- #1
What's the purpose of R20 in this schematic
- Thread starter j777
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Discussion Overview
The discussion revolves around the purpose of R20 in a schematic from an evaluation board, with participants exploring its role in relation to clock connections and resistor values. The conversation includes technical explanations and hypotheses regarding the function of R20 and its comparison to another resistor, R35.
Discussion Character
- Technical explanation
- Debate/contested
Main Points Raised
- Some participants propose that R20 acts as a jumper connecting two clocks, with a zero Ohm resistor commonly used for layout purposes.
- Others argue that R20 may not simply be a jumper, suggesting it could serve a different function based on comparisons to another resistor, R35, in a similar schematic.
- One participant mentions that a 100 Ohm resistor is typically indicative of termination rather than a jumper, contrasting it with the zero Ohm resistor.
- A theory is presented that the zero Ohm resistor allows users to choose between accessing an SD card via SPI or SDIO.
- Another participant states that R20 enables the use of either common or separate offboard clocks, while R35 allows for offboard selection of one of two clocks, with the 100 Ohm resistor providing protection against erroneous connections.
Areas of Agreement / Disagreement
Participants express differing views on the purpose of R20, with no consensus reached regarding its exact function. Multiple competing interpretations remain, particularly concerning the distinction between R20 and R35.
Contextual Notes
Participants reference different resistor values and their implications, but the discussion does not resolve the specific purpose of R20 or the conditions under which it operates.
- #2
mgb_phys
Science Advisor
Homework Helper
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Isn't it just a connection between the two clocks?
It's common to draw a link as a zero Ohm resistor in some packages, it let's the layout software actualy put a real part there so you can use it as a test point, or remove it to test the two clocks separately. On surface mount it's the normal way to do a link.
It's common to draw a link as a zero Ohm resistor in some packages, it let's the layout software actualy put a real part there so you can use it as a test point, or remove it to test the two clocks separately. On surface mount it's the normal way to do a link.
- #3
Proton Soup
- 223
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ja, it's a jumper
- #4
j777
- 148
- 0
I'm not so sure that it's just a jumper...
Here's a snippet from a similar eval board. In this schematic the resistor is R35. Any ideas?
The full schematic for this snippet can be viewed is at http://www.olimex.com/dev/images/ARM/ATMEL/SAM9-L9260-REV-B-sch.gif"
Here's a snippet from a similar eval board. In this schematic the resistor is R35. Any ideas?
The full schematic for this snippet can be viewed is at http://www.olimex.com/dev/images/ARM/ATMEL/SAM9-L9260-REV-B-sch.gif"
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- #5
berkeman
Admin
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Hard to say. 100 Ohms is different from the zero Ohm jumper in your first post. 100 Ohms is more like a termination, but zero Ohms is almost always a jumper for stuff options.
- #6
j777
- 148
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Thanks for all the input.
I still don't know what the 100 Ohm resistor is for but my theory for the zero Ohm resistor is that it gives the user the option of accessing the SD card via SPI or SDIO. Does that make sense?
I still don't know what the 100 Ohm resistor is for but my theory for the zero Ohm resistor is that it gives the user the option of accessing the SD card via SPI or SDIO. Does that make sense?
- #7
Phrak
- 4,266
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R20 allows the user to use either common or separate offboard clocks.
R35 allows offboard selection of one of two clocks. If you happen to erroniously connect both, the 100 ohms will protect the offboard clock drivers.
R35 allows offboard selection of one of two clocks. If you happen to erroniously connect both, the 100 ohms will protect the offboard clock drivers.