ATAK51002-V1

RF RSSI Sniffer Tool for Car Access Systems Chris Wunderlich, George Rueter Introduction The ability to monitor the received signal strength in a radio receiver is a valuable development and diagnostic tool for the engineer. In an automotive application the ablility to monitor this received signal strength indicator (RSSI) value represents what a radio receiver will experience in service, including the effects related to antenna and mounting location. The article describes a simple method to re-configure the Atmel® ATA5830N device, utilizing its Flash program capability, to create an RSSI monitoring tool for labratory and in-vehicle use. The ATA5830N UHF transceiver chip incorporates a highperformance UHF transceiver and a low-power 8-bit AVR® microcontroller on a single die. Figure 1 shows a simplified block diagram of the ATA5830N device. Included in the device is 6KB of Flash memory available to the end user. This article describes an application that utilizes this Flash memory space to develop an application for an RSSI monitor that generates universal synchronous/asynchronous receiver transmitter- (USART-) formatted messages containing RSSI data. From an operational perspective this application lends itself well to several common RF engineering tasks such as: a) RF environment analysis, b) © 2012 / www.atmel.com performance tuning of the receiver section, c) RF component selection and d) antenna performance evaluation. AVCC VS DVCC SRC, FRC Oscillators AVR CPU Rx DSP EEPROM ROM Flash Tx DSP SRAM RF Frontend RFOUT RSSI Measurement Details Supply Reset AVR Perepherals RFIN DATA BUS XTO XTAL Port B (8) Port C (6) PB [0 to 7] (SPI) PC [0 to 5] Figure 1. ATA5830N Block Diagram Flash-based RSSI Application RSSI data is a critical radio reception metric that reflects the level of RF signal energy at a given frequency channel and is useful in evaluating an RF environment. The Flash memory application uses the internal RSSI measuring function of the chip and communicates this value using a general-purpose I/O pin using standard USART communication protocol timing. This article describes how a Flash program application is implemented and demonstrates the flexibility of the internal AVR controller. The Flash application provides a small flexible RSSI measurement tool that can be easily and quickly configured to operate across a wide range of frequencies, providing a simple way to measure RSSI. The software for this demo was developed using the ATAK51002-V1 evaluation kit. The application operates with no external inputs required other than 5V DC power and the RF signal source. The desired radio parameters, such as frequency and intermediate frequency bandwidth (IFBW), are programmed into the part using the EEPROM configuration file. The user selects these values by using an Excel spreadsheet tool that automatically generates the EEPROM file. Once these values are programmed into the EEPROM, the application of power will automatically initiate the self-configuration and execution of the Flash application program. It begins measuring and outputting the 8-bit RSSI data values on pin 17 (PC3) of the device. USART communication parameters are 38400 Baud, 8 data bits, 1 start bit and no parity bit (38400, 8-1-N). Each data byte output requires about 260μs to transmit and occurs about every 50ms. The output data is at logic value Automotive Compilation Vol. 9 voltage levels (about 0V to 5V) and, while not compliant with RS-232 standards, most PCs have no problem in receiving and displaying this data stream using a terminal program. Received signal strength indicator (RSSI) is a measurement of the power present in a received radio signal. In the ATA5830N chip, this function is built into the receiver’s digital signal processing (Rx DSP) section and occurs completely in the digital signal domain, allowing for high relative and absolute RSSI accuracy. In this application, RSSI values are returned as 8-bit values and the signal power at the matched 50Ω antenna input can be calculated in dBm as: RF Power In (dBm) = RSSI/2 – 155 (154 in 868-928MHz band) The ATA5830N datasheet notes the following RSSI measurement specifications: • • • Absolute RSSI accuracy +/- 5.5dBm Relative RSSI accuracy +/- 1dB RSSI resolution 0.5dB The typical RSSI range is a function of the IFBW selected: • • • • IFBW = 25kHz >>> RSSI = -115dBm to -25dBm IFBW = 80kHz >>> RSSI = -110dBm to -25dBm IFBW = 165kHz >>> RSSI = -105dBm to -25dBm IFBW = 366kHz >>> RSSI = -100dBm to -25dBm Software Operational Flow On power-up and after the hardware-generated power-on reset is complete, the AVR controller will execute its main() function in ROM, where the EEPROM configuration settings are loaded and checked. This EEPROM configuration defines all operational parameters of the device, including that a Flash application program is present. When this EEPROM setting is encountered, the application flow jumps to the Flash main(), and the application flow begins. The Flash main() function can be very simple since the ROM software functions will perform most of the work. In this application, the Flash program function loops indefinitely, first requesting ROM-code-based RSSI measurements and then calling Flash code functions that format and output the data. The following is the Flash program loop that drives this application: for(;;){ _WDR; ATA_makeGlobalSystemFlags(); RDCR |= (1