This commit implements automatic module weak links for all built-in
modules, by searching for "ufoo" in the built-in module list if "foo"
cannot be found. This means that all modules named "ufoo" are always
available as "foo". Also, a port can no longer add any other weak links,
which makes strict the definition of a weak link.
It saves some code size (about 100-200 bytes) on ports that previously had
lots of weak links.
Some changes from the previous behaviour:
- It doesn't intern the non-u module names (eg "foo" is not interned),
which saves code size, but will mean that "import foo" creates a new qstr
(namely "foo") in RAM (unless the importing module is frozen).
- help('modules') no longer lists non-u module names, only the u-variants;
this reduces duplication in the help listing.
Weak links are effectively the same as having a set of symbolic links on
the filesystem that is searched last. So an "import foo" will search
built-in modules first, then all paths in sys.path, then weak links last,
importing "ufoo" if it exists. Thus a file called "foo.py" somewhere in
sys.path will still have precedence over the weak link of "foo" to "ufoo".
See issues: #1740, #4449, #5229, #5241.
This patch add basic building blocks for nrf9P60.
It also includes a secure bootloader which forwards all
possible peripherals that are user selectable to become
non-secure. After configuring Flash, RAM and peripherals
the secure bootloader will jump to the non-secure domain
where MicroPython is placed.
The minimum size of a secure boot has to be a flash
block of 32Kb, hence why the linker scripts are
offsetting the main application this much.
The RAM offset is set to 128K, to allow for later
integration of Nordic Semiconductor's BSD socket
library which reserves the range 0x20010000 - 0x2001FFFF.
Add support for pca10059 with REPL over tinyusb USB CDC.
The board also includes a board specific module that will
recover UICR->REGOUT0 in case this has been erased.
This initial support does not preserve any existing bootloader
on the pca10090 in case this was present, and expects to use all
available flash on the device.
Add nrf-port finyusb driver files. USB CDC can be activated
by board configuration files using the MICROPY_HW_USB_CDC.
Updating BLE driver, Makefile, nrfx-glue and main.c to plug
in the tinyusb stack.
The the nrfx driver is aware of chip specific registers, while
the raw HAL abstraction is not. This driver enables use of NVMC
in non-secure domain for nrf9160.
This patch moves the check for MICROPY_PY_MACHINE_TEMP to come
before the inclusion of nrf_temp.h. The nrf_temp.h depends on
the NRF_TEMP_Type which might not be defined for all nRF devices.
It was previously not taking into account that the list of pins was sparse,
so using the wrong index. The boards/X/pins.csv was generating the wrong
data for machine.Pin.board.
As part of this fix rename the variables to make it more clear what the
list contains (only board pins).
mp_compile no longer takes an emit_opt argument, rather this setting is now
provided by the global default_emit_opt variable.
Now, when -X emit=native is passed as a command-line option, the emitter
will be set for all compiled modules (included imports), not just the
top-level script.
In the future there could be a way to also set this variable from a script.
Fixes issue #4267.
Change static LED functions to lowercase names, and trim down source code
lines for variants of MICROPY_HW_LED_COUNT. Also rename configuration for
MICROPY_HW_LEDx_LEVEL to MICROPY_HW_LEDx_PULLUP to align with global PULLUP
configuration.
- IBK-BLYST-NANO: Breakout board
- IDK-BLYST-NANO: DevKit board with builtin IDAP-M CMSIS-DAP Debug JTAG,
RGB led
- BLUEIO-TAG-EVIM: Sensor tag board (environmental sensor
(T, H, P, Air quality) + 9 axis motion sensor)
Also, the LED module has been updated to support individual base level
configuration of each LED. If set, this will be used instead of the
common configuration, MICROPY_HW_LED_PULLUP. The new configuration,
MICROPY_HW_LEDX_LEVEL, where X is the LED number can be used to set
the base level of the specific LED.
The stm32 and nrf ports already had the behaviour that they would first
check if the script exists before executing it, and this patch makes all
other ports work the same way. This helps when developing apps because
it's hard to tell (when unconditionally trying to execute the scripts) if
the resulting OSError at boot up comes from missing boot.py or main.py, or
from some other error. And it's not really an error if these scripts don't
exist.
Adding section about on how to disable use of the linker flag
-flto, by setting the LTO=0 argument to Make. Also, added a
note on recommended toolchains to use that works well with
LTO enabled.
