Hi Frank

If you are looking for more details, kindly visit Huixun.

OK, pleased to hear that, I look forward to receiving the updated datasheet in the future.

We are starting the board design now and after doing a crystal survey have come to the conclusion we will probably have to use a metal can HC-49/US (S being the common SMD version of the ubiquitous HC-49U). Even some of them are spec'd to max 500uW, but you get 1mW. Basically, I cannot find any suitable crystal in resin or metal in a compact or low profile form.

The closest I could find in a compact resin form was the earlier mentioned CTS445 with 300uW max rating and 50ohm, 85C, 12MHz, 12pF. For that I calculate about 179uW. The 12pF is still within the range quoted in the datasheet by TI (Table 5-20). I have made contact with CTS and are waiting a reply. I asked for a recommendation and whether they could provide more information on the max. ESR figures that get quoted to enable a risk assessment. It would be good if I could reliably use the smaller crystal size as it makes routing tracks in the area of the oscillator pins easier. I might try and get hold of a Tektronix CT-1 to measure the crystal current accurately to be on the safe side.

Luckily in this design I think we can also use a larger crystal package. But it seems to me increasing the oscillator drive voltage on the has reduced the number of options for crystals. As I said earlier, the manufacturer's do not seem to release details other than the max ESR rating for the crystals so calculating the loss with that will often be over cautious, just as using a blank 1mW rating for the crystal will result in a crystal maybe 11.5 x 4.7 x 4.2 mm in size, compared to the 64pin DSP's 11.2 x 11.2mm.

One final word,  Frank can you confirm then that the 20MHz crystal used on the launchpad does not then fulfil the 1mW recommendation. I think that is important, as many people look to that as a reference design and I am sure many just adopt the crystal circuit as given. I am certainly guilty of that as well in the past.

Kind Regards

Andrew Green

Hi Frank

Thanks for your input  - I have found the discussion very useful.

Yes, we always use a damping resistor, but I have not included its effect in the calculations I have made just yet.  One parameter that would help there is to know the output impedance of the oscillator. Then one could calculate its effect accurately before making measurements. In any case we do have a Network Analyser here. It is not a real high end type, but the Omicron Bode 100 gives in general very good results.

CTS 445I23C12M  12MHz 16pF

Co = 3.27pF

Rm = 15.94ohm  (Motional Resistance)

fs = 11.MHz

fp = 12.006MHz

Cs = 3.978fF

Ls = 44.22mH

ESR = Rm * (1 + Co/Cl)^2 = 23ohm,   for Cl = 16pF

As expected this is less than the 50ohm maximum in the datasheet. The problem is that it is just one sample, and we do not know how the ESR varies  over the lifetime, only that it is less than the given maximum when driven correctly.

If one applies the Drive Level Formula  (from TI SWRA495F) DL = ESR*(pi * f * (Cl + Cs) * Vpp)^2  that gives  91µWatt for Vpp 3.3V. But for how long? I can imagine for many users they could run with a 100µW max. crystal and never have a problem, depending on the product on time, market lifetime, and temperature.

I think with either a 12pF calibrated crystal, or even a 16pF one loaded with just 12pF, I am well on the safe side with this CTS crystal as the DL is even lower. I will certainly report back what CTS says, but I am not holding my breath.

I would add of course a 12MHz Crystal will not give a 100MHz SysClk only 99MHz (or is there a trick I am missing?). The nice thing about 12 to me seems that the product of ESRmax* f ^2 seems the lowest for some of the crystals I have seen. This reduces the DL.

CTS 445:

10 MHz - < 12 MHz,  ESRmax = 100 Ohms  ->  f^2*ESR = 100*100 =

12 MHz - < 16 MHz, ESRmax = 50 Ohms     ->  f^2*ESR = 144*50 =

16 MHz - < 30 MHz, ESRmax = 40 Ohms     ->  f^2*ESR = 256*40 =

Addendum 1:

So I just made some measurements on the Launchpad with its 20MHz Crystal for different damping resistors (R31) and measured the following p-p voltages on the Pierce oscillator input pin X1 (C39), using the calibrated opamp-buffer described at the start of the post.

