Rainbow Electronics T5754 - Manual (Page 4)

4
T5754
4511D–RKE–08/02
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
³
1 ms, 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
³
1 ms, 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%, C
5
= 6.8 pF ± 5%, a switch port with CS
witch
= 3 pF ± 10%, stray
capacitances on each side of the crystal of C
Stray1
= C
Stray2
= 1 pF ± 10%, a parallel
capacitance of the crystal of C
0
= 3.2 pF ± 10% and a crystal with C
M
= 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
M4xCx9x 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