Atmel AVR2092: REB232ED - Hardware User Manual Features • High-performance, 2.4GHz, RF-CMOS Atmel® AT86RF232 radio transceiver targeted for IEEE® 802.15.4, ZigBee®, and ISM applications - Industry leading 104dB link budget - Ultra-low current consumption - Ultra-low supply voltage (1.8V to 3.
Disclaimer Typical values contained in this application note are based on simulations and testing of individual examples. Any information about third-party materials or parts was included in this document for convenience. The vendor may have changed the information that has been published. Check the individual vendor information for the latest changes.
Atmel AVR2092 3 Overview The radio extender board is assembled with an Atmel AT86RF232 radio transceiver [1] and two ceramic antennas, and demonstrates the unrivaled hardware-based antenna diversity feature, which significantly improves radio link robustness in harsh environments. The radio extender board was designed to interface to the Atmel microcontroller development or evolution platforms.
Functional description The block diagram of the REB232ED radio extender board is shown in Figure 4-1. The power supply pins and all digital I/Os of the radio transceiver are routed to the 2 x 20-pin expansion connector to interface to a power supply and a microcontroller. The Atmel AT86RF232 antenna diversity (AD) feature supports the control of two antennas (ANT0/ANT1). A digital control pin (DIG1) is used to control an external RF switch selecting one of the two antennas.
Atmel AVR2092 4.1.1 Atmel ATmega1281 configuration Table 4-1. Default expansion connector mapping (ATmega1281 configuration). Pin# Function Pin# Function 1 GND 2 GND 3 n.c. 4 n.c. 5 n.c. 6 n.c. 7 n.c. 8 n.c. 9 n.c. 10 n.c. 11 n.c. 12 n.c. 13 n.c. 14 n.c. 15 n.c. 16 n.c. 17 XT1 (MCLK) 18 n.c.
Pin# Function Pin# Function 29 PB1 (MCLK) 30 PB0 (open) 31 PD7 (SLPTR) 32 PD6 (DIG2) 33 PD5 (TP2) 34 PD4 (open) 35 PD3 (TP3) 36 PD2 (IRQ) 37 PD1 (TP4) 38 PD0 (open) 39 GND 40 EE#WP (write protect EEPROM) 4.2 ID EEPROM To identify the board type by software, an optional identification (ID) EEPROM is populated. Information about the board, the node MAC address and production calibration values are stored here.
Atmel AVR2092 Table 4-3 shows a detailed description of the EEPROM data structure. Table 4-3. ID EEPROM mapping. Address Name Type Description 0x00 MAC address uint64 MAC address for the 802.15.4 node, little endian byte order 0x08 Serial number uint64 Board serial number, little endian byte order 0x10 Board family uint8 Internal board family identifier 0x11 Revision uint8[3] Board revision number ##.##.
4.3 Supply current sensing The power supply pins of the radio transceiver are protected against overvoltage stress and reverse polarity at the EXPAND1 pins (net CVTG, net DGND) using a Zener diode (D1) and a thermal fuse (F1) (see Exhibit Appendix A). This is required because the Atmel STK500 will provide 5V as default voltage, and the board can also be mounted with reverse polarity. Depending on the actual supply voltage, the diode D1 can consume several milliamperes.
Atmel AVR2092 The reference frequency is also available at pin CLKM of the radio transceiver and, depending on the related register setting; it is divided by an internal prescaler. CLKM clock frequencies of 16MHz, 8MHz, 4MHz, 2MHz, 1MHz, 250kHz, or 62.5kHz are programmable (refer to [1]). The CLKM signal is filtered by a low-pass filter to reduce harmonic emissions within the 2.4GHz ISM band.
5 PCB layout description This section describes critical layout details to be carefully considered during a PCB design. The PCB design requires an optimal solution for the following topics: • Create a solid ground plane for the antenna.
Atmel AVR2092 5.1 PCB detail 1 – balanced RF pin fan out Figure 5-2. Board layout – RF pin fan out. The Atmel AT86RF232 antenna port should be connected to a 100Ω load with a small series inductance of 1nH to 2nH. This is achieved with the connection fan out in between the IC pins and the filter balun combination B1. The trace width is kept small at 0.2mm for a length of approximately 1.5mm. The REB232ED is a two-layer FR4 board with a thickness of 1.5mm.
5.2 PCB detail 2 – RF switch Figure 5-3. Board layout – RF switch. RF RF RF The RF switch requires a solid grounding to achieve the full isolation and RF filter capacitors for the control pins. A parasitic inductance within the ground connection may reduce the RF isolation of the switch in the off state. To achieve a hard, low-impedance ground connection, vias are placed on each side of the ground pad. Additionally, the ground pad is connected to the top layer ground plane.
Atmel AVR2092 Figure 5-4. Board layout – XTAL section. The reference crystal and load capacitors C36/37 form the resonator circuit. These capacitors are to be placed close to the crystal. The ground connection in between the capacitors should be a solid copper area right underneath the crystal, including the housing contacts. 5.4 PCB detail 4 – transceiver analog GND routing With the Atmel AT86RF232, consider pins 3, 6, 27, 30, 31, and 32 as analog ground pins.