Rd = 0  gave 3.21Vpp

Rd = 220R gave 2.87Vpp

Rd= 470R = 2.63Vpp

Rd = 1K  = 2.25Vpp

Rd = 1K5 = 2.0Vpp

(all voltages above were scaled by 0.95 for bandwidth of measuring opamp at 20MHz)

Clearly this reduced voltage will help as Frank pointed out. But what should be then tested is the start-up time and safety margin to ensure the oscillator always starts reliably. Normally this involves adding resistance in series with the crystal until it stops oscillating, then dividing this value by the ESRmax. Depending on which guideline you follow this should be a minimum of  3 - 10, with 5 being an often quoted figure. I would have done that but the crystal on the Launchpad is a bit difficult to remove.

Addendum2:

I managed to remove the 20MHz ECS Crystal from the Launchpad with a hot-air desoldering tool, and made the following measurements with a damping resistor Rd at the oscillator output and various values of Rx in series with the crystal to check the safety margin. In all cases oscillation was observed to occur. The oscillation start times were not recorded.

ECS-200-18-30B-AGN-TR 20MHz 18pF

Rx=0, Rd=220, Vpp at X1 = 2.76V

Rx=0, Rd=1K, Vpp at X1 = 2.12V

Rx=220, Rd=220, Vpp at X1 = 0.99V

Rx=220, Rd=470, Vpp at X1 = 0.81V

Rx=220, Rd=1K, Vpp at X1 = 0.54V

(all voltages above were scaled by 0.95 for bandwidth of measuring opamp at 20MHz)

CTS 445I23C12M  12MHz 16pF  (leaving  the two 15pF load capacitors in the  Launchpad unchanged)

Want more information on he-49u crystal quartz resonator? Feel free to contact us.

Rx=0, Rd=220, Vpp at X1 = 3.09V

Rx=220, Rd=220, Vpp at X1 = 1.75V

Rx=220, Rd=470, Vpp at X1 = 1.64V

Rx=470, Rd=470, Vpp at X1 = 1.04V

(all voltages above were scaled by 0.97 for bandwidth of measuring opamp at 12MHz)

Kind Regards

Andrew Green

Hi Frank

Thanks for the tip about measuring the start-up time and the comparator - I will definitely look into it. I got burnt a bit with the with start-up time.I had Rd=470ohms and adjusted the oscillator startup wait delay in my test code, but then forget to tell the software guys. Some boards they were developing the "real" code on started within the TI library defined time, and others did not. But we resolved it quickly enough and before we were in production.

Just measured the following on the Launchpad Crystal ECS-200-18-30B-AGN-TR (ESR max = 40, Cload = 18pF, DLmax = 100uW) using our Bode 100

Rm = 9.83ohms

Cp = 3.25pF

fs = 19.997MHz

fp = 20.024MHz

Cs = 8.77fF

Ls = 7.22mH

ESR (with Cload = 18pF) = Rm*(1+Co/Cl)^2 = 13.7ohm

ESR (with Cload = 12pF) = Rm*(1+Co/Cl)^2 = 15.8ohm

So in this case the ESR is much less than the 40ohm maximum in the datasheet, so the crystal on the Launchpad is probably being driven close to the specified 100uW maximum, but maybe not over it. But at the risk of sounding repetitive that's the problem we have as designers - the crystal manufacturers only provide the maximum value and no statistics. But with a judicial choice of Rd, I think there are solutions available without resorting to a 1mW Crystal.

Addendum 1:

I used the CTS 12MHz Crystal with Rd=470ohm and routed XCLKOUT to  GIPO16 as per your suggestion. After connecting the 5V power to the Launchpad you see that X2 goes high, and a few msec later it drops sharply to about 1V, then after about 500usec the oscillation starts to build up. I triggered a one-shot on this falling edge. I have attached two scope captures. Green Trace = X2, Purple Trace= GPIO16.

From the green X2 falling transition to 1V until the start of XCLKOUT appearing on GPIO16 is 2msec. Can I assume that is the oscillator startup delay needed, or is it actually longer than that still, and I have to use the "Register" method  you described. The purple trace reaches 51% duty almost immediately it starts, as seen in the zoomed in second attachment.

I also added 200ohm in series with the crystal to verify if a safety margin of "about" 5 for worst case ESR was there. There was no difference in the waveforms to speak of, and the 2msec delay until XCLKOUT appeared remained the same. Leaving Rx=200R in, and changing Rd to 1K or 0ohm surprisingly  also had no effect - the 2msec remained the same.

