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External Oscillator - chossing right capacitors | Cypress Semiconductor

External Oscillator - chossing right capacitors

Summary: 4 Replies, Latest post by Gautam Das on 18 Nov 2011 04:07 AM PST
Verified Answers: 1
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user_60984833's picture
11 posts


we are having a problems running
CY8C3866LTI-068 with external oscillators.

In our devices we are using
12 MHz external oscillator (core), and 32 kHZ external oscillator (RTC). 

Can someone help me and
explain what capacitors I should use with them?

In documentations its just
written that they should be but there is no information about right capacitance.
I expect that it should be some kind of equation..

On developer kit I can see
that capacitors next to external oscillator are just on schematic, marked as
"not loaded". On board there is none of them.

When we were doing tests
with different capacitors (using equations that we thought that should work) it
sometimes work, sometimes not.

ps. there is no problems
when i switch it to use internal oscillator.


Thanks in advance for

kiku's picture
Cypress Employee
64 posts


Please refer to the App Note on the webpage ( which explains how to select the Capacitors for External Oscillators..
Which Development board are you using? -001 or -030?



user_60984833's picture
11 posts

Thank You for answering!

We are using our custom design but we just did check on dev kit 001 how it looks like. We read this document and choose "right" capacitors. Some of devices did work with these capacitors, some with different and some without capacitors at all. Because of short timeframe we had to swap to internal oscillator which was working without problems. We will try this again next time - during another design (probably next year).

During my experiments with it i was wondering what kind of influence did the change of voltage configuration in cydwr file (System tab) have. Our device is battery powered and it shoudl work from 3.6v down to 1.8v - i wasn't sure what i supouse to write there. Changes there makes some of our devices working with external oscillator..

user_460349's picture
1362 posts

I had a similar problem with a PSoC 1 project, I just use the internal osc as that was good enought at that time.

Would like to know if other people has problem with the internal osc of PSoC3 as we are going to use PSoC5 in one of the project ( I know it may be different but just wondering if they would behave the same). We also notice that some oscilaltors actually has build in capacitors as well, so not all osc are the same.

dasg's picture
Cypress Employee
730 posts

Hi vac at work,


Setting of the right values in the "Voltage Configuration" window is important as it determines the reference voltage when you are using Delta Sigma ADC with Input Range selected as "Vssa to Vdda".


When APIs such as ADC_CountsTo_mVolts( ) or ADC_CountsTo_uVolts( ) are used, the Vdda value is taken from the Voltage Configuration tab.

If there is mismatch in the voltage set in the configuration tab and the actual voltage the device is powered with, when Vdda is set as reference, the output got from the APIs like ADC_CountsTo_mVolts( ) will be scaled by a factor.

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