Texas Instruments 3138 155 232931 - Manual (Page 43)

CC2420
SWRS041B Page 43 of 89
20 Radio control state machine
CC2420 has a built-in state machine that is
used to switch between different
operational states (modes). The change of
state is done either by using command
strobes or by internal events such as SFD
detected in receive mode.
The radio control state machine states are
shown in Figure 25. The numbers in
brackets refer to the state number
readable in the FSMSTATE status register.
Reading the FSMSTATE status register is
primarily for test / debug purposes.
Before using the radio in either RX or TX
mode, the voltage regulator and crystal
oscillator must be turned on and become
stable. The voltage regulator and crystal
oscillator start-up times are given in the
Electrical Specifications section on page
9.
The crystal oscillator is controlled by
accessing the SXOSCON / SXOSCOFF
command strobes. The XOSC16M_STABLE
bit in the status register returned during
address transfer indicates whether the
oscillator is running and stable or not (see
Table 5). This status register can be polled
when waiting for the oscillator to start.
For test purposes, the frequency
synthesizer (FS) can also be manually
calibrated and started by using the
STXCAL
command strobe register. This
will not start a transmission before a
STXON
command strobe is issued. This is
not shown in Figure 25.
Enabling transmission is done by issuing a
STXON
or STXONCCA command strobe.
Turning off RF can be accomplished by
using one of the SRFOFF or SXOSCOFF
command strobe registers.
After reset the CC2420 is in Power Down
mode. All configuration registers can then
be programmed in order to make the chip
ready to operate at the correct frequency
and mode. Due to the very fast start-up
time,
CC2420 can remain in Power Down
until a transmission session is requested.
As also described in the 4-wire Serial
Configuration and Data Interface section
on page 27, the crystal oscillator must be
running (IDLE) in order to have access to
the RAM and FIFOs.