13

Atmel ATA5781 [Summary DATASHEET]

9286BS–RKE–06/13

With the single pole double throw (SPDT) switch the RF signal from the antenna is switched to RFIN in RXMode.

user ROM for the Atmel ATA5782. 1024-byte EEPROM, 1024-byte SRAM, and other peripherals are supporting the receiver

handling. Two GPIO ports, PB[7:0] and PC[5:0], are available for external digital connections, for example, as an alternate

function the SPI interface is connected to port B. The Atmel ATA5781/2/3 is controlled by the EEPROM configuration and SPI

commands and the functional behavior is mainly determined by firmware in the ROM. Much of the configuration can be modified

by the EEPROM settings. The firmware running on the AVR gives access to the hardware functionality of the Atmel

ATA5781/2/3. Extensions to this firmware can be added in the 20KB of Flash memory for the Atmel ATA5781. The Rx DSP

registers are addressed directly and accessible from the AVR. A set of sequencer state machines is included to perform Rx path

operations (such as enable, disable, receive) which require a defined timing parallel to the AVR program execution.

The power management contains low-dropout (LDO) regulators and reset circuits for the supply voltages VS, AVCC, and DVCC

of the Atmel ATA5781/2/3. In OFFMode all the supply voltages AVCC and DVCC are switched off to achieve very low current

consumption. The Atmel ATA5781/2/3 can be powered up by activating the PWRON pin or one of the NPWRON[6:1] pins

because they are still active in OFFMode. The AVCC domain can be switched on and off independently from DVCC. The Atmel

ATA5781/2/3 includes two idle modes. In IDLEMode(RC) only the DVCC voltage regulator, the FRC and SRC oscillators are

active and the AVR uses a power-down mode to achieve low current consumption. The same power-down mode can be used

during the inactive phases of the PollingMode. In IDLEMode(XTO) the AVCC voltage domain as well as the XTO are

additionally activated.

An integrated watchdog timer is available to restart the Atmel ATA5781/2/3 when it is not served within the configured time-out

period.

3.2

Receive Path

3.2.1

Overview

The receive path consists of a low-noise amplifier (LNA), mixer, IF amplifier, analog-to-digital converter (ADC), and an Rx digital

signal processor (Rx DSP). The fractional-N PLL and the XTO deliver the local oscillator frequency in RXMode. The receive

path is controlled by the RF front-end registers.

Two separate LNA inputs, one for Low-Band and one for High-Band, are provided to obtain optimum performance matching for

each frequency range and to allow multi-band applications. A radio frequency (RF) level detector at the LNA output and a

switchable damping included into the single-pole double-trough (SPDT) switch is used in the presence of large blockers to

achieve enhanced system blocking performance.

The mixer converts the received RF signal to a low intermediate frequency (IF) of about 250kHz. A double-quadrature

architecture is used for the mixer to achieve high image rejection. Additionally, the third-order suppression of the local oscillator

(LO) harmonics makes receiving without a front-end SAW filter less critical, such as in a car key fob application.

An IF amplifier provides additional gain and improves the receiver sensitivity by 2-3dB. Because of built-in filter function, the in-

band compression is degraded by 10dB, while the out-of-band compression remains unchanged.

The ADC converts the IF signal into the digital domain. Due to the high effective resolution of the ADC, the channel filter and

received signal strength indicator (RSSI) can be realized in the digital signal domain. Therefore, no analog gain control (AGC)

potentially leading to critical timing issues or analog filtering is required in front of the ADC. This leads to a receiver front end

with excellent blocking performance up to the 1dB compression point of the LNA and mixer, and a steep digital channel filter can

be used.

The Rx DSP performs the channel filtering and converts the digital output signal of the ADC to the baseband for demodulation.

Due to the digital realization of these functions the Rx DSP can be adapted to the needs of many different applications. Channel

bandwidth, data rate, modulation type, wake-up criteria, signal checks, clock recovery, and many other properties are

configurable. The RSSI value is realized completely in the digital signal domain, enabling very high relative and absolute

accuracy that is only deteriorated by the gain errors of the LNA, mixer, and ADC.

Two independent receive paths A and B are integrated in the Rx DSP after the channel filter and allow the use of different data

rates, modulation types, and protocols without the need to power up the receive path more than once to decide which signal

should be received. This results in a reduced polling current in several applications.

The integration of remote keyless entry (RKE), passive entry and go (PEG) and tire-pressure monitoring systems (TPM) into

one module is simplified because completely different protocols can be supported and a low polling current is achieved. It is

even possible to configure different receive RF bands for different applications by using the two LNA inputs. For example, a

TPM receiver can be realized at 433.92MHz while a PEG system uses the 868MHz ISM band with multi-channel

communication.