Removing linker script for nrf52840 s140 v6.0.0 as pca10056
target board now points to the new v6.1.1. Also, removing the
entry from the download_ble_stack.sh script.
Removing linker script for nrf52832 s132 v6.0.0 as all target
boards now points to the new v6.1.1. Also, removing the entry
from the download_ble_stack.sh script.
This patch makes sure that the char_data.props is first
assigned a value before other flags are OR'd in.
Resolves compilation warning on possible unitialized variable.
This patch makes sure that advertisment data is located in
persistent static RAM memory throughout the advertisment.
Also, setting m_adv_handle to predifined
BLE_GAP_ADV_SET_HANDLE_NOT_SET value to indicate first time
usage of the handle. Upon first advertisment configuration
this will be populated with a handle value returned by the
stack (s132/s140).
After new layout of nordicsemi.com the direct links to
command line tools (nrfjprog) has changed to become dynamic.
This patch removes the old direct links to each specific OS
variant and is replaced with one single link to the download
landing page instead.
Don't exclude the Timer instance 1 entry from machine_timer_obj[] when
using soft PWM. The usage is already checked when creating the Timer,
so just create an empty entry.
If needed these parameters can be added back and made functional one at a
time. It's better to explicitly not support them than to silently allow
but ignore them.
Due to new webpages at nordicsemi.com, the download links
for Bluetooth LE stacks were broken.
This patch updates the links to new locations for the current
targets.
For s132 and s140, GAP_ADV_MAX_SIZE was currently set to
BLE_GATT_ATT_MTU_DEFAULT, which is 23. The correct value
should have been 31, but there are no define for this in
the s132/s140 header files as for s110.
Updating define in ble_drv.c to the correct value of 31.
The NRF52 define only covers nrf52832, so update the define checks
to use NRF52_SERIES to cover both nrf52832 and nrf52840.
Fixed machine_hard_pwm_instances table in modules/machine/pwm.c
This enables PWM(0) to PWM(3), RTCounter(2), Timer(3) and Timer(4),
in addition to NFC reset cause, on nrf52840.
Adding MICROPY_FATFS as makefile flag in order to explicitly
include oofatfs files to be compiled into the build.
The flag is set to 0 by default. Must be set in addition to
MICROPY_VFS and MICROPY_VFS_FAT in mpconfigport.h.
Temporarly solving the issue of
"differ from the size of original declaration [-Werror=lto-type-mismatch]
until linker is fixed in upcomming release of gcc.
Bug is reported by others, and will be fixed in next version of arm-gcc.
However, this patch makes it possible to use modmusic and modimage
with current compilers.
Alternativly, the code can be compiled with LTO=0, but uses valuable 9K
more on this already squeezed target (microbit).
Support added for s132/s140 v6 in linker scripts and boards.
Support removed for s132 v2/3/5.
Download script updated to fetch new stacks and removed the
non-supported ones.
ble_drv.c updated to only handle s110 v8, and s132/s140 v6.
ubluepy updated to continue scanning after each individual scan
report reported to the module to keep old behaviour of the
Scanner class.
This patch generalize the feather52 target to be a board without
an in-built Bluetooth stack or bootloader giving all flash memory to
micropython code.
This way the feather52 target can run any supported Bluetooth LE
stack the port supports for other nrf52832 targets. Hence, this
make Makefiles/linker scripts and BLE driver support easier
to maintain in the future.
Current adoption on top of nrfx only reads the GPIO->IN register.
In order to read back an output state, nrf_gpio_pin_out_read has
to be called.
This patch concatinate the two read functions such that, if
either IN or OUT register has a value 1 it will return this,
else 0.
Updating lib/nrfx submodule to latest version of master to get
the new GPIO API to read pin direction.
(nrfx: d37b16f2b894b0928395f6f56ca741287a31a244)
Cleaning up use of "pyb" module.
Moving the file to a new folder and updating the
makefile accordingly. New module created called
"board" to take over the functionality of the legacy
"pyb" module.
Updating outdated documentation referring to pyb.Pin,
to now point to machine.Pin.
As EasyDMA variant of SPI(M) might clock out an additional byte
in single byte transactions this patch moves the nrf52832 to
use SPI and not SPIM to get more stable data transactions.
Ref: nrf52832 rev2 errata v1.1, suggested workaround is:
"Use the SPI module (deprecated but still available) or
use the following workaround with SPIM ..."
