CC2500 Datasheet, PDF (39/76 Pages) Texas Instruments – Single Chip Low Cost Low Power RF Transceiver

Overflow
margin
FIFO_THR=13
CC2500
56 bytes
RXFIFO
FIFO_THR=13
Underflow
margin
8 bytes
TXFIFO
Figure 15: Example of FIFOs at threshold
21 Frequency Programming
The frequency programming in CC2500 is
designed to minimize the programming
needed in a channel-oriented system.
To set up a system with channel numbers, the
desired channel spacing is programmed with
the
MDMCFG0.CHANSPC_M
and
MDMCFG1.CHANSPC_E registers. The channel
spacing registers are mantissa and exponent
respectively.
The base or start frequency is set by the 24 bit
frequency word located in the FREQ2, FREQ1
and FREQ0 registers. This word will typically
be set to the centre of the lowest channel
frequency that is to be used.
The desired channel number is programmed
with the 8-bit channel number register,
CHANNR.CHAN, which is multiplied by the
channel offset. The resultant carrier frequency
is given by:
( ( )) ( ) f_carrier =
f _XOSC
2¹⁶
⋅
FREQ
+ CHAN
⋅
256 + CHANSPC _ M
⋅ 2^{CHANSPC _ E−2}
With a 26 MHz crystal the maximum channel
spacing is 405 kHz. To get e.g. 1 MHz channel
spacing one solution is to use 333 kHz
channel spacing and select each third channel
in CHANNR.CHAN.
The preferred IF frequency is programmed
with the FSCTRL1.FREQ_IF register. The IF
frequency is given by:
f _IF
=
f _XOSC
2¹⁰
⋅
FREQ _
IF
Note that the SmartRF^® Studio software
automatically calculates the optimum
FSCTRL1.FREQ_IF register setting based on
channel spacing and channel filter bandwidth.
If any frequency programming register is
altered when the frequency synthesizer is
running, the synthesizer may give an
undesired response. Hence, the frequency
programming should only be updated when
the radio is in the IDLE state.
Preliminary Data Sheet (rev.1.1.) SWRS040
Page 39 of 77