APGRD001 Microchip Technology, APGRD001 Datasheet - Page 7

• 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

LC ANTENNA TUNING
As shown in Equations 2 and 3, the induced coil volt-
age is maximized when the LC circuit is tuned precisely
to the incoming carrier frequency. In practical applica-
tions, however, the LC resonant frequency differs from
transponder to transponder due to the tolerance varia-
tion of the LC components. To compensate the error
due to the component tolerance, the PIC16F639 has
an internal resonant capacitor bank per channel. The
capacitor value can be programmed up to 63 pF with
1 pF per step. Figure 7 shows an example of the
capacitance tuning using the Configuration register bits
(6 bits). The capacitance is monotonically increased
with the Configuration register bits. Refer to the
PIC12F635/PIC16F636/639
(DS41232) for more details (see “References”).
The capacitance can be effectively tuned by monitoring
the RSSI current output. The RSSI output is propor-
tional to the input signal strength. Therefore, the higher
RSSI output will be monitored the closer the LC circuit
is tuned to the carrier frequency.
The total capacitance adds up as the Configuration
register bits step up . The resulting internal capacitance
is added to the present capacitor values of the LC
circuit. The LC resonant frequency will shift to lower by
adding the internal resonant capacitor.
FIGURE 7:
© 2007 Microchip Technology Inc.
70
60
50
40
30
20
10
0
0
20
Bit Setting (steps)
CAPACITANCE TUNING VS.
BIT SETTING
40
Device
60
Data
80
Ch. X
Ch. Y
Ch. Z
Sheet
Battery Back-up and Batteryless Modes
In real-life applications, there is the chance that the
battery can be momentarily disconnected from the
circuit by accident, for example, if the unit is dropped
onto a hard surface. If this should happen, the data
stored in the MCU may not be recovered correctly. To
protect the battery from accidental misplacement,
users may consider using a battery back-up circuit. The
battery back-up circuit provides a temporary V
voltage to the transponder. The circuit is recommended
for sophisticated transponders, but may not be a
necessary mechanism for all applications. In Figure 2,
D4 and C1 form the battery back-up circuit. C1 is fully
charged when the battery is connected and provides
the V
The Batteryless mode is the case when the transpon-
der is operating without the battery. In Figure 2, diodes
D1, D2, D3 and C1 form a power-up circuit for battery-
less operation. When the transponder coil develops
voltages, the coil current flows through the diodes, D1
and D2, and charges the capacitor, C1, which can pro-
vide the V
is useful when the PIC16F639 is used for anti-collision
transponder applications, where batteryless operation
is preferred. The value of the capacitor, C1, for Battery-
less mode is from a few μF to a few Farad (F)
depending on the application.
DD
when the battery is momentarily disconnected.
DD
for the transponder. The power-up circuit
AN1024
DS01024B-page 7
DD