APGRD001 Microchip Technology, APGRD001 Datasheet - Page 9

• 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

TRANSPONDER CIRCUIT
Figure B-1, in Appendix B: “Transponder”, shows an
example of the PKE transponder circuit which has been
used for customer training and device demonstration
purposes.
The transponder circuit has three external LC resonant
circuits, 5 push button switches, a 433.92 MHz
resonator for UHF data transmission and components
for Battery Back-up mode.
Each LC resonant circuit is connected to the LC input
and LCCOM pins. The air coil antenna is connected to
the LCX input and the two ferrite-rod inductors are
connected to the LCY and LCZ pins. The LCCOM pin
is a common pin for all three antenna connections,
which is grounded via C11 and R9. Each resonant
antenna must be tuned to the carrier frequency of the
base station unit for the best signal reception
conditions. The internal capacitor of each channel can
be used to tune the antenna for the best performance.
When the device is powered up initially, the digital
section programs the Configuration registers of the
AFE using the SPI (CS, SCLK/ALERT, SDIO).
The AFE is very sensitive to environmental noise due
to its high input sensitivity (~3 mV
appropriate care to prevent excess AC noise along the
PCB traces. Capacitors C6 and C12 are used for noise
filtering for the V
Diodes D1 and D2, and capacitor C5 are for the Battery
Back-up mode. Diodes D2, D3 and D7 and capacitor
C5 are for Batteryless mode. A larger C5 value is
needed for stable Batteryless mode operation. Capac-
itor C5 holds the charges from the battery and from the
coil voltage through diodes D3 and D7. The stored
charge on C5 can keep the PIC16F639 device pow-
ered when the battery is momentarily disconnected.
Diodes D3 and D7 are connected across the air coil,
which develops the strongest coil voltage among the
three external LC resonant antennas.
Once a valid input signal is detected, the digital MCU
section is waken up and transmits a response if the
command is valid.
The transponder can send responses using an internal
modulator (LF talk-back) or an external UHF
transmitter. The analog input channel has an internal
modulator (transistor) per channel between the input
and the LCCOM pins. The internal modulator is turned
on and off if the AFE receives Clamp-On and Clamp-
Off commands from the digital MCU section,
respectively. The antenna voltage is clamped or
unclamped depending upon the Clamp-On or Clamp-
Off command, respectively. This is called LF talk-back,
which is used for proximity range applications only. The
base station can detect the changes in the transponder
antenna voltage and reconstruct the modulation data.
© 2007 Microchip Technology Inc.
DD
and V
DDT
pins, respectively.
PP
); therefore, take
See the PIC12F635/PIC16F636/639 Device Data
Sheet (DS41232) for more details of the LF talk-back
(see “References”).
The transponder uses a UHF transmitter for long range
applications. An On-Off-Keying UHF transmitter is
formed by the UHF (433.92 MHz) resonator U2 and
power amplifier Q1. The values of capacitors C2 and
C3 are approximately 20 pF range each, but are layout
dependent. The L1, which is typically formed by a metal
trace on the PCB, is a UHF antenna and its efficiency
increases significantly by increasing its loop area.
The UHF transmitter section is turned on when the
MCU I/O pin outputs a logic level high; otherwise it is
turned off. The output of RC5 is the modulation data of
the UHF signal and can be reconstructed by the UHF
receiver in the base station.
AN1024
DS01024B-page 9