stm32lib now provides system_stm32XXxx.c source files for all MCU variants,
which includes SystemInit and prescaler tables. Since these are quite
standard and don't need to be changed, switch to use them instead of custom
variants, making the start-up code cleaner.
The SystemInit code in stm32lib was checked and is equivalent to what is
removed from the stm32 port in this commit.
Without this you often don't get any DNS server from your network provider.
Additionally, setting your own DNS _does not work_ without this option set
(which could be a bug in the PPP stack).
This is a start to make a more consistent machine.RTC class across ports.
The stm32 pyb.RTC class at least has the datetime() method which behaves
the same as esp8266 and esp32, and with this patch the ntptime.py script
now works with stm32.
If both FS and HS USB peripherals are enabled for a board then the active
one used for the REPL will now be auto-detected, by checking to see if both
the DP and DM lines are actively pulled low. By default the code falls
back to use MICROPY_HW_USB_MAIN_DEV if nothing can be detected.
When going out of memory-mapped mode to do a control transfer to the QSPI
flash, the MPU settings must be changed to forbid access to the memory
mapped region. And any ongoing transfer (eg memory mapped continuous read)
must be aborted.
The Cortex-M7 CPU will do speculative loads from any memory location that
is not explicitly forbidden. This includes the QSPI memory-mapped region
starting at 0x90000000 and with size 256MiB. Speculative loads to this
QSPI region may 1) interfere with the QSPI peripheral registers (eg the
address register) if the QSPI is not in memory-mapped mode; 2) attempt to
access data outside the configured size of the QSPI flash when it is in
memory-mapped mode. Both of these scenarios will lead to issues with the
QSPI peripheral (eg Cortex bus lock up in scenario 2).
To prevent such speculative loads from interfering with the peripheral the
MPU is configured in this commit to restrict access to the QSPI mapped
region: when not memory mapped the entire region is forbidden; when memory
mapped only accesses to the valid flash size are permitted.
When compiled with hard float the system should enable FP access when it
starts or else FP instructions lead to a fault. But this minimal port does
not enable (or use) FP and so, to keep it minimal, switch to use soft
floating point. (This became an issue due to the recent commit
34c04d2319 which saves/restores FP registers
in the NLR state.)
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.
Commit 9e68eec8ea introduced a regression
where the PID of the USB device would be 0xffff if the default value was
used. This commit fixes that by using a signed int type.
Entering a bootloader (ST system bootloader, or custom mboot) from software
by directly branching to it is not reliable, and the reliability of it
working can depend on the peripherals that were enabled by the application
code. It's also not possible to branch to a bootloader if the WDT is
enabled (unless the bootloader has specific provisions to feed the WDT).
This patch changes the way a bootloader is entered from software by first
doing a complete system reset, then branching to the desired bootloader
early on in the start-up process. The top two words of RAM (of the stack)
are reserved to store flags indicating that the bootloader should be
entered after a reset.
WIFI_REASON_AUTH_FAIL does not necessarily mean the password is wrong, and
a wrong password may not lead to a WIFI_REASON_AUTH_FAIL error code. So to
improve reliability connecting to a WLAN always reconnect regardless of the
error.
This updates ESP IDF to use v3.3-beta3. And also adjusts README.md to
point to stable docs which provide a link to download the correct toolchain
for this IDF version, namely 1.22.0-80-g6c4433a-5.2.0
Previously the end of the heap was the start (lowest address) of the stack.
With the changes in this commit these addresses are now independent,
allowing a board to place the heap and stack in separate locations.
With this the user can select multiple logical units to expose over USB MSC
at once, eg: pyb.usb_mode('VCP+MSC', msc=(pyb.Flash(), pyb.SDCard())). The
default behaviour is the original behaviour of just one unit at a time.
Eventually these responses could be filled in by a function to make their
contents dynamic, depending on the attached logical units. But for now
they are fixed, and this patch fixes the MODE SENSE(6) responses so it is
the correct length with the correct header.
