ATA8741 ATMEL Corporation, ATA8741 Datasheet - Page 7

• Manufacturer Part Number: ATA8741
• Manufacturer: ATMEL Corporation

3. Functional Description
9140B–INDCO–07/09
For a typical application 3 to 4 interconnections between the AVR and the transmitter are
required (see
microcontroller to generate an XTAL-based transmitter signal. The ENABLE line is used to start
the transmitter’s XTO, PLL, and clock. The PA_ENABLE line is used to enable the power ampli-
fier in ASK and FSK mode. In FSK mode a fourth line is necessary to modulate the load capacity
of the XTAL. To wake up the system from standby mode at least one key input is required. After
pressing the key, the microcontroller starts up with the internal RC oscillator. For TX operation
user software must control ENABLE, PA_ENABLE, and XTAL load capacity as described in the
following section.
If ENABLE = L and PA_ENABLE = L the transmitter and the microcontroller (MCU) are in
standby mode, reducing the power consumption so that a lithium cell can be used as power sup-
ply for several years.
If ENABLE = H and PA_ENABLE = L, the XTO, PLL, and the CLK driver from the transmitter are
activated. The crystal oscillator together with the PLL from the RF transmitter typically require
< 3 ms until the PLL is locked and the clock output (Pin 8) is stable.
If ENABLE = H and PA_ENABLE = H, the XTO, PLL, CLK driver, and the power amplifier (PA)
are switched on. ASK modulation is achieved by switching on and off the power amplifier via
PA_ENABLE. FSK modulation is achieved by switching on and off an additional capacitor
between the XTAL load capacitor and GND, thus changing the reference frequency of the PLL.
This is done using a MOS switch controlled by a microcontroller output. The power amplifier is
switched on via PA_ENABLE = H.
The MCU has to wait at least > 3 ms after setting ENABLE = H, before the external clock can be
used. The external clock is connected via the timer0 input pin that clocks the USI from the MCU
to achieve an accurate data transfer. The frequency of the internal RC oscillator is affected by
ambient temperature and operating voltage.
The USI provides two serial synchronous data transfer modes, with different physical I/O ports
for the data output. The two wire mode is used for ASK and the three wire mode is used for FSK.
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 are 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.
Figure 1-1 on page 2
and
Figure 1-2 on page
3). The CLK line is used to allow the
ATA8741
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