Current nrfx SPIM driver does not contain this workaround,
and in the meanwhile moving back to SPI fixes the issue.
Also, tabbing the nrfx_config.h a bit to make it more readable.
This patch moves the check of SPI configuration before
including any SPI header files. As targets might disable SPI
support, current code ends up in including SPIM if not SPI
is configured. Hence, this is why the check whether the module is
enabled should be done before including headers.
dfu-gen .PHONY target is run unconditionally as first build
target when included, and might fail if the hex file is not
yet generated.
To prevent this, the dfu-gen and dfu-flash targets are moved
to the main Makefile and only exposed if feather52 is the
defined BOARD.
Update configuration define from
MICROPY_HW_HAS_BUILTIN_FLASH to MICROPY_MBFS.
MICROPY_MBFS will enable the builtin flash as
part of enabling the micro:bit FS.
Clang understands only -fshort-enums, not --short-enums. As
--short-enums isn't even mentioned in the gcc man page, I think this
alias exists more for backwards compatibility.
Removing unused nrf52832_512k_64k_s132_5.0.0.ld.
Adding new linker script s132_5.0.0 following new
linker script scheme.
Updating ble_drv.c to handle de-increment of
outstanding tx packets on hvx for s132 v5.
After nrfx 1.0.0 a new macro was introduced to do a common
hardware timeout. The macro function triggers a counter of
retries or a timeout in us. However, in many cases, like in
nrfx_adc.c the timeout value is set to 0, leading to a infinite
loop in mp_hal_delay_us. This patch prevents this from happening.
Path of error:
nrfx_adc.c -> NRFX_WAIT_FOR -> NRFX_DELAY_US -> mp_hal_delay_us.
This patch also opens up for all arguments to be set as positional
arguments such that an external user of the make_new function can set
provide all parameters as positional arguments.
machine/i2c already uses mp_hal_get_pin_obj which
points to pin_find function in order to locate correct
pin object to use.
The pin_find function was recently updated to also
being able to locate pins based on an integer value,
such that pin number can be used as argument to object
constructors.
This patch modfies and uniforms pin object lookup for
SPI, music and pwm.
Renaming config for enabling random module with hw
random number generator from MICROPY_PY_HW_RNG to
MICROPY_PY_RANDOM_HW_RNG to indicate which module it
is configuring.
Also, disabling the config by default in mpconfigport.h.
Adding the enable of RNG in all board configs.
Moving ifdef in modrandom, which test for the config being
set, earlier in the code. This is to prevent un-necessary
includes if not needed.
Increase the maximum number of queued notifications from 1 to 6. This
massively speeds up the NUS console - especially when printing large
amounts of text. The reason is that multiple transfers can be done in a
single connection event, in ideal cases 6 at a time.
This patch moves all nrf52 targets to use SPIM backend
for SPI which features EasyDMA. The main benefit of doing
this is to utilize the SPIM3 on nrf52840 which is
EasyDMA only peripheral.
This patch ads irq method to the pin object. Handlers
registered in the irq method will be kept as part of the
ROOT_POINTERS.
In order to resolve which pin object is the root of the
IRQ, the pin_find has been extended to also be able to
search up Pin objects based on mp_int_t pin number.
This also implies that the Pin.new API is now also supporting
creation of Pin objects based on the integer value of the
pin instead of old style mandating string name of the Pin.
All boards have been updated to use real pin number from
0-48 instead of pin_Pxx for UART/SPI and music module pins.
UART/SPI/modmusic has also been updated to use pin number
provided directly or look up the Pin object based on the
integer value of the pin (modmusic).
Pin generation has been updated to create a list of pins, where
the board/cpu dicts are now refering to an index in this list
instead of having one const declaration for each pin. This new
const table makes it possible to iterate through all pins generated
in order to locate the correct Pin object.
In order to be able to support GPIO1 port on nrf52840
the port has been removed from the Pin object.
All pins on port1 will now be incrementally on top of
the pin numbers for gpio0. Hence, Pin 1.00 will become
P32, and Pin 1.15 will become P47.
The modification is done to address the new gpio HAL
interface in nrfx, which resolves the port to be
configured base on a multiple of 32.
The patch also affects the existing devices which does
not have a second GPIO port in the way that the
port indication A and B is removed from Pin generation.
