APGRD001 Microchip Technology, APGRD001 Datasheet - Page 10

• Manufacturer Part Number: APGRD001
• Description: REF DESIGN PKE PIC16F639
• Manufacturer: Microchip Technology
• Type: PKE, RKEr

Specifications of APGRD001

• Frequency: 125kHz
• Processor To Be Evaluated: PIC16F639
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With/related Products: PIC16F639, PIC16F636, PIC18F2680
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant

AN1024
BASE STATION CIRCUIT
Figure C-1 and Figure C-2, in Appendix C: “Base
Station”, show an example circuit of the base station,
which has been used for customer training and device
demonstration purposes.
The base station unit consists of a microcontroller,
125 kHz transmitter/receiver and an UHF receiver
module.
The base station transmits a 125 kHz low-frequency
command
transponders in the field via UHF or LF talk-back. After
transmitting the LF commands, it checks whether there
is any response through LF or UHF link.
The 125 kHz transmitter generates a carrier signal
based on the MCU’s Pulse-Width Modulator (PWM)
output. The power of the125 kHz square pulses from
the MCU is boosted by the current driver, U1. The
square pulse output from U1 becomes sine waves as it
passes through an LC series resonant circuit that is
formed by L1, C2, C3 and C4. L1 is an air-core inductor
and is used for the 125 kHz LF antenna.
The antenna radiation becomes maximized when the
LC series resonant circuit is tuned to the frequency of
the PWM signal. At the resonant frequency, the
impedance of the LC circuit is minimized. This results
in a maximum load current through L1 and therefore
produces strong magnetic fields. Users may tune the
LC circuit by monitoring the coil voltage across L1.
The components after diode D1 are used to receive the
LF talk-back signal from the transponder. When the
transponder responds with LF talk-back, there will be
changes in the coil voltage (across L1) due to the mag-
netic fields originated by the voltage on the transponder
coil. Since the voltage on the transponder coil is initially
caused by the voltage of the base station antenna (L1),
the return voltage has 180º phase difference with
respect to the originating voltage. Therefore, at a given
condition, the voltage across L1 changes with the coil
voltage of the transponder coil.
FIGURE 9:
DS01024B-page 10
Base Station
Transmits:
(Step 1)
Transponder
Transmits:
(Step 2)
Base Station
Transmits:
(Step 3)
Transponder
Transmits:
(Step 4)
Base Station:
and
AGC Stabilization Pulse + Wake-up Filter + 10 bits (ID Command + Parity + Stop Bit)
Header + ID (32 bits) + 4 Parity bits
AGC Stabilization Pulse + Wake-up Filter + IFF Command (8 bits) +Challenge (32 bits) + 5 Parity bits +
Stop Bit (46 bits)
Header + Response (32 bits) + 4 Parity bits (36 bits)
Display message on LCD
receives
EXAMPLE OF HANDSHAKE BETWEEN BASE STATION AND TRANSPONDER
responses
from
the
The change in the coil voltage (across L1) can be
detected through an envelope detector and low-pass
filter formed by D1 and C5. The detected envelope
passes through active gain filters, U2A and U2B. The
demodulated analog output is fed into the comparator
input pin of the MCU for pulse shaping. The output of
the comparator is available on TP6 and decoded by the
MCU.
U4 is the 433.92 MHz ASK receiver module. This
receiver module detects the transponder’s UHF
responses. The digital output from this module is fed
into the MCU for decoding. An antenna (a few inches
long) is typically attached to the antenna pad of the
module to receive a signal in stable condition. Since the
receiver module is next to the LF transmitter section,
which produces strong fields, the module typically
outputs noise. Therefore, it may require an adequate
firmware routine to filter out the noise inputs.
The base station unit displays the data on the LCD or
turns on the buzzer each time valid data is received.
FIRMWARE EXAMPLES
Firmware examples, including HTML documentation
for the transponder and the base station units are avail-
able in an archived file (see Appendix A: “Source
Code”).
The main firmware files for the transponder and the
base station are PIC16F639_Transponder.asm and
PIC16F639_BaseStation.asm, respectively.
The firmware does not use the K
algorithm. Contact Microchip sales for assistance if you
want to use K
Figure 9 shows an example of the handshake between
the base station and the transponder.
Figure 10 shows a communication example between
the transponder and base station units by using the
firmware.
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