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ItsyBitsy M4 Eagle schematic: SERCOM4 is NEOPIX DMA MOSI?
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ItsyBitsy M4 Eagle schematic: SERCOM4 is NEOPIX DMA MOSI?

by kerryveenstra on Wed Jan 16, 2019 8:30 am

Hello,

Thank you for extensive documention on the ItsyBitsy M4 as well as the support in this forum regarding the SAMD51's SERCOM blocks. The posts have been helpful.

There is a comment in quadrant B2 of the ItsyBitsy M4 Eagle schematic that says

Code: Select all | TOGGLE FULL SIZE
SERCOM0: (avail on A1, A4, A5, D2)
SERCOM1: SPI
SERCOM2: I2C
SERCOM3: UART
SERCOM4: (NEOPIX DMA MOSI)
SERCOM5: (avail on D10-D13)

Regarding SERCOM4, is this comment correct? I see a DotStar LED as part L2 in quadrant B6, but I don't see a NeoPixel LED anywhere. Also, SERCOM4 appears to be connected to either SAMD51 pins [PB08/09/10/11] or [PA13/12/14/15], which are not related to the schematic's DotStar LED.

I'm asking because I'm making a list of which pins I can use for extra UART TX pins. I understand from other posts that I probably will need to monkey with "variant" files.

So far I think I have these pins available for UART transmit data:

  • SERCOM0: available with transmit data on pin A4
  • SERCOM3: already used for UART with transmit data on TX_D1
  • SERCOM4: available with transmit data on pin A2 (or will using this interfere with other pins of the IOSET that drive the QSPI chip?)
  • SERCOM5: available with transmit data on pin D12

Is this correct, or have I misunderstood? Is SERCOM4 available, or as the comment in the schematic suggests, is it used for something else?

Thank you

kerryveenstra
 
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Re: ItsyBitsy M4 Eagle schematic: SERCOM4 is NEOPIX DMA MOSI

by adafruit_support_mike on Thu Jan 17, 2019 2:37 am

The ItsyBitsy M4 uses a DotStar (APA102) instead of a NeoPixel (WS2812). The major difference in terms of signals is that a DotStar requires DATA and CLOCK lines.

The DotStar appears in the schematic in the upper-right corner, around location 6B.

Looking at the signal mappings, it appears the notes in the comment are out of sync with the hardware. The DotStar is controlled by SERCOM5, and SERCOM4 is used for the QSPI Flash memory chip. The lines at the top are correct though. The complete list would be:

- SERCOM0 is broken out to pins A1, A4, A5, and D2.
- SERCOM1 handles SPI through MOSI, MISO, and SCK.
- SERCOM2 handles I2C through SDA and SCL.
- SERCOM3 handles Serial communication through TX and RX.
- SERCOM4 handles QSPI to the Flash Memory chip.
- SERCOM5 handles the DotStar.

It does look like some of the secondary pin options for SERCOM4 and SERCOM5 are broken out to pins at the edge of the board, but you'd have to disable the existing signal connections to use them.

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Re: ItsyBitsy M4 Eagle schematic: SERCOM4 is NEOPIX DMA MOSI

by kerryveenstra on Thu Jan 17, 2019 8:13 am

Thank you for the clarification!

kerryveenstra
 
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Re: ItsyBitsy M4 Eagle schematic: SERCOM4 is NEOPIX DMA MOSI

by westfw on Thu Jan 17, 2019 11:28 pm

- SERCOM4 handles QSPI to the Flash Memory chip.

Isn't QSPI a separate (non-sercom) interface on all the samd51s?
Unless the QSPI driver requires a sercom AND the dedicated pins, it looks to me like SERCOM4 would theoretically still be available on SCL/SDA or A2/A3
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Re: ItsyBitsy M4 Eagle schematic: SERCOM4 is NEOPIX DMA MOSI

by adafruit_support_mike on Fri Jan 18, 2019 4:58 am

Yeah, I think you're right.

I just checked the datasheet, and the QSPI module is a separate peripheral from the SERCOMs. It looks like they just mapped it to some of the same pins as SERCOM4, probably as a matter of hardware efficiency.. those pins were already set up for high-speed signals.

Good catch.. thank you!

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