SCSI can support multiple logical units over the one interface (in this
case over USBD MSC) and here the MSC code is reworked to support this
feature. At this point only one LU is used and the behaviour is mostly
unchanged from before, except the INQUIRY result is different (it will
report "Flash" for both flash and SD card).
To use it a board should define MICROPY_PY_USSL=1 and MICROPY_SSL_MBEDTLS=1
at the Makefile level. With the provided configuration it adds about 64k
to the build.
It doesn't work to tie the polling of an underlying NIC driver (eg to check
the NIC for pending Ethernet frames) with its associated lwIP netif. This
is because most NICs are implemented with IRQs and don't need polling,
because there can be multiple lwIP netif's per NIC driver, and because it
restricts the use of the netif->state variable. Instead the NIC should
have its own specific way of processing incoming Ethernet frame.
This patch removes this generic NIC polling feature, and for the only
driver that uses it (Wiznet5k) replaces it with an explicit call to the
poll function (which could eventually be improved by using a proper
external interrupt).
This adds support for SD cards using the ESP32's built-in hardware SD/MMC
host controller, over either the SDIO bus or SPI. The class is available
as machine.SDCard and using it can be as simple as:
uos.mount(machine.SDCard(), '/sd')
If the board-pin name is left empty then only the cpu-pin name is used, eg
",PA0". If the board-pin name starts with a hyphen then it's available as
a C definition but not in the firmware, eg "-X1,PA0".
The patch solves the problem where multiple Timer objects (e.g. multiple
Timer(0) instances) could initialise multiple handles to the same internal
timer. The list of timers is now maintained not for "active" timers (where
init is called), but for all timers created. The timers are only removed
from the list of timers on soft-reset (machine_timer_deinit_all).
Fixes#4078.
The board config option MICROPY_HW_USB_ENABLE_CDC2 is now changed to
MICROPY_HW_USB_CDC_NUM, and the latter should be defined to the maximum
number of CDC interfaces to support (defaults to 1).
Set the active MPU region to the actual size of SDRAM configured and
invalidate the rest of the memory-mapped region, to prevent errors due to
CPU speculation. Also update the attributes of the SDRAM region as per ST
recommendations, and change region numbers to avoid conflicts elsewhere in
the codebase (see eth usage).
I2C can't be enabled in prj_base.conf because it's a board-specific
feature. For example, if a board doesn't have I2C but CONFIG_I2C=y then
the build will fail (on Zephyr build system side). The patch here gets the
qemu_cortex_m3 build working again.
This enables going back to previous wrapped lines using backspace or left
arrow: instead of just sticking to the beginning of a line, the cursor will
move a line up.
On MCUs that have an I2C TIMINGR register, this can now be explicitly set
via the "timingr" keyword argument to the I2C constructor, for both
machine.I2C and pyb.I2C. This allows to configure precise timing values
when the defaults are inadequate.
Previously the hardware I2C timeout was hard coded to 50ms which isn't
guaranteed to be enough depending on the clock stretching specs of the I2C
device(s) in use.
This patch ensures the hardware I2C implementation honors the existing
timeout argument passed to the machine.I2C constructor. The default
timeout for software and hardware I2C is now 50ms.
Recent gcc versions (at least 9.1) give a warning about using "sp" in the
clobber list. Such code is removed by this patch. A dedicated function is
instead used to set SP and branch to the bootloader so the code has full
control over what happens.
Fixes issue #4785.
This also fixes deleting the PPP task, since eTaskGetState() never returns
eDeleted.
A limitation with this patch: once the PPP is deactivated (ppp.active(0))
it cannot be used again. A new PPP instance must be created instead.
This allows figuring out the number of bytes in the memoryview object as
len(memview) * memview.itemsize.
The feature is enabled via MICROPY_PY_BUILTINS_MEMORYVIEW_ITEMSIZE and is
disabled by default.
