So, wow, 6 SERCOMs, totally configurable - best thing ever! So what's the catch?
It's just a little one really - when setting up the SERCOM you have to configure what pins it will use (you have many options) and it takes a little time to look up the exact multiplexing setup and make sure you have it right. Not a huge deal, but has to be done right.
Multiplex Table
If you look in the ATSAMD21 datasheet, there's a big table in section 6 called Multiplexing and Considerations, and it says...
Each pin is by default controlled by the PORT as a general purpose I/O and alternatively it can be assigned to one of the peripheral functions A, B, C, D, E, F, G or H. To enable a peripheral function on a pin, the Peripheral Multiplexer Enable bit in the Pin Configuration register corresponding to that pin (PINCFGn.PMUXEN, n = 0-31) in the PORT must be written to one.
The selection of peripheral function A to H is done by writing to the Peripheral Multiplexing Odd and Even bits in the Peripheral Multiplexing register (PMUXn.PMUXE/O) in the PORT
Which basically means - each pin has different capabilities, such as digital I/O, analog input, timer/pwm output, and sercom connectivity. On most chips, there's only one pin that can act as the I2C clock pin, but on this chip, there's quite a few pins that can be selected. So the trick is figuring out which pin you want to have that capability and 'MUXing it'
Here's how to read the table, here we've got the first page:
The chip we're using is the ATSAMD21G note that the J series does have more pins so for example, pins 5-10 on the ATSAMD21J (PB04 thru PB07) don't apply to our chips.
In columns C and D, you can see the SERCOM pads available. I've also circled in blue the matching I/O Pins.
Each SERCOM has 4 possible pads: SERCOM#/PAD[x] where # can be 0 thru 5 (6 total SERCOMs), and x can be 0 thru 3 (4 total pads).
Some pins can only act as a single SERCOM's pads - for example PA00 is the first pin in the table and can only act as SERCOM1.PAD[0]
On the other hand, some pins can do dual duty: PA08 lower on the list can act as either SERCOM #0's PAD 0 or it can act as SERCOM #2's PAD 0.
Arduino's MUX Table
To map the raw pin name to the Arduino Zero 'board pin' you can read the table in variants.cpp (in the board package)
/* * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * + Pin number + ZERO Board pin | PIN | Label/Name | Comments (* is for default peripheral in use) * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | | Digital Low | | | * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | 0 | 0 -> RX | PA11 | | EIC/EXTINT[11] ADC/AIN[19] PTC/X[3] *SERCOM0/PAD[3] SERCOM2/PAD[3] TCC0/WO[3] TCC1/WO[1] * | 1 | 1 <- TX | PA10 | | EIC/EXTINT[10] ADC/AIN[18] PTC/X[2] *SERCOM0/PAD[2] TCC0/WO[2] TCC1/WO[0] * | 2 | 2 | PA14 | | EIC/EXTINT[14] SERCOM2/PAD[2] SERCOM4/PAD[2] TC3/WO[0] TCC0/WO[4] * | 3 | ~3 | PA09 | | EIC/EXTINT[9] ADC/AIN[17] PTC/X[1] SERCOM0/PAD[1] SERCOM2/PAD[1] *TCC0/WO[1] TCC1/WO[3] * | 4 | ~4 | PA08 | | EIC/NMI ADC/AIN[16] PTC/X[0] SERCOM0/PAD[0] SERCOM2/PAD[0] *TCC0/WO[0] TCC1/WO[2] * | 