RFPIC12F675 MICROCHIP [Microchip Technology], RFPIC12F675 Datasheet - Page 52

• Manufacturer Part Number: RFPIC12F675
• Description: 20-Pin FLASH-Based 8-Bit CMOS Microcontroller with UHF ASK/FSK Transmitter
• Manufacturer: MICROCHIP [Microchip Technology]

rfPIC12F675
9.3
The transmitter crystal oscillator is a Colpitts oscillator
that provides the reference frequency to the PLL. It is
independent of the microcontroller oscillator. An
external crystal or AC coupled reference signal is
connected to the XTAL pin. The transmit frequency is
fixed and determined by the crystal frequency
according to the formula:
Due to the flexible selection of transmit frequency, the
resulting crystal frequency may not be a standard off-
the-shelf value. Therefore, for some carrier frequencies
the designer will have to consult a crystal manufacturer
and have a custom crystal manufactured. For
background information on crystal selection see
Application Note AN588, PICmicro
Oscillator Design Guide, and AN826 Crystal Oscillator
Basics and Crystal Selection for rfPIC™ and
PICmicro
For ASK modulation the crystal can be connected
directly from RFXTAL to ground, or in series with an
additional capacitor to trim the frequency. Figure 9-2
shows how the crystal is connected and Table 9-2
shows how the frequency of a typical crystal changes
with capacitance.
The oscillator is enabled when the RFEN input is high.
It takes the crystal approximately 1 ms to start oscillat-
ing. Higher frequency crystals start-up faster than
lower frequencies. The crystal oscillator start time
(T
crystal building up an oscillation, but also includes the
time for the PLL to lock on the crystal frequency.
TABLE 9-2:
DS70091A-page 50
Characteristics. This start-up time is mainly due to the
ON
Note 1: Standard Operating Conditions (unless otherwise stated) T
) is listed in Table 13-11, Transmitter AC
Crystal Oscillator
®
1000 pF
100 pF
150 pF
470 pF
22 pF
39 pF
Devices.
C1
f
XTAL
f
transmit
XTAL OSC APPROXIMATE FREQ. VS. CAPACITANCE (ASK MODE)
= 13.55 MHz
=
f
RFXTAL
Predicted Frequency
13.551438
13.550563
13.549844
13.549672
13.549548
13.549344
×
®
(MHz)
32
Microcontroller
Preliminary
PPM from 13.55 MHz
9.4
In ASK modulation the data is transmitted by varying
the output power. The DATA
toggling the pin turns the RF output signal on and off. A
simple receiver using a tuned filter and peak detector
diode can capture the data. A more advanced super-
heterodyne receiver such as the rfRXD0420 can
greatly increase the range and reduce susceptibility to
interference.
In ASK mode the DATA
used and should both be tied to ground. An example of
a typical ASK circuit is shown in Figure 9-5. The C1
capacitor can be replaced by a short to simplify the
transmitter if the receiver has a wide enough
bandwidth. For a very narrowband receiver the C1
capacitor may need to be replaced by a trimmer cap to
tune the transmitter to the exact frequency.
FIGURE 9-2:
+106
+42
-12
-24
-33
-48
A
ASK Modulation
= 25°C, RFEN = 1, V
X1
C1
ASK CRYSTAL CIRCUIT
Transmit Frequency (MHz)
 2003 Microchip Technology Inc.
FSK
XTAL
and FSK
rfPIC12F675K/F/H
ASK
(32 * f
DDRF
433.5895
433.5856
433.646
433.618
433.595
433.579
pin enables the PA,
(1)
= 3V,
XTAL
OUT
)
pins are not