The original code called setsockopt(SO_RCVTIMEO/SO_SNDTIMEO) with NULL
timeout structure argument, which is an illegal usage of that function.
The old code also didn't validate the return value of setsockopt, missing
the bug completely.
Before this change, if the USB was reconnected it was possible that some
characters in the TX buffer were retransmitted because tx_buf_ptr_out and
tx_buf_ptr_out_shadow were reset while tx_buf_ptr_in wasn't. That
behaviour is fixed here by retaining the TX buffer state across reconnects.
Fixes issue #4761.
The new function factory_reset_make_files() populates the given filesystem
with the default factory files. It is defined with weak linkage so it can
be overridden by a board.
This commit also brings some minor user-facing changes:
- boot.py is now no longer created unconditionally if it doesn't exist, it
is now only created when the filesystem is formatted and the other files
are populated (so, before, if the user deleted boot.py it would be
recreated at next boot; now it won't be).
- pybcdc.inf and README.txt are only created if the board has USB, because
they only really make sense if the filesystem is exposed via USB.
It's more common to need non-blocking behaviour when reading from a UART,
rather than having a large timeout like 1000ms (the original behaviour).
With a large timeout it's 1) likely that the function will read forever if
characters keep trickling it; or 2) the function will unnecessarily wait
when characters come sporadically, eg at a REPL prompt.
- 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 alternate function pin allocations are different to other NUCLEO-144
boards. This is because the STM32F413 has a very high peripheral count:
10x UART, 5x SPI, 3x I2C, 3x CAN. The pinout was chosen to expose all
these devices on separate pins except CAN3 which shares a pin with UART1
and SPI1 which shares pins with DAC.
Includes:
- Support for CAN3.
- Support for UART9 and UART10.
- stm32f413xg.ld and stm32f413xh.ld linker scripts.
- stm32f413_af.csv alternate function mapping.
- startup_stm32f413xx.s because F413 has different interrupt vector table.
- Memory configuration with: 240K filesystem, 240K heap, 16K stack.
This patch makes pllvalues.py generate two tables: one for when HSI is used
and one for when HSE is used. The correct table is then selected at
compile time via the existing MICROPY_HW_CLK_USE_HSI.
When building with link time optimization enabled it is possible both
gc_collect() and gc_collect_regs_and_stack() get inlined into gc_alloc()
which can result in the regs variable being pushed on the stack earlier
than some of the registers. Depending on the calling convention, those
registers might however contain pointers to blocks which have just been
allocated in the caller of gc_alloc(). Then those pointers end up higher on
the stack than regs, aren't marked by gc_collect_root() and hence get
sweeped, even though they're still in use.
As reported in #4652 this happened for in 32-bit msvc release builds:
mp_lexer_new() does two consecutive allocations and the latter triggered a
gc_collect() which would sweep the memory of the first allocation again.
The machine.WDT() now accepts the "timeout" keyword argument to select the
WDT interval. And the WDT is changed to panic mode which means it will
reset the device if the interval expires (instead of just printing an error
message).
On the STM32F722 (at least, but STM32F767 is not affected) the CK48MSEL bit
must be deselected before PLLSAION is turned off, or else the 48MHz
peripherals (RNG, SDMMC, USB) may get stuck without a clock source.
In such "lock up" cases it seems that these peripherals are still being
clocked from the PLLSAI even though the CK48MSEL bit is turned off. A hard
reset does not get them out of this stuck state. Enabling the PLLSAI and
then disabling it does get them out. A test case to see this is:
import machine, pyb
for i in range(100):
machine.freq(122_000000)
machine.freq(120_000000)
print(i, [pyb.rng() for _ in range(4)])
On occasion the RNG will just return 0's, but will get fixed again on the
next loop (when PLLSAI is enabled by the change to a SYSCLK of 122MHz).
Fixes issue #4696.
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.
