CC2500 Datasheet, PDF (37/76 Pages) Texas Instruments – Single Chip Low Cost Low Power RF Transceiver

CC2500
keep the frequency as accurate as possible,
the RC oscillator will be calibrated whenever
possible, which is when the XOSC is running
and the chip is not in the SLEEP state. When
the power and XOSC is enabled, the clock
used by the WOR timer is a divided XOSC
clock. When the chip goes to the sleep state,
the RC oscillator will use the last valid
calibration result. The frequency of the RC
oscillator is locked to the main crystal
frequency divided by 750.
19.6 Timing
The radio controller controls most timing in
CC2500, such as synthesizer calibration, PLL
lock and RT/TX turnaround times. Timing from
IDLE to RX and IDLE to TX is constant,
dependent on the auto calibration setting.
RX/TX and TX/RX turnaround times are
constant. The calibration time is constant
18739 clock periods. Table 28 shows timing in
crystal clock cycles for key state transitions.
Power on time and XOSC start-up times are
variable, but within the limits stated in Table 7.
Note that in a frequency hopping spread
spectrum or a multi-channel protocol the
calibration time can be reduced from 721 µs to
approximately 150 µs. This is explained in
Section 30.2.
Description
Idle to RX, no calibration
Idle to RX, with calibration
Idle to TX/FSTXON, no calibration
Idle to TX/FSTXON, with calibration
TX to RX switch
RX to TX switch
RX or TX to IDLE, no calibration
RX or TX to IDLE, with calibration
Manual calibration
XOSC
periods
2298
~21037
2298
~21037
560
250
2
~18739
~18739
26 MHz
crystal
88.4 µs
809 µs
88.4 µs
809 µs
21.5 µs
9.6 µs
0.1 µs
721 µs
721 µs
Table 28: State transition timing
19.7 RX Termination Timer
CC2500 has optional functions for automatic
termination of RX after a programmable time.
The main use for this functionality is wake-on-
radio (WOR), but it may be useful for other
applications. The termination timer starts when
in RX state. The timeout is programmable with
the MCSM2.RX_TIME setting. When the timer
expires, the radio controller will check the
condition for staying in RX; if the condition is
not met, RX will terminate. After the timeout,
the condition will be checked continuously.
The programmable conditions are:
• MCSM2.RX_TIME_QUAL=0:
Continue
receive if sync word has been found
• MCSM2.RX_TIME_QUAL=1:
Continue
receive if sync word has been found or
preamble quality is above threshold (PQT)
If the system can expect the transmission to
have started when enabling the receiver, the
MCSM2.RX_TIME_RSSI function can be used.
The radio controller will then terminate RX if
the first valid carrier sense sample indicates
no carrier (RSSI below threshold). See Section
17.4 on page 31 for details on Carrier Sense.
For OOK modulation, lack of carrier sense is
only considered valid after eight symbol
periods. Thus, the MCSM2.RX_TIME_RSSI
function can be used in OOK mode when the
distance between “1” symbols is 8 or less.
If RX terminates due to no carrier sense when
the MCSM2.RX_TIME_RSSI function is used,
or if no sync word was found when using the
MCSM2.RX_TIME timeout function, the chip
will always go back to IDLE if WOR is disabled
and back to SLEEP if WOR is enabled.
Otherwise, the MCSM1.RXOFF_MODE setting
determines the state to go to when RX ends.
Note that in wake-on-radio (WOR) mode, the
WOR state is cleared in the latter case. This
means that the chip will not automatically go
back to SLEEP again, even if e.g. the address
field in the packet did not match. It is therefore
recommended to always wake up the
microcontroller on sync word detection when
using WOR mode. This can be done by
selecting output signal 6 (see Table 33 on
page 46) on one of the programmable GDO
output pins, and programming the
microcontroller to wake up on an edge-
triggered interrupt from this GDO pin.
Preliminary Data Sheet (rev.1.1.) SWRS040
Page 37 of 77