Setting up a DMA transfer can take longer than sending out a buffer
byte by byte if the buffer is small.
DMA only shows advantages for large buffers, using it for every transfer
will cause a net slowdown.
Since we did not come up with a good way to determine the treshold based
on the SPI frequency, just use a fixed buffer for now so that DMA can be
used without slowing things down overall.
On samd5x only the RTC can wake the CPU from Deep Sleep (pm modes 0 & 1).
The external interrupt controller is disabled, but we can use the tamper
detection of the RTC.
If an gpio interrupt is configured on one of the five tamper detect pins,
those can be used to wake the CPU from Deep Sleep / Hibernate.
RTT module may get stuck during the init process if the RTC was running from
a previous boot. Unsure we don't remove the peripheral clock during the CPU
init if RTC is active from a previous boot and add a workaround to disable
this module as it will be re-init at some point later by the auto_init module
or by the user's application.
The MPU on the cortex-m23 has some differences with the MPU on the older
cortex-m devices. It is not implemented in the cortex-m MPU driver. This
removes the available feature as it gives a false sense of security by
advertising the feature, but implementing it with noop's
The sam0 MCUs all have a DAC peripheral.
The DAC has a resulution of 10 or 12 bits and can have one or two
output channels.
The output pins are always hard-wired to PA2 for DAC0 and PA5 for DAC1
if it exists.
On the same54-xpro I would only get a max value of ~1V when using the
internal reference, so I configured it to use an external voltage reference.
The external reference pin is hard-wired to PA3, so you'll have to connect
that to 3.3V to get results.
Also adapt the defines to the documentation
- CPUs define up to 4 power modes (from zero, the lowest power mode,
to PM_NUM_MODES-1, the highest)
- >> there is an implicit extra idle mode (which has the number PM_NUM_MODES) <<
Previously on saml21 this would always generate pm_set(3) which is an illegal state.
Now pm_layered will correctly generate pm_set(2) for IDLE modes.
Idle power consumption dropped from 750µA to 368µA and wake-up from standby is also
possible. (Before it would just enter STANDBY again as the mode register was never
written with the illegal value.)
Creating an `exti_config` array for a new MCU manually is tedious and error prone.
Luckiely all information is already availiable in the vendor files.
Credit for this discovery & method goes to @Sizurka
The file was generated with
```C
int main(void) {
puts("static const int8_t exti_config[PORT_GROUPS][32] = {");
for (unsigned port = 1; port < 5; ++port) {
printf("#if PORT_GROUPS >= %d\n{\n", port);
for (unsigned pin = 0; pin < 32; ++pin) {
printf("#ifdef PIN_P%c%02uA_EIC_EXTINT_NUM\n", '@' + port, pin);
printf(" PIN_P%c%02uA_EIC_EXTINT_NUM,\n", '@' + port, pin);
printf("#else\n -1,\n#endif\n");
}
printf("},\n#endif\n\n");
}
puts("};");
return 0;
}
```
No changes in generated code are expected, but this makes adding new members
of the sam0 CPU families much easier.
