PIC16LF1824T39A_12 Datasheet, PDF (326/418 Pages) Microchip Technology – 20-Pin Flash Microcontrollers with XLP Technology

PIC16LF1824T39A
28.4 Frequency Selection and
Configuration
The RF transmitter is capable of generating many of
the popular RF frequencies that are permitted within
the Radio Regulations of the country the finished
product will be sold. The RF frequency configuration is
performed by determining which frequency band,
selecting the crystal frequency, and setting the
frequency value in the Frequency register DF<18:0>. If
FSK modulation is used, the frequency deviation is set
in the Application register DA<12:5>. See
Section 28.2.2 “RF Transmitter Registers” for
information on Configuration register settings.
28.4.1 BAND SELECTION
The Band bit, DA13, in the Application register
configures the RF transmitter for a range of frequencies
for a given crystal frequency, as shown in Table 28-6.
TABLE 28-6: FREQUENCY BAND SELECTION
Band Setting DA<13>
0
1
Frequency Band (f_RF)
310 - 450 MHz
312 - 450 MHz
338 - 450 MHz
863 - 870 MHz
902 - 924 MHz
863 - 870 MHz
902 - 928 MHz
Crystal Frequency (f_XTAL)
22 MHz
24 MHz
26 MHz
22 MHz
24 MHz
26 MHz
28.4.2 CRYSTAL SELECTION
Once the frequency band has been selected, the
choice of crystal frequency is flexible provided the
crystal meets the specifications summarized in
Table 28-7. The boundaries of the Frequency register
DF<18:0> are followed as shown in Figure 28-7, and
RF transmit frequency error is acceptable (see
Section 28.4.3 “Frequency Calculation”).
TABLE 28-7: CRYSTAL RESONATOR SPECIFICATIONS
Symbol
Description
Min.
Typ.
Max.
Unit
FXTAL
Crystal Frequency
22
CL
Load Capacitance
—
ESR
Equivalent Series Resistance
—
—
26
15
—
—
100
MHz
pF
Ohms
The crystal frequency tolerance and frequency stability
over the operating temperature range depends on the
system frequency budget. Typically, the receiver crystal
frequency tolerance, stability, and receiver bandwidth
will have the greatest influence. For OOK modulation,
the transmitted RF signal (f_RF) should remain inside the
receiver bandwidth, otherwise signal degradation will
occur. For FSK modulation, f_RF should remain inside
the receiver bandwidth and within 0.5 * f_DEV.
As a general practice, do not choose a RF transmit
signal (f_RF) with an integer or near integer multiple of
f_XTAL. This will result in higher noise and spurious
emissions.
28.4.3 FREQUENCY CALCULATION
Once the frequency band and crystal frequency are
selected, the RF transmit signal (f_RF) is calculated by
setting the Frequency register DF(18:0) bits according
to the formula shown in Figure 28-7. If the calculated
value for DF(18:0) is not an integer, there will be an
associated transmit frequency error. Ensure that this
error is within the acceptable system frequency budget.
Similarly, the frequency deviation is calculated as
shown in Figure 28-7.
DS41657A-page 326
Preliminary
 2012 Microchip Technology Inc.