This newer peripheral supports DMA (through EasyDMA) and should
generally be faster. Importantly for some operations: interrupts (within
255 byte buffers) will not interfere with the SPI transfer.
The nrf52 series is all very similar and copying the code only makes it
harder to maintain the code or to add more chips in the nrf52 series
(for example, the nrf52833 as used in the micro:bit v2).
This commit also has a small improvement regarding pins: it now includes
chip-level pin names (P0.00, P0.01, etc) to the machine package.
Let's use the same default frequency everywhere, for consistency.
It could be any frequency, but 4MHz is already used for other chips and
it seems like a reasonable frequency to me (not too fast for most chips
but still reasonably fast). Oh, and 4MHz is slow enough that it can be
inspected by a Saleae Logic 4 (that sadly has been discontinued).
Instead of only allowing a limited number of speeds, use the provided
speed as an upper bound on the allowed speed. The reasoning is that
picking a higher speed than requrested will likely result in malfunction
while picking a lower speed will usually only result in slower
operation.
This behavior matches the ESP32 at least.
Only some pins (notably including GPIO2 aka machine.LED) have GPIO for
the default function 1. Other pins (such as GPIO 15) had a different
function by default. Function 3 means GPIO for all the pins, so always
use that when configuring a pin to use as a GPIO pin.
In the future, the mux configuration will need to be updated for other
functions such as SPI, I2C, etc.
This ensures that stdout (println etc) keeps working in interrupts.
Generally you shouldn't print anything in an interrupt. However,
printing things for debugging is very useful and printing panic messages
can be critical when the code doesn't work for some reason.
This is only very minimal support. More support (such as tinygo flash,
or peripheral access) should be added in later commits, to keep this one
focused.
Importantly, this commit changes the LLVM repo from llvm/llvm-project to
tinygo-org/llvm-project. This provides a little bit of versioning in
case something changes in the Espressif fork. If we want to upgrade to
LLVM 11 it's easy to switch back to llvm/llvm-project until Espressif
has updated their fork.
This also fixes a bug: the Bluefruit doesn't have a low frequency
crystal. Somehow non-SoftDevice code still worked. However, the
SoftDevice won't initialize when this flag is set incorrectly.
