Rainbow Electronics AT86RF230 - Manual (Page 22)

22
AT86RF230
5131A-ZIGB-06/14/06
The BATMON can be configured using the register 0x11 (BATMON). BATMON_VTH[3:0] sets the threshold
voltage. It is programmable with a resolution of 75 mV in the upper voltage range (BATMON_HR = 1) and with a
resolution of 50 mV in the lower voltage range (BATMON_HR = 0). The signal-bit BATMON_OK indicates the
current value of the battery voltage:
• If BATMON_OK is ”0”, the battery voltage is lower than the threshold voltage
• If BATMON_OK is “1”, the battery voltage is higher than the threshold voltage
Furthermore, an interrupt (IRQ7) is automatically generated when the battery voltage falls below the programmed
threshold (see control register 0x0F and 0x0E). The interrupt appears only when BATMON_OK changes from “1” to
”0”. No interrupt will be generated when:
• the battery voltage is under the default 1.8V threshold at power up (BATMON_OK was never ”1”), or
• a new threshold is set, which is above the current battery voltage (BATMON_OK remains ”0”).
After setting a new threshold, the value BATMON_OK should be read out to verify the current supply voltage value.
When the battery voltage is close to the programmed threshold voltage, noise or temporary voltage drops can
generate a lot of unwanted interrupts initiated by a toggling BATMON_OK signal. To avoid this:
• disable the IRQ7-bit in IRQ mask register after the first interrupt and treat the battery as empty, or
• set a lower threshold value after the first interrupt.
Note that the battery monitor is inactive during PON and SLEEP modes, see control register 0x01 (TRX_STATUS).
6.6. Crystal Oscillator
The crystal oscillator generates the reference frequency for the AT86RF230. All other internally-generated
frequencies in the transceiver are derived from this unique frequency. Therefore the overall system performance is
mainly based on the accuracy of this reference frequency. The external components of the crystal oscillator should
be selected carefully and the related board layout should be done meticulously.
The register 0x12 (XOSC_CTRL) provides access to the control signals of the oscillator. Basically, two operating
modes are supported. A reference frequency can be fed to the internal circuitry by using an external clock
reference or by setting up the integrated oscillator as described in Figure 6-3.
Using the internal oscillator, the oscillation frequency strongly depends on the load capacitance seen by the crystal
between the crystal pins XTAL1 and XTAL2. The total load capacitance must be equal to the specified load
capacitance CL of the crystal itself. It consists of the external capacitors CX and parasitic capacitances connected
to the XTAL nodes. In Figure 6-3, all parasitic capacitances, such as PCB stray capacitances and the pin input
capacitance, are summarized to C
PAR
. Additional internal trimming capacitors C
TRIM
are available. Any value in the
range from 0 pF to 4.8 pF with a 0.3 pF resolution is selectable using the register bits XTAL_TRIM[3:0]. To
calculate the total load capacitance, the following formula can be used CL = 0.5*(CX+C
TRIM
+C
PAR
).
The trimming capacitors provide the possibility of an easy adjustment of frequency changes caused by production
process variations or by tolerances of the external components. Note that the oscillation frequency can be reduced
only by increasing the trimming capacitance. The frequency deviation caused by one unit of C
TRIM
decreases with
increasing crystal load capacitor values.
An amplitude control circuit is included to ensure stable operation with different operating conditions and different
crystal types. A high current during the amplitude build-up phase guarantees a low start-up time. At stable
operation, the current is reduced to the amount necessary for a robust operation. This also keeps the drive level of
the crystal low.
Generally, crystals with a higher load capacitance are less sensitive to parasitic pulling effects caused by external
component variations or by variations of board and circuit parasitics. On the other hand, a larger crystal load
capacitance results in a longer start-up time and a higher steady state current consumption.