Mega4808 External Low Frequency Crystal Oscillator?

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ka7ehk
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Posted by ka7ehk: Thu. Sep 27, 2018 - 09:52 PM
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Greetings,

 

I would like to run the RTC in a Mega4809 at 32KHz to get 10Hz ticks. I think the hardware makes this possible.

 

BUT, it is NOT clear that it will accept a 32,000Hz crystal. At various points in the Manual, it refers to 32.768KHz and, at others, 32KHz. In the Data Sheet, it calls out that external resonator oscillator as TYPICALLY 32.768KHz.

 

Does anyone know whether it will actually work at 32.000KHz? If it does work, then whether or not the crystal requirements (such as ESR) are consistent for both frequencies?

 

Many thanks

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

Tags:

AVR Microcontrollers, megaAVR and tinyAVR, AVR4100, ATmega4809
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gchapman
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Posted by gchapman: Thu. Sep 27, 2018 - 10:16 PM
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ka7ehk wrote:
If it does work, then whether or not the crystal requirements (such as ESR) are consistent for both frequencies?
from mega4809 Xplained Pro schematic page 4 of 6 :

32 kHz CRYSTAL

Crystal datasheet:

Ccrystal = 7pF

max ESR = 70kOhm

Accuracy ±20ppm

 

ATmega4809 datasheet:

Cpara = TBD

max ESR = TBD

 

PCB capacitance: Cpcb = 0.5pF (estimated)

 

Estimated load:

C = 2x(Ccrystal- Cpara - Cpcb)

C = 2x(7pF - 1pF(estimated) - 0.5pF)

C = 11pF

 

Verification from KYOCERA Corporation:

C305 = 10pF and C306 = 13pF.

 

Negative resistance: -465 kOhm

Accuracy @ vcc=3.3V: 3.4 ppm

Accuracy @ vcc=5.0V: -0.3 ppm

ST3215SB32768C0HPWBB

Kyocera Corporation

http://www.microchip.com/developmenttools/ProductDetails/ATMEGA4809-XPRO

 

"Dare to be naïve." - Buckminster Fuller

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gchapman
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Posted by gchapman: Thu. Sep 27, 2018 - 10:39 PM (Reply to #2)
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compared Kyocera ST3215SB32768C0HPWBB vs Kyocera CX2016DB32000F0FPLCC (32000Hz)

drive level is 0.1microW vs 10microW

For that one pair, the answer is no.

 

"Dare to be naïve." - Buckminster Fuller

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igendel
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Posted by igendel: Thu. Sep 27, 2018 - 10:46 PM
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I wonder how will the MCU ever be able to tell, all by itself, whether it's connected to a 32.768KHz or 32.0KHz crystal :-)

 

Just remember that if you're going to use the RTC module, it has very limited functionality in the current silicon revision. From the Errata:

 

1.3 RTC
1.3.1 Any Write to the RTC.CTRLA Register Resets the RTC and PIT Prescaler
Any write to the RTC.CTRLA register resets the RTC and PIT prescaler.
Work around
None.
 

angry

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Who-me
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Posted by Who-me: Thu. Sep 27, 2018 - 10:47 PM
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ka7ehk wrote:

Does anyone know whether it will actually work at 32.000KHz? If it does work, then whether or not the crystal requirements (such as ESR) are consistent for both frequencies?

32kHz and 32.768kHz are close enough, to be considered the same on a simple biased inverter-Oscillator Circuit.  ie ESR specs etc would be considered the same.

 

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ka7ehk
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Posted by ka7ehk: Fri. Sep 28, 2018 - 02:16 AM
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Thanks, folks

 

Jim

 

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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gchapman
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Posted by gchapman: Fri. Sep 28, 2018 - 05:59 PM
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ka7ehk wrote:
BUT, it is NOT clear that it will accept a 32,000Hz crystal.
09's datasheet has 

  • tstartup = 300ms typ for 7.5pF crystal (very low power oscillator?)
  • CL = 7.5pF .. 12.5pF (crystal's capacitance)
  • ESR <= 80Kohm for 7.5pF crystal, 40Kohm for 12.5pF

Not meant to be completely clear (though data is more complete than PB megaAVR) as there's AVR4100 to aid in measuring and calculating the 32KHz crystal oscillator's safety factor given a crystal.

Verification from KYOCERA Corporation:

Crystal manufacturers can recommend one or more crystals from their catalog given a PCBA with the MCU mounted on it per their layout for the crystal(s)

32KHz is low enough for inexpensive test and measurement equipment; AVR4100 is on a STK600 though may be easier to create a PCBA specific for this evaluation.

ka7ehk wrote:
In the Data Sheet, it calls out that external resonator ...
Couldn't find 'resonator' in the datasheet though some of those may work (some on mega328PB boards)

 

https://www.microchip.com/wwwproducts/en/ATMEGA4809

Microchip Technology

Application Notes

AN2648 AVR4100: Selecting and Testing 32 KHz Crystal Oscillators for AVR Microcontrollers

http://www.microchip.com//wwwAppNotes/AppNotes.aspx?appnote=en592059

https://abracon.com/index.php?p=news/choose-the-right-quartz-crystal-for-your-energy-saving-mcu-with-this-simple-test

https://www.mouser.com/new/abracon/abraconpierce/

https://www.mouser.com/ProductDetail/ABRACON/PAS-BC3WK?qs=%2fha2pyFaduhBnuB8Lw6hLEzINxkVt4mGul4Z3yZdhd0%3d

 

Edit: 1st Mouser URL

 

"Dare to be naïve." - Buckminster Fuller

Last Edited: Fri. Sep 28, 2018 - 06:03 PM
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mannen
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Posted by mannen: Sat. Sep 29, 2018 - 01:48 PM
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btw

when you click "ATmega4809 Xplained Pro Design Documentation"

there you can download the schematic and BOM and PCB footprint for a complete design that is definitely working and that uses this kind of crystal if I remember correctly. 

This should save some time for making footprint.

 

https://www.microchip.com/Develo...

 

edit: remembered wrong, they use 32.768kHz

Last Edited: Sat. Sep 29, 2018 - 01:53 PM
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gchapman
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Posted by gchapman: Sat. Sep 29, 2018 - 08:11 PM
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ka7ehk wrote:
Does anyone know whether it will actually work at 32.000KHz?
DIY

schematic :

J300 for the AVR4100 rheostat

XC300 - remove 32KHz crystal and solder the DUT 32000Hz crystal

C305, C306 - are 0402 (not easy but do-able); might be acceptable as-is for first trials

 

layout :
 

J300 at top

XC300 - 3.2mm long by 1.5mm wide; the ground metal under it may be an issue if the 32000Hz crystal is an "odd" size

C305, C306 - 0402; between the mega4809 and XC300

 

Compared mega324PB and mega4809 power save and power down currents; mega4809 is about half that of mega324PB.

Am certain mega4809 TOSC startup will be an concern,

Reasons : 1. TOSC crystal data in mega4809 datasheet versus none in mega324PB, 2. significantly reduced power save current

May you be fortunate in locating a compatible 32000Hz crystal.

 

"Dare to be naïve." - Buckminster Fuller

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ka7ehk
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Posted by ka7ehk: Sat. Sep 29, 2018 - 08:23 PM
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Thanks

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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