This means that the port which was earlier addressed
as PA0 is now P0, and PA31 is P31.
Also, this patch removes the gpio member which earlier
pointed to the perihperal GPIO base address. This is not
needed anymore, hence removed.
With all the variation in chips and boards it's tedious to copy and
redefine linker scripts for every option. Making linker scripts more
modular also opens up more possibilities, like enabling/disabling the
flash file system from the Makefile - or even defining it's size from a
Makefile argument (FS_SIZE=12 for a 12kB filesystem if tight on space).
Summarized this squashed PR replaces the hal/ folder in the port. This has been replaced the official
HAL layer from Nordic Semiconductor; https://github.com/NordicSemiconductor/nrfx.
A Git submodule has been added under lib/nrfx, for the nrfx dependency.
The drivers / modules has been updated to use this new HAL layer; nrfx at v1.0.0.
Also, header files and system files for nrf51/nrf52x chip variants has been deleted from the device/ folder, only keeping back the startup files written in C. All other files are now fetched from nrfx.
3 new header files in the ports/nrf/ folder has been added to configure nrfx (nrfx_config.h), logging (nrfx_log.h) and glue nrfx together with the drivers and modules from micropython (nrfx_glue.h).
The PR has been a joint effort from @aykevl (Ayke van Laethem) and @glennrub.
For reference, the commit log will be kept to get an overview of the changes done:
* ports/nrf: Initial commit for moving hal to Nordic Semiconductor BSD-3 licensed nrfx-hal.
* ports/nrf: Adding nrfx, Nordic Semiconductor BSD-3 hal layer, as git submodule checked out at lib/nrfx.
* ports/nrf/modules/machine/uart: Fixing bug which set hwfc to parity excluded, always resulting in no flow control, hence corrupted output. Also adding an extra loop on uart_tx_char to prevent any tx when any ongoing tx is in progress.
* ports/nrf/i2c: Moving I2C over to nrfx driver.
* ports/nrf/modules/machine/i2c: Alignment. Renaming print function param 'o' to 'self_in'
* ports/nrf/spi: Updating SPI machine module to use nrfx drivers.
* ports/nrf: Renaming modules/machine/rtc.c/.h to rtcounter.c/.h to not confuse the peripheral with Real-Time Clock:
* ports/nrf: Updating various files after renaming machine module RTC to RTCounter.
* ports/nrf: Renaming RTC to RTCounter in modmachine globals dict table. Also updating object type name to reflect new module name.
* ports/nrf: Fixing leftovers after renaming rtc to rtcounter.
* ports/nrf: Early untested adoption of nrfx_rtc in RTCounter. Untested.
* nrf/modules/machine/i2c: Improve keyword argument handling
* ports/nrf/modules/temp: Updating Temp machine module to use nrfx defined hal nrf_temp.h. Moving logic of BLE stack awareness to machine module.
* ports/nrf/boards/pca10040: Enable machine Temp module.
* nrf/modules/machine/rtcounter: Remove magic constants.
* ports/nrf: Adding base support for nrfx module logging. Adding option to disable logging of UART as it might log its own setup over UART while the peripheral is not yet set up. Logging of UART could make sense if other transport of log is used.
* ports/nrf: updating nrfx_log.h with more correct parenthisis on macro grouping.
* ports/nrf: Updating nrfx logging with configuration to disable logging of UART module. The pattern can be used to turn off other modules as well. However, for now UART is the only module locking itself by logging before the peripheral is configured. Logging is turned off by default, can be enabled in nrfx_config.h by setting NRFX_LOG_ENABLED=1.
* ports/nrf/modules/random: Updating modrandom to use nrfx hal for rng. Not using nrfx-driver for this peripheral as its blocking mode would do the trick on RNG. Moving softdevice aware code from legacy hal to modrandom.c.
* nrf: Enable Peripheral Resource Sharing.
This enables TWI and SPI to be enabled at the same time.
* nrf/Makefile: Define MCU sub variant (e.g. NRF51822/NRF51422)
* nrf: Port TIMER peripheral to nrfx HAL.
* nrf/modules/machine/uart: Optimize UART module
For a nRF51, this results in a size reduction of:
.text: -68 bytes
.data: -56 bytes
* nrf/modules/machine/uart: Don't use magic index numbers.
* nrf/modules/machine/uart: Fix off-by-one error.
