STM not respecting recommended design guide for HSE crystal

[Maz D]

Posted on August 16, 2017 at 18:24

I am trying to choose a

crystal 

for the STM32F7. The only problem being is that I cant find a component that respects the recommended values found in the following arm

http://www.st.com/content/ccc/resource/technical/document/application_note/c6/eb/5e/11/e3/69/43/eb/CD00221665.pdf/files/CD00221665.pdf/jcr:content/translations/en.CD00221665.pdf

. Even STM for it's STM32F779I-EVAL board does not respect the gain_margin.

If I have understood correctly there is one main rule governing the choice for a crystal so that the oscillator starts and reaches a stable phase.

gainmargin = gm / gmcrit

gm is the oscillator transconductance specified in the STM32F779 datasheet.

• Gm_crit_max = 0.001

gmcrit is computed from oscillation-loop passive components parameters.

• ESR is the equivalent series resistance 
• C0 is the crystal shunt capacitance
• CL is the crystal nominal load capacitance.
• F is the crystal nominal oscillation frequency

The one used for the eval board is this one :

https://www.eltech.spb.ru/files/item/NX3225HA-STD-CRT-1.pdf

But I found gainmargin = 1.674730 when it's supposed to be >5

Does anyone know why?

#crsytal #oscillator #quartz #stm32f7 #hse

[Maz D]

Posted on August 16, 2017 at 18:30

Seems as though I am not the only one asking the same question. But the answear is not clear as to why there are no crystals that exist that respect the design rules.

https://community.st.com/0D50X00009XkaJBSAZ

 

https://community.st.com/0D50X00009Xke67SAB

 

[Tesla DeLorean]

Posted on August 16, 2017 at 22:17

I suspect the F7 design contains a lot of hindsight..

[STOne-32 · Accepted solution]

Posted on August 17, 2017 at 00:51

Dear all, 

Having a crystal compatible with our HSE is very important else there is a risk of no oscillation in worst case conditions. 

That NDK crystal is specific for us (STMICROELECTRONICS ) and is not from their public website. Look to the exact reference at the end. 

Cl =6.5pF

C0 = 1.3pF

Esr max = 50 Ohm

F=25MHz

Gm margin = 5/0.3 = 16 >>> 5.

Please do not confuse gm _crit and gm_crit_max  there is a factor of 5 between them.

Cheers 

STOne -32 

[Tesla DeLorean]

Posted on August 17, 2017 at 01:02

>>

That NDK crystal is specific for us (STMICROELECTRONICS ) and is not from their public website.

Does that have an orderable (full qualified) part number? I'd suppose it is in the BOM, but in case it is a special order part it would be good to know not to use an unsuitable, but higher running, substitute. People are apt to copying a known working design. Can you provide the spec sheet for that part?

[Maz D]

Posted on August 17, 2017 at 15:03

I think I finally understood... it's in the datasheet but it really needs to be clarified.

As stated in the the design guide p 13:

http://www.st.com/resource/en/application_note/cd00221665.pdf

• http://www.st.com/resource/en/application_note/cd00221665.pdf

• http://www.st.com/resource/en/application_note/cd00221665.pdf

This is because the following conversion needs to be done before when considering the g_mcrit that was calculated (p14)

http://www.st.com/resource/en/application_note/cd00221665.pdf

For example, with

http://www.st.com/resource/en/datasheet/stm32f100cb.pdf

First I multiple by 5 the 

Gm_crit_max 

5*Gm_crit_max = 

g_m => 5 x 1mA/V = 5

Then I divide by gm_crit that I had calculated for my quartz

gm/gm_crit = gain_margin => 5 / 0.3 = 16

[STOne-32]

Posted on August 17, 2017 at 16:30

Hi Clive,

Of course this part is orderable and will be at inventory at distributors, you need to ask the partner the right specification. For NDK each time we put a crystal on our Boards, they codify it with a special naming, so if a customer will re-ask for it, they know it is already qualified for that board and that STM32 MCU from STMicro. {Their strategy} and may be different from other partners.

here is attached the datasheet of his cousin 24MHz, just frequency will change.

We as ST, we work with all Crystal Manufacturers to provide World Wide substitute of all part-numbers based on customer preferred solution : You can see our AN2867 with all major crystal partners.

Cheers,

STOne-

________________ Attachments : NX3225HA_24MHz EXS00A-CH00342.pdf : https://st--c.eu10.content.force.com/sfc/dist/version/download/?oid=00Db0000000YtG6&ids=0680X000006HyUf&d=%2Fa%2F0X0000000b8f%2FiUMop_9APUXOW9IXG3V1cca335sOsKYi6g6QKDfEIaA&asPdf=false

[Tesla DeLorean]

Posted on August 17, 2017 at 16:53

I understand that crystal vendors can make parts to specs, but they also tend to have very high running existing parts on the shelf which would likely have significantly better availability with lower lead times.

For people designing STM32 boards there is also a strong preference to use parts they already stock.

[STOne-32]

Posted on August 17, 2017 at 18:52

Indeed, That is our purpose.   if you look to our datasheet you can see Reference to AN2867  always.  But we need to put one Physical crystal on our boards one day .   That does not mean that it is the preferred or the best one for us or our board subco.

Cheers,

STOne-32.