Previous revisions of microbit-dal would handle the case of a bonding request
when the bond table was full by attempting to purge the bonding table on
demand. This suffered two flaws:
1) Nordic's device manager does not support the purging of the bonding table
while a connection is active (including a pairing request).
2) Some devices (e.g. Android 4.4.2 take two entries in the bonding table
when paired), thus making predictions of how 'full' the table is challenging.
This patch employs a simpler replacement policy. The maximum size of the bond
table has been increased to 6 (from 4). However, if the bond table contains 4
or more entries when entering pairing mode it will be purged in advance of any
pairing requetss being initiated.
The BLE stack locks out the CPU for long periods of time when necessary. This
can be alleviated by eans of a flag that release the CPU between radio events.
This can cause issues from applications with tighter timing requirements.
This patch only enables this MUTEX when a BLE connection is active, rather
than automatically at startup. Therefore, application that do not actively use
BLE will not suffer this penalty.
Integration of new radio mode that permits simple local area multicast communication
directly between micro:bits using the device's on board radio hardware.
It should be noted that this implementation build directly on the nrf51822
hardware. It is not a BLE protocol, and must (currently) be operated with BLE
A standard on the wire configuration is defined, to promote future
interoperability between projects and languages. A lightweight
packet format is also defined and adhered to, so as to promote future
extensibility without sacrificing compatibility. The notion of groups is also
introduced, that enable multiple groups of children to work in the same area
without compromising each other's projects.
In adition to a low level packet radio driver, two higher level protocols are
defined that allow simple user datagram communication between micro:bits and
an EventBus protocol that allows the micro:bit's MessageBus to be extended to
BLE maintains metadata on its characterisitics, some of which record whether
or not a central device (phone/tablet) wishes to receive notifications when a
given BLE characteristic changes value. One such characterisitic is used as
part of a core BLE service to indicate of the BLE profile of a device has
IOS is heavily reliant on this feature, and expects the state of this metadata
to be persistent across device reboots. This patch ensures that this metadata
is always set to allow Service Changed indications to be propagated to central
MicroBitCompass initiates a calibration procedure on demand if a heading is
requested from an uncalibrated device. The BLE MagnetometerService also
attempts to retrieve a heading periodically, but it is not good behaviour to
initiate calibration as a response to this.
This patch introduces a check such that the BLE MagnetometerService will only
attempt to read heading data from a previously calibrated compass.
BLE central devices (such as mobile phones/tablets) are permitted to cache the
list of services and characteristics provided by a BLE peripheral such as the
micro:bit. This was causing compatibility problems with IOS during FOTA
flashing, as the the list of BLE services may change after flashing.
The Nordic BLE stack contains a fixed size bond table - a list of devices that
have paired with our device (in the case the micro:bit). The default mbed
wrapper around the nordic stack does not account for the case where the bond
table is full.
This patch implements basic bounds checking within the MicroBitBLEManager so
that this table cannot overrun. If a new device attempts to pair when the bond
table is full, the table is emptied prior to completing bonding.
Added configuration option in MicroBitConfig to allow the default power level
to be defined. Based on experiments undertaken by the BBC and subsequent policies defined by
the BBC's child protection panel, to is set by default it's lowest level (0),
in order to protect children's privacy.
It should be noted that this is 30dB below the device's default setting, so will
inevitably leave the device more susceptible to interference, and therefore
significantly reduce the reliability of the device. This will likely present
itself through increased packet loss, reduced thorughout, lost data and
connection drop outs. The normal BLE power level on commercial devices
translates to power level of 6.
The author has witnessed an inability to connect to a micro:bit from a
distance of 40cm under domestic conditions using a Samsung S5 mini
running Android 4.4.2.
Whilst Android devices permit explicit initiation of the BLE pairing process,
IOS based devices are only capable of implicit initiation of pairing based
on accessing a protected characteristic. Adding read capability to the
ControlPoint characteristic provides a safe and convenient way for IOS devices
to initiate pairing.
- BLUEZONE mode renamed to PairingMode (by popular request).
- Added timeout of 90 seconds before automatic device reboot.
- Move from a scroll to a print based algorithm to improve the amount of time key is displayed fully.
- Added compulsory 'click to bond' button press.
- Improved response time of tick/cross being displayed.
Changes to default BLE name. During normal operation, all micro:bits now have the same name: BBC micro:bit
When pairing mode (AKA BLUEZONE) is entered, device name details re appended as before, to give BBC micro:bit [<name>]
This is to provide a degree of anonymity to users, to make tracking of kids more difficult.
This option is now configured when the BLE stack is initialised. This ensures that SoftDevice is never
starved of CPU during periods of criticality. This does lock out the CPU for application use, but prevents
MIC failures caused by __disable_irq() operations (as found in the mbed Ticker API for example).
Minor changes to BLE profile services (cleanup prior to merge with master branch):
- Added temperature period characteristic to match design pattern of other sensors.
- Updated endinaness of MicroBitLEDService Matrix to be more intuitive.
- Cleaned up Microbit.cpp by removing unused reference to BLE services.
- Exposed serial number and naming information thr BLE DeviceInformation and API.
- Updated BLE advertisement period to be alightly more repsonsive (200ms vs 1000ms).
- BUGFIX: micro:bit name is now always generated, regardless of BLE services.
- BUGFIX: Scroll Period data length.
All message bus listeners with the micro:bit runtie are non blocking, and safe to be called
in an interrupt context. As such, these have now all been declared MESSAGE_BUS_LISTENER_IMMEDIATE
which should mitigate all unbounded event queueing effects.