Timing - Texas Instruments CC2500 TK Manual

Event 1 follows Event 0 after a programmed
timeout.
The time between two consecutive Event 0 is
programmed with a mantissa value given by
WOREVT1.EVENT0 and WOREVT0.EVENT0,
and an exponent
WORCTRL.WOR_RES. The equation is:
750
 
t EVENT
Event 0
f
XOSC
The Event 1 timeout is programmed with
WORCTRL.EVENT1. Figure 18 shows the
timing relationship between Event 0 timeout
and Event 1 timeout.
Figure 18: Event 0 and Event 1 Relationship
The time from the CC2500 enters SLEEP state
until the next Event 0 is programmed to
appear (t in Figure 18) should be larger
SLEEP
than 11.08 ms when using a 26 MHz crystal
and 10.67 ms when a 27 MHz crystal is used.
If t is less than 11.08 (10.67) ms there is a
SLEEP
chance that the consecutive Event 0 will occur
750

128
f
XOSC
too early. Application Note AN047 [3] explains
in detail the theory of operation and the
different registers involved when using WOR,
as well as highlighting important aspects when
using WOR mode.
19.5.1 RC Oscillator and Timing
The frequency of the low-power RC oscillator
used for the WOR functionality varies with
temperature and supply voltage. In order to
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
value set by


5 WOR _ RES
0 2
seconds
SWRS040C
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.
In applications where the radio wakes up very
often, typically several times every second, it
is possible to do the RC oscillator calibration
once and then
(WORCTRL.RC_CAL=0) to reduce the current
consumption. This requires that RC oscillator
calibration values are read from registers
and RCCTRL1_STATUS
RCCTRL0_STATUS
and written back to RCCTRL0 and RCCTRL0
respectively. If the RC oscillator calibration is
turned off it will have to be manually turned on
again if temperature and supply voltage
changes.
Refer to Application Note AN047 [3] for further
details.
19.6

Timing

The radio controller controls most timing in
CC2500 , such as synthesizer calibration, PLL
lock time and RX/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 31.2.
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
Table 28: State Transition Timing
CC2500
turn off calibration
XOSC 26 MHz
Periods Crystal
88.4 μs
2298
809 μs
~21037
88.4 μs
2298
809 μs
~21037
21.5 μs
560
9.6 μs
250
0.1 μs
2
721 μs
~18739
721 μs
~18739
Page 42 of 89