Am I misunderstanding I/O multiplexing (or is this a Datasheet error)?

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JohanEkdahl
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Posted by JohanEkdahl: Tue. Nov 28, 2017 - 01:43 PM
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I am looking at the Atmel SAM D20E / SAMD 20G / SAM D20J data sheet (Atmel-42129P-SAM-D20_Datasheet_Complete-09/2016), and specifically at Table 7-1 Port Function Multiplexing.

 

I was under the impression that for each SERCOM/PAD it would only be routed to one pin under "function C" (SERCOM) and one pin under "function D" (SERCOMALT).

 

In the table I find two sets of entries for the SERCOM4 PADs (PB08-09-PB10-PB11 and PA12-13-14-15). Similar duplicates exists for e.g. SERCOM3 and SERCOM5.

 

I have a hard time believing such a blatant error would pass through corrective reading before publishing, so what would be the point in "duplicating" the routing in this way, having the in- and out-put of one SERCOM on several sets of pins?

 

EDIT/PS: I confess not having read every word of the data sheet yet. On my way, though...

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Atmel SMART ARM-based MCUs and MPUs, Atmel Cortex-M MCUs, solved, SERCOM USART, ATSAMD20
Last Edited: Tue. Nov 28, 2017 - 06:53 PM
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frog_jr
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This reply has been marked as the solution. 
Posted by frog_jr: Tue. Nov 28, 2017 - 01:59 PM
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You must remember that you select the function of each pin individually, not as a single unit.

So you can configure each SERCOM pin as required.

David (aka frog_jr)

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JohanEkdahl
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Posted by JohanEkdahl: Tue. Nov 28, 2017 - 03:51 PM
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Doh! Of-course. blush 

 

This is what happens to me when I take in too muck information in too short a time for my brain to cope with. Assumptions and misconceptions roam freely between my ears.. wink

 

The more general question, for understanding, would be "what parameters are behind how the set of possible "routings" are determined?" Why the duplication of e.g. SERCOM4 routing possibilities? Did the Atmel people have specific options/applications in mind, or is it simply what reasonably could be designed? (Hope you get what I'm asking - I am aware the question might sound weird...)

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awneil
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Posted by awneil: Tue. Nov 28, 2017 - 04:17 PM
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JohanEkdahl wrote:
muck information

laugh

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awneil
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Posted by awneil: Tue. Nov 28, 2017 - 04:20 PM
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JohanEkdahl wrote:
"what parameters are behind how the set of possible "routings" are determined?"

Nope - I never could fathom that.

 

Especially, IIRC, some of the omissions.

angry

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frog_jr
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Posted by frog_jr: Tue. Nov 28, 2017 - 05:53 PM
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I found it useful when the pins could be adjusted to accommodate  other needed functions. In one instance, I found it allowed for a bit more "elegant" board layout.

awneil wrote:
Especially, IIRC, some of the omissions.angry
yes

I still tend to find combinations where no twiddling of the pins can get all of the desired functions, so off to a larger package or another device.

David (aka frog_jr)

Last Edited: Tue. Nov 28, 2017 - 05:54 PM
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JohanEkdahl
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Posted by JohanEkdahl: Tue. Nov 28, 2017 - 06:59 PM
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awneil wrote:

JohanEkdahl wrote:
muck information

laugh

Both fun and serious: I'm not sure what has happened to my typing lately, but I do that more and more. Not hitting the adjacent key but hitting "similar" keys, not only phonetically similar (but that happens too). 'k' instead of 'h', 'c' instead of 's' (in Swedish both are often "sss"-sounding. The weirder one is 'e' instead of 'a' or vice verse - I'm heving problems with my kayboard typing. It's like there is some badly programmed logic in my psyche - a corrupted LUT? Time for a shrink?

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frog_jr
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Posted by frog_jr: Tue. Nov 28, 2017 - 08:07 PM
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JohanEkdahl wrote:
I'm heving problems with my kayboard typing.
Is is probably just that you need a new keyboard!wink

David (aka frog_jr)

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jerryr
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Posted by jerryr: Tue. Nov 28, 2017 - 10:08 PM
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JohanEkdahl wrote:
"what parameters are behind how the set of possible "routings" are determined?"

Back to serious things after this flippant discussion

If it's anything like the SAMD21 you can configure how the pads are used with the bits SERCOMn->CTRLA.TXPO and .RXPO 

I could give you a short example of code (not ASF) for setting up a SERCOM to talk to a GPS module

Jerry

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awneil
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Posted by awneil: Tue. Nov 28, 2017 - 10:33 PM (Reply to #9)
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jerryr wrote:
If it's anything like the SAMD21

Yes, it is.

you can configure how the pads are used with the bits SERCOMn->CTRLA.TXPO and .RXPO 

Yes, we know that - but you're missing the point.