Figure 5-5. Board layout – transceiver GND. 7 6 3 32 31 30 12 27 16 18 21 The soldering technology used allows the placement of small vias (0.15mm drill) within the ground paddle underneath the chip. During reflow soldering, the vias get filled with solder, having a positive effect on the connection cross section. The small drill size keeps solder losses within an acceptable limit. During the soldering process vias should be open on the bottom side to allow enclosed air to expand. 5.
Atmel AVR2092 Figure 5-6. Board layout – GND and shield. Besides the function to provide supply ground to the individual parts, the ground plane has to be considered as a counterpart for the antenna. Such an antenna base plate is required to achieve full antenna performance. It has to be a continuous, sustained metal plate for that purpose. The shield, covering the electronic section will help to form this antenna base plate.
Figure 5-7. Board layout – transceiver RF tuning. During tuning, the best compromise in between RX and TX performance has to be found. Tuning should be done for the receiver first. First step should be to verify the 50Ω matching at U1, pin 5. After that, the capacitor position and value can be slightly varied. The reception performance should be measured using a packet error test. Typical tuning capacitor values are 1pF ±0.5pF.
Atmel AVR2092 Because the antenna has to operate in an environment different from that of the manufacturer’s evaluation board, the correct frequency tuning has to be verified. The antenna distance, required for optimum diversity operation, provides enough board space to use a low-cost tuning method based on a transmission line and capacitors. The actual tuning procedure is explained in Section 5.7.3, page 20. Figure 5-8. Initial antenna tuning and test board.
large metal plate. The graph shows receive signal strength variations caused by the interference of reflected waves reaching the receiver via different propagation paths. From Figure 5-10, page 19 one can conclude key parameters for such an indoor scenario: 1. For one antenna, multipath fades can exceed 30dB. 2. For the 2.4GHz ISM band, a local fading minimum is typically below 5cm (~2in). This number is expected considering the wavelength.
Atmel AVR2092 Figure 5-10. Local fading effects in an indoor multipath environment. 5.7.2 Antenna design-in This section describes the antenna design-in and the implementation of the antenna tuning structure. An overview of the layout can be found in Figure 5-11. Figure 5-11. Antenna PCB environment and tuning structure. 2 1 4 3 The antenna is available from two sources: 1. Johanson 2450AT45A100. 2. Würth 7488910245.
The antenna test board, as specified by the manufacturers, has a ground plane size of 20mm x 40mm, an antenna placed in a 12mm x 20mm FR4 area, and an FR4 substrate height of 0.8mm. This is the expected environment where the antenna performance should be equivalent to the datasheet values. On the REB232ED, the environment differs considerably because the FR4 height is 1.5mm, the antenna is placed in a triangular corner and the ground plane geometry is different, too.
Atmel AVR2092 The second step is to calibrate the network analyzer (NWA) to the 50Ω connector as usual. After normal calibration, the reference point for the NWA is at the cable SMA connection. To determine the tuning elements, the reference point has to be moved to the antenna feed point using the extended length parameter inside the NWA. To determine this parameter, a hard short is required at the antenna feed line end.
Figure 5-13. Antenna without tuning elements. The first tuning step will use the series capacitor to tune the band center down to a pure resistive behavior. The band center is crossing the 30 degree (1/3 x Z0) line. Therefore, the tuning capacitor can be determined by: C= 1 2πfXc with Xc = 1 * 50Ohm 3 f = 2.450GHz We get a capacitance of 3.89pF, and can simply use a 3.9pF value. The result of this tuning step can be seen in Figure 5-14, page 23.
Atmel AVR2092 Figure 5-14. Antenna tuning with series capacitor. The final tuning step will use a shunt capacitor to correct the antenna load impedance. A 0.5pF capacitor has been used to tune the antenna resonance frequency to the band center. If the antenna resonance frequency is too low, the capacitor needs to be moved towards the antenna, and vice versa. Figure 5-15. Final tuning.
Figure 5-15, page 23, and Figure 5-16 show the final result as a diagram and on the board. Figure 5-16. Antenna tuning with series and shunt capacitor. In most cases, it is beneficial to tune the antenna a little towards higher frequencies. The reason is that environmental changes in most cases tune the antenna down to lower frequencies. Such environmental changes can be any kind of object that is situated near the antenna, such as a housing or table surface.
Atmel AVR2092 multipath environments. The dual antenna setup has access to many more propagation modes than a single antenna. By switching from one antenna to the other, the physical antenna location is changing because of the antenna distance and on top of that the wave polarization is changing as well. The achieved propagation path effect of this switch was already illustrated in Figure 5-10, page 19. Figure 5-17. Radiated measurement for Azimut -50deg, Phi -35deg, Polarization 65deg.
