2.2.1 Typical 5V Antenna Diversity Application with External Microcontroller

Figure 2-3 shows a typical vehicle side antenna diversity application circuit with an external host microcontroller running from a single 5V voltage regulator. The digital I/O pins are always part of the VS power domain, and therefore, set to 5V operation. All internal voltages are generated by the embedded power management circuitry. The DC-DC Converter is set up and activated to supply the analog front end and the power amplifier to substantially reduce the required power in the 5V domain.

The ATA5835 is powered on when the host sets the PWRON pin PB4 to ‘high’. After the internal start-up procedure, the ATA5835 enters the IDLEMode, which is signaled to the host by a system ready event on PB6. The system is now ready for SPI communication with the host. The ATA5835 can also be EEPROM-configured to directly switch to RXMode, PollingMode or TXMode after start-up.

The RF antenna switch is set up with two antennas at SPDT_ANT1 and SPDT_ANT3 for an antenna diversity application. The harmonic suppression can be improved by using an optional filter on the antenna side. If a SAW filter is used in the transmit path, the user has to ensure that the RF peak voltage on the pins, SPDT_ANTx and SPDT_TX, stays in the range between -0.3V and VS_PA + 0.3V. The antenna diversity algorithm itself is implemented in firmware and will automatically check the signal quality on both antennas whenever the RXMode is activated or a polling channel is checked. The better antenna is selected for telegram reception and stored for transmission of a successive Acknowledge message. This algorithm can significantly improve the service availability in scattered and disturbed environments when choosing an appropriate positioning and orientation of the antennas.