5 | ~5 | PA15 | | EIC/EXTINT[15] SERCOM2/PAD[3] SERCOM4/PAD[3] *TC3/WO[1] TCC0/WO[5] * | 6 | ~6 | PA20 | | EIC/EXTINT[4] PTC/X[8] SERCOM5/PAD[2] SERCOM3/PAD[2] *TCC0/WO[6] * | 7 | 7 | PA21 | | EIC/EXTINT[5] PTC/X[9] SERCOM5/PAD[3] SERCOM3/PAD[3] TCC0/WO[7] * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | | Digital High | | | * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | 8 | ~8 | PA06 | | EIC/EXTINT[6] ADC/AIN[6] AC/AIN[2] PTC/Y[4] SERCOM0/PAD[2] *TCC1/WO[0] * | 9 | ~9 | PA07 | | EIC/EXTINT[7] ADC/AIN[7] AC/AIN[3] PTC/Y[5] SERCOM0/PAD[3] *TCC1/WO[1] * | 10 | ~10 | PA18 | | EIC/EXTINT[2] PTC/X[6] +SERCOM1/PAD[2] SERCOM3/PAD[2] *TC3/WO[0] TCC0/WO[2] * | 11 | ~11 | PA16 | | EIC/EXTINT[0] PTC/X[4] +SERCOM1/PAD[0] SERCOM3/PAD[0] *TCC2/WO[0] TCC0/WO[6] * | 12 | ~12 | PA19 | | EIC/EXTINT[3] PTC/X[7] +SERCOM1/PAD[3] SERCOM3/PAD[3] TC3/WO[1] *TCC0/WO[3] * | 13 | ~13 | PA17 | LED | EIC/EXTINT[1] PTC/X[5] +SERCOM1/PAD[1] SERCOM3/PAD[1] *TCC2/WO[1] TCC0/WO[7] * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | | Analog Connector | | | * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | 14 | A0 | PA02 | A0 | EIC/EXTINT[2] *ADC/AIN[0] DAC/VOUT PTC/Y[0] * | 15 | A1 | PB08 | A1 | EIC/EXTINT[8] *ADC/AIN[2] PTC/Y[14] SERCOM4/PAD[0] TC4/WO[0] * | 16 | A2 | PB09 | A2 | EIC/EXTINT[9] *ADC/AIN[3] PTC/Y[15] SERCOM4/PAD[1] TC4/WO[1] * | 17 | A3 | PA04 | A3 | EIC/EXTINT[4] *ADC/AIN[4] AC/AIN[0] PTC/Y[2] SERCOM0/PAD[0] TCC0/WO[0] * | 18 | A4 | PA05 | A4 | EIC/EXTINT[5] *ADC/AIN[5] AC/AIN[1] PTC/Y[5] SERCOM0/PAD[1] TCC0/WO[1] * | 19 | A5 | PB02 | A5 | EIC/EXTINT[2] *ADC/AIN[10] PTC/Y[8] SERCOM5/PAD[0] * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | | Wire | | | * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | 20 | SDA | PA22 | SDA | EIC/EXTINT[6] PTC/X[10] *SERCOM3/PAD[0] SERCOM5/PAD[0] TC4/WO[0] TCC0/WO[4] * | 21 | SCL | PA23 | SCL | EIC/EXTINT[7] PTC/X[11] *SERCOM3/PAD[1] SERCOM5/PAD[1] TC4/WO[1] TCC0/WO[5] * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | |SPI (Legacy ICSP) | | | * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | 22 | 1 | PA12 | MISO | EIC/EXTINT[12] SERCOM2/PAD[0] *SERCOM4/PAD[0] TCC2/WO[0] TCC0/WO[6] * | | 2 | | 5V0 | * | 23 | 4 | PB10 | MOSI | EIC/EXTINT[10] *SERCOM4/PAD[2] TC5/WO[0] TCC0/WO[4] * | 24 | 3 | PB11 | SCK | EIC/EXTINT[11] *SERCOM4/PAD[3] TC5/WO[1] TCC0/WO[5] * | | 5 | | RESET | * | | 6 | | GND | * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | | EDBG | | | * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | 30 | | PB22 | EDBG_UART TX | *SERCOM5/PAD[2] * | 31 | | PB23 | EDBG_UART RX | *SERCOM5/PAD[3] * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | 32 | | PA22 | EDBG_SDA | Pin 20 (SDA) * | 33 | | PA23 | EDBG_SCL | Pin 21 (SCL) * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | 34 | | PA19 | EDBG_MISO | EIC/EXTINT[3] *SERCOM1/PAD[3] SERCOM3/PAD[3] TC3/WO[1] TCC0/WO[3] * | 35 | | PA16 | EDBG_MOSI | EIC/EXTINT[0] *SERCOM1/PAD[0] SERCOM3/PAD[0] TCC2/WO[0] TCC0/WO[6] * | 36 | | PA18 | EDBG_SS | EIC/EXTINT[2] *SERCOM1/PAD[2] SERCOM3/PAD[2] TC3/WO[0] TCC0/WO[2] * | 