This patch makes the DAC driver simpler and removes the need for the ST
HAL. As part of it, new helper functions are added to the DMA driver,
which also use direct register access instead of the ST HAL.
Main changes to the DAC interface are:
- The DAC uPy object is no longer allocated dynamically on the heap,
rather it's statically allocated and the same object is retrieved for
subsequent uses of pyb.DAC(<id>). This allows to access the DAC objects
without resetting the DAC peripheral. It also means that the DAC is only
reset if explicitly passed initialisation parameters, like "bits" or
"buffering".
- The DAC.noise() and DAC.triangle() methods now output a signal which is
full scale (previously it was a fraction of the full output voltage).
- The DAC.write_timed() method is fixed so that it continues in the
background when another peripheral (eg SPI) uses the DMA (previously the
DAC would stop if another peripheral finished with the DMA and shut the
DMA peripheral off completely).
Based on the above, the following backwards incompatibilities are
introduced:
- pyb.DAC(id) will now only reset the DAC the first time it is called,
whereas previously each call to create a DAC object would reset the DAC.
To get the old behaviour pass the bits parameter like: pyb.DAC(id, bits).
- DAC.noise() and DAC.triangle() are now full scale. To get previous
behaviour (to change the amplitude and offset) write to the DAC_CR (MAMP
bits) and DAC_DHR12Rx registers manually.
If MICROPY_HW_RTC_USE_BYPASS is enabled the RTC startup goes as follows:
- RTC is started with LSE in bypass mode to begin with
- if that fails to start (after a given timeout) then LSE is reconfigured
in non-bypass
- if that fails to start then RTC is switched to LSI
Use uos.dupterm for REPL configuration of the main USB_VCP(0) stream on
dupterm slot 1, if USB is enabled. This means dupterm can also be used to
disable the boot REPL port if desired, via uos.dupterm(None, 1).
For efficiency this adds a simple hook to the global uos.dupterm code to
work with streams that are known to be native streams.
These macros are unused, and they can conflict with other entities by the
same name. If needed they can be provided as static inline functions, or
just functions.
Fixes issue #4559.
Adds support for hardware i2c to the zephyr port. Similar to other ports
such as stm32 and nrf, we only implement the i2c protocol functions
(readfrom and writeto) and defer memory operations (readfrom_mem,
readfrom_mem_into, and writeto_mem) to the software i2c implementation.
This may need to change in the future because zephyr is considering
deprecating its i2c_transfer function in favor of i2c_write_read; in this
case we would probably want to implement the memory operations directly
using i2c_write_read.
Tested with the accelerometer on frdm_k64f and bbc_microbit boards.
For gpio_hold_en() to work properly (not draw additional current) pull
up/down must be disabled when hold is enabled. This patch makes sure this
is the case by reworking the pull constants to be a bit mask.
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.
Previously specifying None as the pull value would leave the pull up/down
state unchanged. This change makes it so -1 leaves the state unchanged and
None makes the pin float, as per the docs.
In this port JavaScript is the underlying "machine" and MicroPython is
transmuted into JavaScript by Emscripten. MicroPython can then run under
Node.js or in the browser.
Functions in these files may be needed when certain features are enabled
(eg dual core mode), even if the linker does not give a warning or error
about unresolved symbols.
During make, makemoduledefs.py parses the current builds c files for
MP_REGISTER_MODULE(module_name, obj_module, enabled_define)
These are used to generate a header with the required entries for
"mp_rom_map_elem_t mp_builtin_module_table[]" in py/objmodule.c
To use HSI instead of HSE define MICROPY_HW_CLK_USE_HSI as 1 in the board
configuration file. The default is to use HSE.
HSI has been made the default for the NUCLEO_F401RE board to serve as an
example, and because early revisions of this board need a hardware
modification to get HSE working.
This demonstrates how to use external QSPI flash in XIP (execute in place)
mode. The default configuration has all extmod/ code placed into external
QSPI flash, but other code can easily be put there by modifying the custom
f769_qspi.ld script.