Figure 5-18. Some main radiation directions with polarization angle, seen from Antenna A1. All three figures show the same 3D model from different directions. The board has a rather smooth radiation characteristic but these spots have been selected to show the varying polarization. Figure 5-19. Some main radiation directions with polarization angle, seen from Antenna A2.
Atmel AVR2092 Figure 5-20. Some main radiation directions with polarization angle, seen from the PCB bottom side. There is one radiation direction where no diversity effect exists. The direction is A=0, F=180 and P=0. For that case both antennas have a polarization that is turned by 180deg against each other. However, it is still horizontal and the antenna gain is similar for both antennas. Table 5-1. Measured radiation power for different radiation directions.
6 Mechanical description The REB232ED is manufactured using a low-cost, two-layer printed circuit board. All components and connectors are mounted on the top side of the board. The format was defined to fit the EXPAND1 connector on the Atmel AVR STK500 / STK501 microcontroller evaluation board. The upright position was chosen for best antenna performance. Table 6-1. REB232ED mechanical dimensions.
Atmel AVR2092 7 Electrical characteristics 7.1 Absolute maximum ratings Stresses beyond the values listed in Table 7-1 may cause permanent damage to the board. This is a stress rating only and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of this manual are not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
No. Parameter Condition 7.3.4 Supply current MCU @ 16MHz (int. RC 32MHz), transceiver in state TRX_ON 28 mA 7.3.5 Supply current MCU @ 16MHz (int.
Atmel AVR2092 8 Abbreviations AD - Antenna diversity AES - Advanced encryption standard BB - Baseband CBB/REB-CBB - Controller base board ETSI - European Telecommunications Standards Institute EVM - Error Vector Magnitude FCC - Federal Communications Commission ISM - Industrial, scientific and medical (frequency band) LNA - Low-noise amplifier MAC - Medium access control NWA - Network analyzer PA - Power amplifier PDI - Program/debug interface PER - Packet error rate
D C B A MAC S/N S/N SNr RF1 RF2 SEL GND 1 PD2 PB4 PB5 PB6 PB7 PB1 PD7 PD6 GND GND DEVDD 1M 23849 R24 DEVDD NC NC SLPTR RSTN MCLK TP1 TP2 TP3 TP4 NC R3 U6A 3 2 5 1 RSTN 4 6 NC7WV04 23860 GND U6B VCC 16 2 GND DEVDD 19514 1 U4A GND 5 6 4 8 2 1 3 SEL_TRX NC7WP32 23861 GND U4B 7 NC7WP32 23861 VCC 1 17 2 3561 J3 4428 PD0 PB0 PB2 PB3 PB1 PD6 PB4 PD4 PB5 2229 D1 IRQ SEL MOSI MISO SCK MCLK SLPTR DIG2 19512 1 3 5 7 9 11 13 15 17
Atmel AVR2092 A.2 Assembly drawing Figure 8-2. REB232ED assembly drawing.
A.3 Bill of materials Table 8-1. Bill of materials. Qty. Designator Description Footprint Manuf. Part# Manufacturer Comment 1 X2 RF Connector MS-147 MS147 CL358-150-5-06 Hirose Build in antenna switch, 50Ω 1 X1 Pin header 2x20 90 degree JP_2x20_90°_ Top_Invers 1007-121-40 CAB HEADER-20X2 1 U5 EEPROM MiniMap-8-2X3 AT25010AY610YH-1.8 Atmel AT25010A 1 U4 Logic gate MO-187 NV7WP32K8X Fairchild NC7WP32K8X 1 U3 802.15.4 2.
Atmel AVR2092 Appendix B – Radio certification The REB232ED, mounted on a REB controller base board (REB-CBB), has received regulatory approvals for modular devices in the United States and European countries. B.1 United States (FCC) Compliance Statement (Part 15.19) The device complies with Part 15 of the FCC rules.
B.2 Europe (ETSI) If the device is incorporated into a product, the manufacturer must ensure compliance of the final product to the European harmonized EMC and low-voltage/safety standards. A Declaration of Conformity must be issued for each of these standards and kept on file as described in Annex II of the R&TTE Directive.
Atmel AVR2092 EVALUATION BOARD/KIT IMPORTANT NOTICE This evaluation board/kit is intended for use for FURTHER ENGINEERING, DEVELOPMENT, DEMONSTRATION, OR EVALUATION PURPOSES ONLY. It is not a finished product and may not (yet) comply with some or any technical or legal requirements that are applicable to finished products, including, without limitation, directives regarding electromagnetic compatibility, recycling (WEEE), FCC, CE or UL (except as may be otherwise noted on the board/kit).
9 Table of contents Features ............................................................................................... 1 1 Introduction ...................................................................................... 1 2 Disclaimer......................................................................................... 2 3 Overview ........................................................................................... 3 4 Functional description................................................
Atmel AVR2092 B.2 Europe (ETSI)............................................................................................... 36 References......................................................................................... 36 EVALUATION BOARD/KIT IMPORTANT NOTICE ........................... 37 9 Table of contents ...........................................................................
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