Addendum 2:

I would add that the violet XCLKOUT trace is configured as PLLSYSCLK. It happened to be the PLL was set to 5 and the XCLKOUT divider  to 8 so the frequency = 12MHz x 5/8 = 7.5MHz.

Cheers

Andrew

Hi Frank

OK, I have some results that seem to make better sense. This is what we did:

Setup that XCLKOUTso that it could be viewed and this time was configured to OSCCLK. Then in the SysXtalOscSel() Routine in (fx_sysctrl.c) where the Crystal oscillator is activated we toggled a gpio pin as suggested by you. The following results are for the CTS 12MHz Crystal with Rx=200ohms in series with crystal to simulate a "worst" case condition. I actually forgot to take it out,  made the measurements and thought oh what the heck. I made measurements for Rd = 0, 470R, 1K. Trying to find the 45-55% duty point is very subjective but you can see the influence of Rd. I have attached four scope captures. In the following one the blue curves were saved for Rd=470, and the Y/G/V are for Rd=0ohms. Y =  GPIO toggled after oscillator is enabled, G = Oscillator X1 input, V = OSCCLK output on GPIO16.  Notice I shifted the saved blue waveforms around for better viewing. I measured for 0ohms 900usec to reach 45-55%, and for 470ohm 960usec.

In the following two captures you see 1) how OSCCLK does not start with 50% duty (as you foresaw) and later when it does reach the target range.

The last capture below shows the case for Rd=470 (saved in blue) compared to Rd=1K in Y/G/V. For Rd=1K I measured a time of usec.

So it confirms that increasing/adding a damping resistor does increase the start-up time needed, but it is not necessarily much longer. Also even with 200ohms added in series with crystal and Rd = 1K the oscillator started.

Frank, I think we should close this issue now (it was very interesting,  but I have spent much too much time on it). I am happy that I have a solution that seems to meet Crystal Drive Level requirements and Oscillator Safety Margins, and in a footprint that is not too big. Still no reply from CTS. If someone else decides to use a crystal from CTS, or decides the crystal on the launchpad fulfils their requirements, then in both cases the responsibility lies with them. As it stands I will be using something that is still outside the 1mW specification in the TI datasheet. It is my responsibilty to ensure it works.

Kind Regards

Andrew Green

Hi Frank

OK, I have some results that seem to make better sense. This is what we did:

Setup that XCLKOUTso that it could be viewed and this time was configured to OSCCLK. Then in the SysXtalOscSel() Routine in (fx_sysctrl.c) where the Crystal oscillator is activated we toggled a gpio pin as suggested by you. The following results are for the CTS 12MHz Crystal with Rx=200ohms in series with crystal to simulate a "worst" case condition. I actually forgot to take it out,  made the measurements and thought oh what the heck. I made measurements for Rd = 0, 470R, 1K. Trying to find the 45-55% duty point is very subjective but you can see the influence of Rd. I have attached four scope captures. In the following one the blue curves were saved for Rd=470, and the Y/G/V are for Rd=0ohms. Y =  GPIO toggled after oscillator is enabled, G = Oscillator X1 input, V = OSCCLK output on GPIO16.  Notice I shifted the saved blue waveforms around for better viewing. I measured for 0ohms 900usec to reach 45-55%, and for 470ohm 960usec.

In the following two captures you see 1) how OSCCLK does not start with 50% duty (as you foresaw) and later when it does reach the target range.

The last capture below shows the case for Rd=470 (saved in blue) compared to Rd=1K in Y/G/V. For Rd=1K I measured a time of usec.

So it confirms that increasing/adding a damping resistor does increase the start-up time needed, but it is not necessarily much longer. Also even with 200ohms added in series with crystal and Rd = 1K the oscillator started.

Frank, I think we should close this issue now (it was very interesting,  but I have spent much too much time on it). I am happy that I have a solution that seems to meet Crystal Drive Level requirements and Oscillator Safety Margins, and in a footprint that is not too big. Still no reply from CTS. If someone else decides to use a crystal from CTS, or decides the crystal on the launchpad fulfils their requirements, then in both cases the responsibility lies with them. As it stands I will be using something that is still outside the 1mW specification in the TI datasheet. It is my responsibility to ensure it works.

Kind Regards

Andrew Green

If you want to learn more, please visit our website hc-49u-jacket quartz crystal.

Checking your browser before you access eBay.

Your browser will redirect to your requested content shortly. Please wait...