A board can now use the make variables TEXT0_SECTIONS and TEXT1_SECTIONS to
specify the linker sections that should go in its firmware. Defaults are
provided which give the existing behaviour.
esp_wifi_connect will return ESP_OK for the normal path of execution which
just means the reconnect is started, not that it is actually reconnected.
In such a case wifi.isconnected() should return False until the
reconnection is complete. After reconnect a GOT_IP event is called and it
will change wifi_sta_connected back to True.
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.
Currently all usages of mp_hal_pin_config_alt_static() set the pin speed to
"high" (50Mhz). The SDRAM interface typically runs much faster than this
so should be set to the maximum pin speed.
This commit adds mp_hal_pin_config_alt_static_speed() which allows setting
the pin speed along with the other alternate function details.
A few RTC constants weren't being parsed properly due to whitespace
differences, and this patch makes certain whitespace optional. Changes
made:
- allow for no space between /*!< and EXTI, eg for:
__IO uint32_t IMR; /*!<EXTI Interrupt mask register, Address offset: 0x00 */
- allow for no space between semicolon and start of comment, eg for:
__IO uint32_t ALRMASSR;/*!< RTC alarm A sub second register, Address offset: 0x44 */
As mentioned in #4450, `websocket` was experimental with a single intended
user, `webrepl`. Therefore, we'll make this change without a weak
link `websocket` -> `uwebsocket`.
If opening of /dev/mem has failed an `OSError` is appropriately raised, but
the next time `mem8/16/32` is accessed the invalid file descriptor is used
and the program gets a SIGSEGV.
Replaces "PYB: soft reboot" with "MPY: soft reboot", etc.
Having a consistent prefix across ports reduces the difference between
ports, which is a general goal. And this change won't break pyboard.py
because that tool only looks for "soft reboot".
Adds support for 3 Cortex-M boards, selectable via "BOARD" in the Makefile:
- microbit, Cortex-M0 via nRF51822
- netduino2, Cortex-M3 via STM32F205
- mps2-an385, Cortex-M3 via FPGA
netduino2 is the default board because it's supported by older qemu
versions (down to at least 2.5.0).
Instead of checking each callback (currently storage and dma) explicitly
for each SysTick IRQ, use a simple circular function table indexed by the
lower bits of the millisecond tick counter. This allows callbacks to be
easily enabled/disabled at runtime, and scales well to a large number of
callbacks.
This is a good board to demonstrate the use of Mboot because it only has a
USB HS port exposed so the native ST DFU mode cannot be used. With Mboot
this port can be used.
If a custom bootloader is enabled (eg mboot) then machine.bootloader() will
now enter that loader. To get the original ST DFU loader pass any argument
to the function, like machine.bootloader(1).
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.
Python defines warnings as belonging to categories, where category is a
warning type (descending from exception type). This is useful, as e.g.
allows to disable warnings selectively and provide user-defined warning
types. So, implement this in MicroPython, except that categories are
represented just with strings. However, enough hooks are left to implement
categories differently per-port (e.g. as types), without need to patch each
and every usage.
With clock bypass enabled the attached SD card is clocked at the maximum
48MHz. But some SD cards are unreliable at these rates. Although it's
nice to have high speed transfers it's more important that the transfers
are reliable for all cards. So disable this clock bypass option.
This way the UART REPL does not need the MicroPython heap and exists
outside the MicroPython runtime, allowing characters to still be received
during a soft reset.
Auto-detection of the crystal frequency is convenient and allows for a
single binary for many different boards. But it can be unreliable in
certain situations so in production, for a given board, it's recommended to
configure the correct fixed frequency.
Configuration for the build is now specified using sdkconfig rather than
sdkconfig.h, which allows for much easier configuration with defaults from
the ESP IDF automatically applied. sdkconfig.h is generated using the new
ESP IDF kconfig_new tool written in Python. Custom configuration for a
particular ESP32 board can be specified via the make variable SDKCONFIG.