37 | | PA17 | EDBG_SCK | EIC/EXTINT[1] *SERCOM1/PAD[1] SERCOM3/PAD[1] TCC2/WO[1] TCC0/WO[7] * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | 38 | ATN | PA13 | EDBG_GPIO0 | EIC/EXTINT[13] SERCOM2/PAD[1] SERCOM4/PAD[1] *TCC2/WO[1] TCC0/WO[7] * | 39 | | PA21 | EDBG_GPIO1 | Pin 7 * | 40 | | PA06 | EDBG_GPIO2 | Pin 8 * | 41 | | PA07 | EDBG_GPIO3 | Pin 9 * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | |32.768KHz Crystal | | | * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- * | | | PA00 | XIN32 | EIC/EXTINT[0] SERCOM1/PAD[0] TCC2/WO[0] * | | | PA01 | XOUT32 | EIC/EXTINT[1] SERCOM1/PAD[1] TCC2/WO[1] * +------------+------------------+--------+-----------------+-------------------------------------------------------------------------------------------------------- */
To save me from having to constantly look it up, here's a table of all the ATSAMD21G's SERCOM MUXs:
Pin Arduino 'Pin' SERCOM SERCOM alt ----------------------------------------- PA00 Used by xtal SERCOM1.0 PA01 Used by xtal SERCOM1.1 PB08 A1 SERCOM4.0 PB09 A2 SERCOM4.1 PA04 A3 SERCOM0.0 PA05 A4 SERCOM0.1 PA06 D8 SERCOM0.2 PA07 D9 SERCOM0.3 PA08 D4 SERCOM0.0 SERCOM2.0 PA09 D3 SERCOM0.1 SERCOM2.1 PA10 D1 SERCOM0.2 SERCOM2.2 PA11 D0 SERCOM0.3 SERCOM2.3 PB10 D23 / MOSI SERCOM4.2 PB11 D24 / SCK SERCOM4.3 PA12 D22 / MISO SERCOM2.0 SERCOM4.0 PA13 Used by EDBC SERCOM2.1 SERCOM4.1 PA14 D2 SERCOM2.2 SERCOM4.2 PA15 D5 SERCOM2.3 SERCOM4.3 PA16 D11 SERCOM1.0 SERCOM3.0 PA17 D13 SERCOM1.1 SERCOM3.1 PA18 D10 SERCOM1.2 SERCOM3.2 PA19 D12 SERCOM1.3 SERCOM3.3 PA20 D6 SERCOM5.2 SERCOM3.2 PA21 D7 SERCOM5.3 SERCOM3.3 PA22 D20 / SDA SERCOM3.0 SERCOM5.0 PA23 D21 / SCL SERCOM3.1 SERCOM5.1 PA24 Used by USB SERCOM3.2 SERCOM5.2 PA25 Used by USB SERCOM3.3 SERCOM5.3 PB22 D30 / EDBG TX SERCOM5.2 PB23 D31 / EDBG RX SERCOM5.3 PA30 Used by SWCLK SERCOM1.2 PA31 Used by SWDIO SERCOM1.3 PB02 A5 SERCOM5.0 PB03 D25 / RX LED SERCOM5.1
Note that a bunch of these pins are simply *not available* if you've got a Zero or Feather M0, because they are used for programming or LEDs, or not brought out to headers in some other way, so lets remove them & sort by pin
Pin Arduino 'Pin' SERCOM SERCOM alt ----------------------------------------- PA11 D0 SERCOM0.3 SERCOM2.3 PA10 D1 SERCOM0.2 SERCOM2.2 PA14 D2 SERCOM2.2 SERCOM4.2 PA09 D3 SERCOM0.1 SERCOM2.1 PA08 D4 SERCOM0.0 SERCOM2.0 PA15 D5 SERCOM2.3 SERCOM4.3 PA20 D6 SERCOM5.2 SERCOM3.2 PA21 D7 SERCOM5.3 SERCOM3.3 PA06 D8 SERCOM0.2 PA07 D9 SERCOM0.3 PA18 D10 SERCOM1.2 SERCOM3.2 PA16 D11 SERCOM1.0 SERCOM3.0 PA19 D12 SERCOM1.3 SERCOM3.3 PA17 D13 SERCOM1.1 SERCOM3.1 PB08 A1 SERCOM4.0 PB09 A2 SERCOM4.1 PA04 A3 SERCOM0.0 PA05 A4 SERCOM0.1 PB02 A5 SERCOM5.0 PA22 D20 / SDA SERCOM3.0 SERCOM5.0 PA23 D21 / SCL SERCOM3.1 SERCOM5.1 PA12 D22 / MISO SERCOM2.0 SERCOM4.0 PB10 D23 / MOSI SERCOM4.2 PB11 D24 / SCK SERCOM4.3
Predefined SERCOMs
OK so first things we want to sort out is what SERCOMs are already taken, this tutorial will assume you won't 'bash' existing SERCOMs (altho you are free to, if you want to give up the hardware serial UART for I2C, say).
