CC1101
SWRS061H Page 51 of 98
change the signal that is output on the GDO0
pin. The default setting is to output a clock
signal with a frequency of CLK_XOSC/192.
However, to optimize performance in TX and
RX, an alternative GDO setting from the
settings found in Table 41 on page 62 should
be selected.
19.1.1 Automatic POR
A power-on reset circuit is included in the
CC1101
. The minimum requirements stated in
Table 18 must be followed for the power-on
reset to function properly. The internal power-
up sequence is completed when CHIP_RDYn
goes low. CHIP_RDYn is observed on the SO
pin after CSn is pulled low. See Section 10.1
for more details on CHIP_RDYn.
When the
CC1101
reset is completed, the chip
will be in the IDLE state and the crystal
oscillator will be running. If the chip has had
sufficient time for the crystal oscillator to
stabilize after the power-on-reset, the SO pin
will go low immediately after taking CSn low. If
CSn is taken low before reset is completed,
the SO pin will first go high, indicating that the
crystal oscillator is not stabilized, before going
low as shown in Figure 26.
Figure 26: Power-On Reset
19.1.2 Manual Reset
The other global reset possibility on
CC1101
uses the SRES command strobe. By issuing
this strobe, all internal registers and states are
set to the default, IDLE state. The manual
power-up sequence is as follows (see Figure
27):
Set SCLK = 1 and SI = 0, to avoid
potential problems with pin control mode
(see Section 11.3).
Strobe CSn low / high.
Hold CSn low and then high for at least 40
µs relative to pulling CSn low
Pull CSn low and wait for SO to go low
(CHIP_RDYn).
Issue the SRES strobe on the SI line.
When SO goes low again, reset is
complete and the chip is in the IDLE state.
CSn
SO
XOSC Stable
XOSC and voltage regulator switched on
SI
SRES
40 us
Figure 27: Power-On Reset with SRES
19.2 Crystal Control
The crystal oscillator (XOSC) is either
automatically controlled or always on, if
MCSM0.XOSC_FORCE_ON is set.
In the automatic mode, the XOSC will be
turned off if the SXOFF or SPWD command
strobes are issued; the state machine then
goes to XOFF or SLEEP respectively. This
can only be done from the IDLE state. The
XOSC will be turned off when CSn is released
(goes high). The XOSC will be automatically
turned on again when CSn goes low. The
state machine will then go to the IDLE state.
The SO pin on the SPI interface must be
pulled low before the SPI interface is ready to
be used as described in Section 10.1.
If the XOSC is forced on, the crystal will
always stay on even in the SLEEP state.
Crystal oscillator start-up time depends on
crystal ESR and load capacitances. The
electrical specification for the crystal oscillator
can be found in Section 4.4.
Note that the above reset procedure is
only required just after the power supply is
first turned on. If the user wants to reset
the
CC1101
after this, it is only necessary to
issue an SRES command strobe.
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