The esp32.common.ld file is also now generated using the standard ESP IDF
ldgen.py tool.
When the ESP IDF builds a project it puts all separate components into
separate .a library archives. And then the esp32.common.ld linker script
references these .a libraries by explicit name to put certain object files
in iRAM.
This patch does a similar thing for the custom build system used here,
putting all IDF .o's into their respective .a. So a custom linker script
is no longer needed.
ISR's no longer need to be in iRAM, and the ESP IDF provides an option to
specify that they are in iRAM if an application needs lower latency when
handling them. But we don't use this feature for user interrupts: both
timer and gpio ISR routines are registered without the ESP_INTR_FLAG_IRAM
option, and so the scheduling code no longer needs to be in iRAM.
The new compile-time option is MICROPY_HW_USB_MAX_POWER_MA. Set this in
the board configuration file to the maximum current in mA that the board
will draw over USB. The default is 500mA.
The new compile-time option is MICROPY_HW_USB_SELF_POWERED. Set this
option to 1 in the board configuration file to indicate that the USB device
is self powered. This option is disabled by default (previous behaviour).
It can be that LSEON and LSERDY are set yet the RTC is not enabled (this
can happen for example when coming out of the ST DFU mode on an F405 with
the RTC not previously initialised). In such a case the RTC is never
started because the code thinks it's already running. This patch fixes
this case by always checking if RTCEN is set when booting up (and also
testing for a valid RTCSEL value in the case of using an LSE).
One can't use pthread calls in a signal handler because they are not
async-signal-safe (see man signal-safety). Instead, sem_post can be used
to post from within a signal handler and this should be more efficient than
using a busy wait loop, waiting on a volatile variable.
This aligns more closely with the hardware, that there are two, fixed HW
SPI peripherals. And it allows to recreate the HW SPI objects without
error, as well as create them again after a soft reset.
Fixes issue #4103.
In order to suit the more common 800KHz by default (instead of 400KHz), and
also have the same behaviour as the esp8266 port.
Resolves#4396.
Note! This is a breaking change. Anyone that has previously used the
NeoPixel class on an ESP32 board may be affected.
The original behaviour of open-drain-high was to use the open-drain mode of
the GPIO pin, and this seems to make driving a DHT more reliable. See
issue #4233.
The ESP IDF system already provides a math library, and that one is likely
to be better tuned to the Xtensa architecture. The IDF components are also
tested against its own math library, so best not to override it. Using the
system provided library also allows to easily switch to double-precision
floating point by changing MICROPY_FLOAT_IMPL to MICROPY_FLOAT_IMPL_DOUBLE.
So that the user can explicitly deactivate UART(0) if needed. See
issue #4314.
This introduces some risk to "brick" the device, if the user disables the
REPL without providing an alternative REPL (eg WebREPL), or any way to
reenable it. In such a case the device needs to be erased and
reprogrammed. This seems unavoidable, given the desire to have the option
to use the UART for something other than the REPL.
Without the static qualifier these objects will be kept by the linker even
if they are unused. So this patch saves some RAM when these features are
unused by a board.
If there are many short reads to a socket in a row (eg by readline) then
releasing and acquiring the GIL each time will give very poor throughput.
So first poll the socket to see if it has data, and if it does then don't
release the GIL.
Otherwise, if multiple threads are active, printing data to the REPL may be
very slow because in some cases only one character is output per call to
mp_hal_stdout_tx_strn.
On MCUs other than F4 the ORE (overrun error) flag needs to be cleared
independently of clearing RXNE, even though both are wired to trigger the
same RXNE IRQ. In the case that an overrun occurred it's necessary to
explicitly clear the ORE flag or else the RXNE interrupt will keep firing.
Otherwise IRQs may not be enabled for the user UART.irq() handler. In
particular this fixes the user IRQ_RXIDLE interrupt so that it triggers
even when there is no RX buffer.
Damien George