 

The point is that only a few of all the possible permutations and combinations are actually available - so the question was how (if at all) they decided which ones to put in, and which ones to leave out.

 

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JohanEkdahl
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Posted by JohanEkdahl: Tue. Nov 28, 2017 - 10:58 PM
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jerryr wrote:
If it's anything like the SAMD21 you can configure how the pads are used with the bits SERCOMn->CTRLA.TXPO and .RXPO 

Yes, it's very much like the D21. And, yes, I know how to configure the I/O multiplexing. My initial question was a brain fart due to constipation in my synapses. ;-)

 

Thanks for the code offer, but (as noted in some thread here) I have SERCOM USARTs running just fine (like you, no ASF). Rx working both polled and interrupt driven. Will hook in circular buffers and test out interrupt driven Tx eventually, but right now I'm more eager to get the OLED Xplained extension board working. That requires SERCOM SPI (a different beast!), code to control the SSD1306 display controller which is new to me and then some font stuff too.. After that I'd like to go for SERCOM I2C since I have a separate OLED (also SSD1306) that uses I2C.

 

Both of those was what triggered a new inspection/study of the I/O multiplexing table in the data sheet, and somewhere at that point my brain went into overheating. (-:

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awneil
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Posted by awneil: Tue. Nov 28, 2017 - 11:05 PM (Reply to #11)
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Only a very limited number of pins can do I2C - so that further reduces your options for that case ...

 

Also, some of the pin assignment choices on the XPro board do seem, well - "strange" ...

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JohanEkdahl
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Posted by JohanEkdahl: Tue. Nov 28, 2017 - 11:13 PM
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awneil wrote:
Only a very limited number of pins can do I2C - so that further reduces your options for that case

I know. That was part of the reason for me re-visiting and studying the mux table in the sheet.

 

 

As of January 15, 2018, Site fix-up work has begun! Now do your part and report any bugs or deficiencies here. 

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Posted by jerryr: Wed. Nov 29, 2017 - 07:18 AM (Reply to #10)
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awneil wrote:

jerryr wrote:

Yes, we know that - but you're missing the point.

 

O dear! You're right. My only excuse is that it was late and the brain was almost in hibernation mode.

It's indeed difficult to choose what pin should do what if you want to use a lot of the available pins. Just keep shuffling until something sensible comes out

It's easier with some of the PIC (should I mention that here?) chips, you can assign a function to any pin

Jerry

 

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Posted by JohanEkdahl: Wed. Nov 29, 2017 - 10:30 AM
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jerryr wrote:
It's indeed difficult to choose what pin should do what if you want to use a lot of the available pins.
 

..and, while I might be guilty of choosing the wrong wordings and words my (follow-up) question was not even about how I should choose among the options available but rather:

 

What made Atmel choose the set of options they did? Are there any very thoughts behind it that I am missing? I assume they didn't just "sprinkle it out randomly", and knowing what thoughts/ideas/insights they had in mind might help me choosing how to multiplex things in a better way.

 

I spent the last hour yesterday doing a spread-sheet which combines most of table 7-1 from the data sheet with the pinouts of the SAM D20 Xplained board. Makes for easy sorting on whatever column you like, so I can easily get a list of what is available on the EXTn headers - ordered by n and pin within that. (I know I saw a similar spreadsheet being posted somewhere here the other day, but couldn't locate it again. Gotta love the search functionality of this site. Not... )-: )

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frog_jr
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Posted by frog_jr: Wed. Nov 29, 2017 - 01:51 PM
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JohanEkdahl wrote:
I know I saw a similar spreadsheet being posted somewhere here the other day

https://community.atmel.com/comm...

 

David (aka frog_jr)

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awneil
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Posted by awneil: Wed. Nov 29, 2017 - 02:22 PM (Reply to #16)
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You're welcome!

 

laugh

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awneil
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Posted by awneil: Wed. Nov 29, 2017 - 02:28 PM
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JohanEkdahl wrote:
Makes for easy sorting on whatever column you like, so I can easily get a list of what is available on the EXTn headers - ordered by n and pin within that

Absolutely.

 

And, as I said in the other post, it's a shame that manufacturers don't provide this information in an easily machine-digestible form - for precisely this kind of thing!

 

angry

 

Copying & pasting from the PDF is a pain, and it seems that it is more of a pain now than previously - as Adobe seem to have messed-up copying from PDF tables.

 

angry

 

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Posted by JohanEkdahl: Wed. Nov 29, 2017 - 03:33 PM
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frog_jr wrote:

JohanEkdahl wrote:
I know I saw a similar spreadsheet being posted somewhere here the other day

https://community.atmel.com/comm...

 

In a thread I started myself! [rolls eyes] Ouch..

As of January 15, 2018, Site fix-up work has begun! Now do your part and report any bugs or deficiencies here. 

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