Manual

10
AT86RF401
1424D–RKE–09/02
period. A number of registers are available to adjust the performance of the lock detec-
tor. These include lock delay and unlock delay timers as well as a cycle slip counter.
Bandgap Reference The device uses a 1.2V (nominal) bandgap reference generator to provide consistent
performance over a wide range of input supply voltages. This reference voltage is used
throughout the device.
Brown-out Protection/Low
Battery Detection
The brown-out protection and low battery detection functions consist of a voltage refer-
ence, a sampling block and an autozero comparator. The circuit’s primary operating
mode is brown-out protection.
Brown-out Protection The brown-out protection circuit detects when the level of V
DD
drops below the minimum
voltage that guarantees proper operation. The brown-out voltage for this device is typi-
cally 1.8 volts.
If a brown-out occurs, the device enters a reset state. It stays in this state until either of
the following occurs:
• The level of V
DD
increases ~0.1–0.2 volts above the brown-out voltage. This causes
the device to enter a warm reboot state.
• The level of V
DD
drops to ~0 volts, then increases above the POR level. This places
thedeviceintothe“cold start” mode of operation, identical to battery insertion.
Low Battery Detection The low battery detection feature allows the programmer to select a value for V
DD
at
which a warning is issued to the user. This warning may be utilized to activate an I/O
port, for example.
If low battery detection occurs, Bit 7 of register BL_CONFIG is set. Bit 6 of register
BL_CONFIG is used to indicate that Bit 7 is valid. It is left to the programmer to poll both
bits to ensure the potential warning is valid.
Bits 5–0 of register BL_CONFIG are used to program the low battery detect level. This
warning level is programmable between ~1.5–2.7 volts.
Note: The warning level can be set below the brown-out voltage level.
The formula for calculating the low battery detection threshold is located in Table 3.
Table 3. Low Battery Detection Threshold Formulas (V
REF
is approximately 0.7 volts)
V
DD
Falling
V
DD
Rising
bo_hyst = 1 (large hysteresis) bo_hyst = 0 (small hysteresis)
VDD
3.887 V
REF
×
1
0.887
63
---------------
× BL[5:0]+
-----------------------------------------------------------=
VDD
4.05 V
REF
×
1
0.887
63
---------------
× BL[5:0]+
-----------------------------------------------------------=
VDD
4.22 V
REF
×
1
0.887
63
---------------
× BL[5:0]+
-----------------------------------------------------------=
BL[5:0] 71 3.887
V
REF
V
DD
-------------
1–××=
BL[5:0] 71 4.05
V
REF
V
DD
-------------
1–××=
BL[5:0] 71 4.22
V
REF
V
DD
-------------
1–××=