4

T5750 [Preliminary]

4546B–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 64 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 T5750 is activated by ENABLE = H. PA_ENABLE must remain L for t

≥ 4ms, 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 T5750 is switched

back to standby mode with ENABLE = L.

FSK Transmission

The T5750 is activated by ENABLE = H. PA_ENABLE must remain L for t

≥ 4ms, 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 T5750 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

capacitances on each side of the crystal of C

capacitance of the crystal of C

FSK deviation of ±21.5 kHz typical with worst case tolerances of ±16.8 kHz to

±28.0 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 T5750 with ENABLE = H, and after 4 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