BTIE;                              // Enable BT interrupt

  BTCTL = BT_ADLY_1000;                     // 1s Interrupt

  P1DIR |= 0x30;                            // P1.4 and P1.5 output

 

  // An immedate Osc Fault will occur next

  IE1 |= OFIE;                              // Enable Osc Fault

 

  while(1)

  {

    _BIS_SR(LPM3_bits + GIE);               // Enter LPM3 w/ interrupts

    P1OUT ^= 0x20;                          // Toggle P1.5 using exclusive-OR

  }

}

 

// Basic Timer interrupt service routine

#pragma vector=BASICTIMER_VECTOR

__interrupt void basic_timer(void)

{

  P1OUT ^= 0x20;                            // Toggle P1.5 using exclusive-OR

}

 

#pragma vector=NMI_VECTOR

__interrupt void nmi_ (void)

{

  do

  {

    IFG1 &= ~OFIFG;                         // Clear OSCFault flag

    for (i = 0xFFF; i > 0; i--);            // Time for flag to set

    P1OUT ^= 0x10;                          // Toggle P1.4 using exclusive-OR

  }

  while ((IFG1 & OFIFG));                   // OSCFault flag still set?

  IE1 |= OFIE;                              // Enable Osc Fault

}

 

Hi,

I designed a new board with the ground plane, and other precautions suggested by Jens. Also, I did not stuff the external capacitors, as they could be set by software.

The crystal worked perfectly this time. Thank you so much for helping me out!

I think the problem was that my AC lines were pretty close to the crystal, and were causing interference. As Jeff mentioned, the interference would have caused the FLL to malfunction.

@omeraygor: I used the software given by TI in the Application Note: http://focus.ti.com/mcu/docs/litabsmultiplefilelist.tsp?sectionId=96&tabId=1502&literatureNumber=slaa203c&docCategoryId=1&familyId=914

The crystal worked fine with the given code.

- SF

hi SF

thanks. The board i designed is grounded and also i havent still work with AC line. can you send me a picture of PCB you designed including crystal and MSP. 

 

Thanks.

Omer Aygor

hi 

i have a question more. you said "I designed a new board with the ground plane". i think my problem is same with you. my card has only one side grounded. (bottom side)

top side is not grounded. you know crystal pins and V and I pins is so close. 

both side, top and bottom copper must be grounded around the crystal. 

does the ctystal body touch the ground plane? 

thanks....