The debug hardware serial port
which is used when connecting to the Programming/Debug Port on an Arduino Zero is on SERCOM 5, pads 2 & 3. It's not used on an Adafruit Feather because there's no debug port
PB22 D30 / EDBG TX SERCOM5.2 PB23 D31 / EDBG RX SERCOM5.3
I2C (SDA & SCL)
use SERCOM 3 (pads 0 & 1)
PA22 D20 / SDA SERCOM3.0 SERCOM5.0 PA23 D21 / SCL SERCOM3.1 SERCOM5.1
The SPI port
is on SERCOM 4, and uses pads 0, 2, and 3
PA12 D22 / MISO SERCOM2.0 SERCOM4.0 PB10 D23 / MOSI SERCOM4.2 PB11 D24 / SCK SERCOM4.3
And finally, the
hardware Serial interface (e.g. Serial1)
is on SERCOM 0
PA11 D0 SERCOM0.3 SERCOM2.3 PA10 D1 SERCOM0.2 SERCOM2.2
That leaves you with:
SERCOMs 1 & 2 on an Arduino Zero
and
SERCOMs 1, 2, and 5 on a Feather M0
Available SERCOM & Pins
OK we're nearly done paring down that table to the SERCOMs that we can use and pins that are available.
SERCOM 1
Available on both Zero's and Feathers, you can use pins 10, 11, 12 and 13 for SERCOM 1
Pin Arduino 'Pin' SERCOM SERCOM alt ----------------------------------------- PA18 D10 SERCOM1.2 SERCOM3.2 PA16 D11 SERCOM1.0 SERCOM3.0 PA19 D12 SERCOM1.3 SERCOM3.3 PA17 D13 SERCOM1.1 SERCOM3.1
Pin Arduino 'Pin' SERCOM SERCOM alt ----------------------------------------- PA11 D0 SERCOM0.3 SERCOM2.3 PA10 D1 SERCOM0.2 SERCOM2.2 PA14 D2 SERCOM2.2 SERCOM4.2 PA09 D3 SERCOM0.1 SERCOM2.1 PA08 D4 SERCOM0.0 SERCOM2.0 PA15 D5 SERCOM2.3 SERCOM4.3 PA12 D22 / MISO SERCOM2.0 SERCOM4.0
but some are already used, so while you could theoretically use D0 & D1 those are already used for the Hardware Serial UART, and D22/MISO is used for SPI. So that leaves D2 thru D5
Pin Arduino 'Pin' SERCOM SERCOM alt ----------------------------------------- PA14 D2 SERCOM2.2 SERCOM4.2 PA09 D3 SERCOM0.1 SERCOM2.1 PA08 D4 SERCOM0.0 SERCOM2.0 PA15 D5 SERCOM2.3 SERCOM4.3
SERCOM 5
This one is used for programming on the Zero with EDBG interface but you can use it on the Feather M0 since only native USB is used.
Pin Arduino 'Pin' SERCOM SERCOM alt ----------------------------------------- PA20 D6 SERCOM5.2 SERCOM3.2 PA21 D7 SERCOM5.3 SERCOM3.3 PB02 A5 SERCOM5.0 PA22 D20 / SDA SERCOM3.0 SERCOM5.0 PA23 D21 / SCL SERCOM3.1 SERCOM5.1
Two of those are used by SDA/SCL which, unless you want to reuse for a different SERCOM, you'll only have:
Pin Arduino 'Pin' SERCOM SERCOM alt ----------------------------------------- PA20 D6 SERCOM5.2 SERCOM3.2 PA21 D7 SERCOM5.3 SERCOM3.3 PB02 A5 SERCOM5.0
Note that you don't get access to SERCOM5.1!
Freeing up SERCOM5
If you'd like to free up SERCOM5 on a Zero, and you won't be using the programming port (because that's how Serial data is passed back and forth) you can comment out
Uart Serial( &sercom5, PIN_SERIAL_RX, PIN_SERIAL_TX, PAD_SERIAL_RX, PAD_SERIAL_TX ) ;
and
void SERCOM5_Handler()
{
Serial.IrqHandler();
}
At the end of the variants file (you'll need to dig for where this is downloaded after you've added SAMD support, and you'll also have to re-remove after each board manager update of that package.
Note that you'll definitely not be able to use Serial anymore, you can only use USBSerial. Like I said, its a bit annoying, you may want to grab the Adafruit SAMD package, and 'pretend' like your Zero is a Feather M0, it'll work just fine, and then Serial will point to USBSerial. whee!
