2.3.1 Typical 3V Application with External Microcontroller

Figure 2-3. Typical 3V Application with External Microcontroller
Note:
  1. This connection depends on the setting of the bits VS5V and VS22. For more details, refer to the ATA8510/15 Industrial User's Guide (DS50003142).

The preceding figure shows a remote control application circuit with an external host microcontroller for the 315 MHz or 433.92 MHz band running from a 3V lithium cell. The ATA8510/15 stays in OFFMode until NPWRON1 (PC1) is used to wake it up. In OFFMode, the ATA8510/15 draws typically less than 5 nA (600 nA maximum at 3.6V/85°C).

In OFFMode, all the ATA8510/15 AVR ports PB0 – PB7 and PC0 – PC5 are switched to input. PC0 – PC5 and PB7 have internal pull-up resistors ensuring that the voltage at these ports is VS. PB0-PB6 are tri-state inputs and require additional consideration. PB1, PB2 and PB5 have defined voltages because they are connected to the output of the external µC. PB4 is connected to ground to avoid unwanted power-ups. PB0, PB3 and PB6 do not require external circuitry because the internal circuit avoids transverse currents in OFFMode. The external µC has to tolerate the floating inputs. Otherwise, additional pull-down resistors are required on these floating lines.

Typically, the key fob buttons are connected to the external µC, and the ATA8510/15 wake-up is done by pulling NPWRON1 (pin 15) to ground. If there are not enough ports for button inputs on the µC, it is possible to connect up to four additional buttons to the ports PC2 – PC5. In this case, the occurrence of a port event (button pressed) generates an event on pin 28. The corresponding port event is available in the event registers.

A PCB trace loop antenna is typically used in this type of application. An internal antenna tuning procedure tunes the resonant frequency of this loop antenna to the TX frequency. This is accomplished with an integrated variable capacitor on the ANT_TUNE pin. RF_OUT and RF_IN are optimally matched to the SPDT_TX and SPDT_RX pins of the integrated RX/TX switch. The SPDT_ANT pin has an impedance of 50Ω for both the RX and TX functions. The DC output voltage of the power amplifier is required at the SPDT_TX pin for proper operation. Also, the RFIN pin needs a DC path to ground, which is easily achieved with the matching shunt inductor. The impedance of the loop antenna is transformed to 50Ω with three capacitors, two of them external and one built-in at the ANT_TUNE pin.

Together with the fractional-N PLL within the ATA8510/15, an external crystal is used to fix the RX and TX frequency. Accurate load capacitors for this crystal are integrated to reduce the system part count and cost. Only four supply blocking capacitors are needed to decouple the different supply voltages AVCC, DVCC, VS and VS_PA of the ATA8510/15. The exposed die pad is the RF and analog ground of the ATA8510/15. It is connected directly to AGND via a fused lead. For applications operating in the 868.3 MHz or 915 MHz frequency bands, a High-Band RF input, RFIN_HB, is supplied and must be used instead of RFIN_LB.

The ATA8510/15 is controlled using specific SPI commands via the SPI interface and an internal EEPROM for application-specific configurations.