How To Clear An Interrupt Flag On Arduino Zero

Official Hardware Classic Zero
1

I'm trying to port code I have working for Unos and Megas but I've hit a roadblock on how to clear an interrupt flag on the Zero.

For the Unos and Megas I use the following line of code which I borrowed from Nick Gammons excellent article on Interrupts:

EIFR = bit (INTF1);

I've tried to decipher the datasheet for zero's chip but it's quite a different beast and I've got lost. I believe I need to write a 1 to the EXTINTx bit (for external interrupt x) in the INTFLAG register but it's a four byte register which requires the use of offsets which is where I get lost. Any help would be much appreciated.

Thanks & regards,

2

To set (enable) an external interrupt on a given port pin, in this example digital pin 7, which is port pin PA21 or external interrupt EXTINT[5], (SAMD21 datasheet, Table 6.1 PORT Function Multiplexing):

REG_EIC_INTENSET = EIC_INTENSET_EXTINT5;     // Set EXTINT5

...likewise to clear (disable) the interrupt:

REG_EIC_INTENCLR = EIC_INTENCLR_EXTINT5;     // Clear EXTINT5

To clear the interrupt flag after it's been set:

REG_EIC_INTFLAG = EIC_INTFLAG_EXTINT5;     // Clear the EXTINT5 flag

Before using interrupts on the SAMD21 you'll also have to enable the port multiplexer for that pin, in this case digital pin 7:

PORT->Group[g_APinDescription[7].ulPort].PINCFG[g_APinDescription[7].ulPin].bit.PMUXEN = 1;

... and switch the multiplexer to peripheral A (EIC), again for digital pin 7:

PORT->Group[g_APinDescription[6].ulPort].PMUX[g_APinDescription[6].ulPin >> 1].reg |= PORT_PMUX_PMUXO_A;

Note that the SAMD21's multiplexer registers are arranged as odd and even pairs. There are 16 multiplexer registers for the 32 port pins on Port A. Digital pin 6 is port pin PA20, which is even, while digital pin 7 is PA21, which is odd. The >> 1 is essentially divide by 2, so we're essentially addressing multiplexer register 10 (PA20/2) for the PA20 and PA21 pair.

1 Like
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Thanks very much, MartinL.

4

Hey @MartinL how do you figure Digital pin 7 goes to PA21, from the Schematic it looks like PA21 goes to the ATN jumper. Am I missing somthing?

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lpaulson:
Hey @MartinL how do you figure Digital pin 7 goes to PA21, from the Schematic it looks like PA21 goes to the ATN jumper. Am I missing somthing?

Check the variant files of the board, where every pin number are mapped.

6

That schematic is wrong. That's an older version of the schematic from before Arduino.cc and Arduino.org split, and matches the M0 PRO by Arduino.org.

If you check pins 2 and 4, you will see those are reversed as well.

Adafruit has the correct schematics here:

I guess they posted the Eagle schematic because it is easier for beginners to read than the schematic from what I assume is Altium, but they really should have updated it to be accurate.