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T5754 Datasheet(PDF) 4 Page - ATMEL Corporation

Part No. T5754
Description  UHF ASK/FSK TRANSMITTER
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Maker  ATMEL [ATMEL Corporation]
Homepage  http://www.atmel.com
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T5754 Datasheet(HTML) 4 Page - ATMEL Corporation

 
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4
T5754
4511F–RKE–07/04
Functional
Description
If ENABLE = L and the PA_ENABLE = L, the circuit is in standby mode consuming only
a very small amount of current so that a lithium cell used as power supply can work for
several years.
With ENABLE = H the XTO, PLL and the CLK driver are switched on. If PA_ENABLE
remains L only the PLL and the XTO is running and the CLK signal is delivered to the
microcontroller. The VCO locks to 32 times the XTO frequency.
With ENABLE = H and PA_ENABLE = H the PLL, XTO, CLK driver and the power
amplifier are on. With PA_ENABLE the power amplifier can be switched on and off,
which is used to perform the ASK modulation.
ASK Transmission
The T5754 is activated by ENABLE = H. PA_ENABLE must remain L for t
≥1ms, then
the CLK signal can be taken to clock the microcontroller and the output power can be
modulated by means of Pin PA_ENABLE. After transmission PA_ENABLE is switched
to L and the microcontroller switches back to internal clocking. The T5754 is switched
back to standby mode with ENABLE = L.
FSK Transmission
The T5754 is activated by ENABLE = H. PA_ENABLE must remain L for t
≥1ms, then
the CLK signal can be taken to clock the microcontroller and the power amplifier is
switched on with PA_ENABLE = H. The chip is then ready for FSK modulation. The
microcontroller starts to switch on and off the capacitor between the XTAL load capaci-
tor and GND with an open-drain output port, thus changing the reference frequency of
the PLL. If the switch is closed, the output frequency is lower than if the switch is open.
After transmission PA_ENABLE is switched to L and the microcontroller switches back
to internal clocking. The T5754 is switched back to standby mode with ENABLE = L.
The accuracy of the frequency deviation with XTAL pulling method is about ±25% when
the following tolerances are considered.
Figure 4. Tolerances of Frequency Modulation
Using C
4 = 9.2 pF ± 2%, C5 = 6.8 pF ± 5%, a switch port with CSwitch = 3 pF ± 10%, stray
capacitances on each side of the crystal of C
Stray1 =CStray2 = 1 pF ± 10%, a parallel
capacitance of the crystal of C
0 = 3.2 pF ± 10% and a crystal with CM = 13 fF ± 10%, an
FSK deviation of ±21 kHz typical with worst case tolerances of ±16.3 kHz to ±28.8 kHz
results.
CLK Output
An output CLK signal is provided for a connected microcontroller, the delivered signal is
CMOS compatible if the load capacitance is lower than 10 pF.
Clock Pulse Take-over
The clock of the crystal oscillator can be used for clocking the microcontroller. Atmel’s
ATARx9x has the special feature of starting with an integrated RC-oscillator to switch on
the T5754 with ENABLE = H, and after 1 ms to assume the clock signal of the transmis-
sion IC, so that the message can be sent with crystal accuracy.
~
V
S
XTAL
C
Stray1
C
M
L
M
R
S
C
0
C
Stray2
C
4
C
5
Crystal equivalent circuit
C
Switch


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