For nrf51:
.text: -40 bytes
* nrf/modules/machine/rtcounter: Update for nrfx HAL.
* nrf/modules/machine/i2c: Reduce RAM consumption.
Reductions for the nrf51:
flash: -108 bytes
RAM: -72 bytes
* nrf/mpconfigport: Avoid unnecessary root pointers.
This saves 92 bytes of RAM.
* nrf: Support SoftDevice with nrfx HAL.
* nrf: Add NVMC peripheral (microbitfs) support.
There is no support yet for a SoftDevice.
It also fixes a potentially serious bug in start_index generation.
* nrf/modules/machine/spi: Optimize SPI peripheral.
nrf51:
text: -340 bytes
data: -72 bytes
nrf52:
text: -352 bytes
data: -108 bytes
* nrf/modules/random: Forgot to commit header file.
* nrf: Make nrfx_config.h universal for all boards.
* nrf: Use SoftDevice API for flash access when built for SD
* nrf/drivers/bluetooth: Remove legacy HAL driver includes.
These were not used anymore so can be removed.
* ports/nrf/microbit: Port microbit targets to nrfx HAL
Initial port of microbit modules to use nrfx HAL layer.
Tested display/image and modmusic on micro:bit to verify that
softpwm and ticker for nrf51 is working as expected.
Changing IRQ priority on timer to priority 2, as 1 might collide if
used side by side of SD110 BLE stack.
The patch reserves Timer1 peripheral compile time. This is not ideal
and should be resolved in seperate task.
* nrf/boards/microbit: Remove custom nrfx_config.h from microbit target, adding disablement of timer1 if softpwm is enabled.
* nrf/adc: Update ADC module to use nrfx
* nrf/modules/machine/pwm: Updating machine PWM module to use nrfx HAL driver.
examples/nrf52_pwm.py and examples/nrf52_servo.py tested on pca10040.
* nrf: Removing hal folder and boards nrf5x_hal_conf.h headers.
* nrf/nrfx_glue: Adding direct NVIC access for S110 BLE stack
If SoftDevice s110 has not yet been initialized, the IRQ will not be forwarded to
the application using the sd_nvic* function calls. Hence, direct access to cmsi
nvic functions are used instead if SoftDevice is not enabled.
* nrf/drivers/ticker: Setting IRQ priority 3 on Timer1
SoftDevice fails to initilize if Timer1 has been configured to priority
level 2 before enabling the SD. The timer is set to priority 1, higher than BLE
stack in order to provide better quality of music rendering when used with the
music module. This might be too high, time will show.
* nrf/examples: Updating ubluepy_temp after moving RTCounter to nrfx.
* nrf: delete duplicate files from device folder which can be located in nrfx/mdk.
* nrf/Makefile: Fetch system files from nrfx.
Testing on each device sub-variant to figure out which system file to
use. Reason for this is that nrf52.c is actually defining nrf52832.
Removing NRF_DEFINES parameter setting the device in use into the
same sub-variant test, as NRF52 is unique to nrf52832 when using nrfx.
Without this exclusion of -DNRF52 in compilation for nrf52840, the
device will be interpreted as a nrf52, hence nrf52832.
Also, changing name on variable SRC_NRF_HAL to SRC_NRFX_HAL to
explicitly tell the origin of the file.
* nrf: Updating device #ifdefs to be more open to non-nrf51 targets.
* nrf/modules/machine/uart: Removing second instance of UART for nrf52840 as it only has one non-DMA variant.
* nrf/device: Removing system files as these are now used from nrfx/mdk
* nrf: Moving startup files in device one level up as there is no need for deep hierarchy.
* nrf: Use NRF52_SERIES defined in nrfx/mdk/nrf.h as define value when testing for both nrf52(832) and nrf52840 variants.
* nrf/modules/machine/uart: Enable UART RX by default
Enable rx by default after intiialization of the peripheral.
Else, the nrfx driver will re-enable rx for each byte read
on uart REPL, clearing the EVENT_RXDRDY before second byte,
which again will make second byte get lost and read will get stuck.
This happens if the bytes are transmitted nrf(51) while still
processing the previous byte. Not seen on nrf52, but should
also become an issue at higher speeds.
This patch sets rx to always be enabled. Hence, not clearing the event
between read bytes, and it will be able to detect next byte recieved
upon finishing the first.
