Integration Manual

ANNA-B4 series - System integration manual
UBX-21000517 - R03 Contents Page 14 of 76
C1-Public
⚠ As the temperature sensor is embedded in the radio chip, any intensive processing can cause extra
heat and impact the measurement accuracy. An external sensor is required if more accurate
monitoring of the surrounding area is necessary.
2.5 Low power clock
ANNA-B4 uses a 32.768 kHz low power clock to enable different power modes: active, sleep, and
standby. For further information about power modes, see also the ANNA-B412 data sheet [6].
The clock can be generated from either of the following internal or external clock sources:
• Internal oscillator
• External crystal
• External clock source, TCXO
The u-connectXpress software automatically senses the clock input and uses the source from the
external crystal – if one is available. Otherwise, the software uses the source from the internal
oscillator. This automatic sense functionality adds some additional time during startup (~1 s).
To get the lowest possible current consumption of the ANNA-B4 module in sleep mode, an external
crystal or external clock source is needed. If an external crystal or external clock source is not
connected, the internal oscillator must be used instead. The lower accuracy of the internal oscillator
means that the module must perform a calibration at specific intervals, which causes the current
consumption to increase in sleep mode.
For further information about the different hardware options for the low-power clock source and the
implications those choices have on both the cost and performance of the ANNA-B4 module, see also
Internal oscillator, External crystal, External clock source, TCXO, and Selecting clock source.
☞ For practical guidance on how to configure the oscillator on nRF5 open CPU modules, see the
related application note [8].
☞ An external crystal is also required by some third-party software, like Wirepas Massive.
Internal oscillator
When using the internal oscillator, clock pins XL_1 and XL_2 should be connected to ground, as shown
in Figure 4.
⚠ To ensure +/-250 ppm clock stability, open-CPU application software must be configured to check
the calibration of the internal oscillator at least once every 8 seconds. This configuration is fixed
in u-connectXpress software.
☞ The use of an internal oscillator with ANNA-B4 can minimize the Bill of Materials (BOM) and reduce
the associated production costs of the end product. But, it can also increase power consumption
in standby mode.
External crystal
ANNA-B4 has two input pins for connecting an external crystal as the source for the low-power clock.
The use of an external crystal allows ANNA-B4 to run with the lowest overall power consumption.