* nrf/modules/machine/timer: Fixing defines excluding Timer1 if ticker/softpwm is used.
* nrf: Switching import form mpconfigboard.h to mpconfigport.h in nrfx_config.h as mpconfigboard.h might define default values for defines not set by board specific header.
* nrf/modules/machine/i2c: nrfx integration fixes
Increasing speed to 400K.
Returning Address NACK's as MP error code; MP_ENODEV.
Returning MP_ETIMEOUT on all other error codes from TWI nrfx driver
except the ANACK.
Enabling and disabling the TWI peripheral before and after each transaction.
* nrf/examples: Updating ssd1306_mod.py to split framebuffer transfer into multiple chunks
* nrf/modules/machine/i2c: Return MP_EIO error if Data NACK occurs.
* nrf: Addressing review comments.
* nrf: Updating git submodule and users to nrfx v1.0.0.
* nrf/modules/machine/adc: Update adc module to follow v1.0.0 nrfx API.
* nrf/modules/machine/spi: Implement init and deinit functions
Extending SPI objects with a config member such that
configuration can be kept between new() and init().
Moving initialization done in new() to common init
function shared between the module functions.
If SPI is already configured, the SPI peripheral will
be uninitialized before initalized again.
Adding logic to handle initialization of polarity and
phase. As well, updating default speed to 1M from 500K.
* nrf/modules/machine: Removing unused nrfx includes in machine module header files
When compiling for microbit with LTO=0, a compiler error occurs due to
'ms' variable in the microbit_sleep function has not been initialized.
This patch initialize the variable to 0.
The nrf51x22_256k_16k_s110_8.0.0.ld had a stack size of only 1kB, which
is way too low. Additionally, the indicated _minimum_stack_size (set at
2kB for that chip) isn't respected.
This commit sets the heap end based on the stack size (heap end = RAM
end - stack size) making it much easier to configure.
Additionally, the stack/heap size of nrf52 chips has been set to a more
sane value of 8kB.
Leave it enabled by default on all targets.
This is only possible when using UART-over-BLE (NUS) instead of the
default hardware peripheral. The flash area saved is quite substantial
(about 2.2KB) so this is useful for custom builds that do not need UART.
Feather52 target which is using SD s132 v.2.0.1 cannot compile
due to variable containing RAM start address is not used.
This patch enables the correct sd_ble_enable variant for this SD.
Similar commit to this one:
6e56e6269f
When .text + .data oveflow available flash, the linker may not show an
error. This change makes sure .data is included in the size calculation.
This frees 128 bytes of .bss RAM on the nRF51, at the cost of possibly
more expensive GC cycles. Leave it as-is on the nRF52 as that chip has a
lot more RAM.
This is also done in the micro:bit:
a7544718a7/inc/microbit/mpconfigport.h (L6)
* ports/nrf/boards: Adding linker script for nrf52832 using BLE stack s132 v.5.0.0.
* ports/nrf/drivers/bluetooth: Updating makefile to add BLE_API_VERSION=4 if s132 v5.0.0 is used.
* ports/nrf/drivers/bluetooth: Updating BLE stack download script to also download S132 v5.0.0.
* ports/nrf/drivers/bluetooth: Updating ble_drv.c to handle BLE_API_VERSION=4 (s132 v5.0.0).
* ports/nrf/boards: Updating linker script for nrf52832 with s132 v.5.0.0 bluetooth stack.
* ports/nrf/drivers/bluetooth: Removing commented out code in ble_drv.c
* ports/nrf/drivers/bluetooth: Updating define of GATT_MTU_SIZE_DEFAULT for SD132v5 to be defined using the new name defined in the SD headers in a more generic way.
* ports/nrf/drivers/bluetooth: Cleaning up use of BLE_API_VERSION in the ble_drv.c. Also considering s140v6 API, so not all has been changed to >= if API version 3 and 4 in combo is used. New s140v6 will differ on these, and add a new API not compatible with the API for 3 and 4.
- Rename microbit_module_init to board_module_init0 which is the generic
board module init function.
- Add low priority callback registration of display tick handler in the
module init function.
- Rename init function to ticker_init0.
- Implement ticker_register_low_pri_callback (recycle of unused
set_low_priority_callback function which was unimplemented).
- Add support for registering 2 low pri callbacks. For now, one intended
for microbit display, and one for modmusic.
Damien George