AVR2043: REB231ED - Hardware User Manual Features • High-performance, 2.4GHz, RF-CMOS AT86RF231 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.
AVR2043 4 Functional description The block diagram of the REB231ED 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 AT86RF231 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.
.1.1 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.
AVR2043 Pin# Function Pin# Function 27 PB3 (open) 28 PB2 (RSTN) 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.
Address Name Type Description 0x10 Board family uint8 Internal board family identifier 0x11 Revision uint8[3] Board revision number ##.##.## 0x14 Feature uint8 Board features, coded into seven bits 7 Reserved 6 Reserved 5 External LNA 4 External PA 3 Reserved 2 Diversity 1 Antenna 0 SMA connector 0x15 Cal OSC 16MHz uint8 RF231 XTAL calibration value, register XTAL_TRIM 0x16 Cal RC 3.6V uint8 Atmel ATmega1281 internal RC oscillator calibration value @ 3.
AVR2043 Depending on the actual supply voltage, the diode D1 can consume several milliamperes. This has to be considered when the current consumption of the whole system is measured. In such a case, D1 should be removed from the board. To achieve the best RF performance, the analog (EVDD, AGND) and digital (DEVDD, DGND) supply are separated from each other by a CLC PI-element. Digital and analog ground planes are connected together on the bottom layer, underneath the radio transceiver IC.
redesign of R8 and C36. In case of RC cut-off frequency adjustments, depending on the specific load and signal routing conditions, one may observe performance degradation of channel 26. NOTE Channel 26 (2480MHz) is affected by the following harmonics: 155 x 16MHz or 310 x 8MHz. By default, CLKM is routed to a microcontroller timer input; check the individual configuration resistors in the schematic drawing. To connect CLKM to the microcontroller main clock input, assemble R3 with a 0Ω resistor. 4.
AVR2043 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. The PCB has to be considered as a part of the antenna; it interacts with the radiated electromagnetic wave. • Isolate digital noise from the antenna and the radio transceiver to achieve optimum range and RF performance.
5.1 PCB detail 1 – balanced RF pin fan out Figure 5-2. Board layout – RF pin fan out. The Atmel AT86RF231 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 DC blocking capacitors, C26 and C27. The trace width is kept small at 0.2mm for a length of approximately 1.5mm. The REB231ED is a twolayer FR4 board with a thickness of 1.5mm.
AVR2043 5.2 PCB detail 2 – RF switch Figure 5-3. Board layout – RF switch. The RF switch requires a solid grounding and RF filter capacitors for the control pins. A parasitic inductance may reduce the RF isolation of the switch in the off state. To achieve a hard, low-impedance ground connection, a via is placed on each side of the ground pad. Additionally, the ground pad is connected to the top layer ground plane.
Figure 5-4. Board layout – XTAL section. The reference crystal and load capacitors C34/35 form the resonator circuit. These capacitors are to be placed close to the crystal. The ground connection in between the capacitors should be the crystal housing contact, if available, resulting in a compact, robust and stable resonator. The resonator block is enclosed within ground traces around it and a plane on the bottom side. Do not connect the resonator directly to the plane beneath the block.
AVR2043 bottom ground connection can help to keep this connection separate and prevent the layout tool from flooding across this trace. When designing applications for very harsh environments, for example where the radio transceiver is close to mains power lines and burst and surge requirements already dictate special provisions in the design, the above reference crystal design might not work well. In this case, the reference crystal ground is to be directly connected to top and bottom layers. 5.
5.5 PCB – digital GND routing With the Atmel AT86RF231, consider pins 7, 12, 16, 18, and 21 as digital ground pins. Digital ground pins are not directly connected to the paddle. Digital ground pins may carry digital noise from I/O pad cells or other digital processing units within the chip. In case of a direct paddle connection, impedances of the paddle ground vias could cause a small voltage drop for this noise and may result in an increased noise level transferred to the analog domain. 5.
AVR2043 Figure 5-6. Initial antenna tuning and test board. Besides the antenna tuning, the test board was used to measure the diversity effect and the coupling between the two antennas. The better the two antennas are isolated from each other, the higher is the diversity advantage for the receiver. It has to be considered that the unused antenna is operating against an open line end because the RF switch, U1, has high impedance in the off position.
From Figure 5-8, page 17, 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. Conclusion: an antenna diversity design should place antennas at a distance larger than that. 3. For almost all positions, only one antenna is in a deep fade.
AVR2043 Figure 5-8. Local fading effects in an indoor multipath environment. 5.7.2 Antenna design-in This section describes the antenna design-in detail, and the implementation of the antenna tuning structure. An overview of the layout can be found in Figure 5-9. Figure 5-9. Antenna PCB environment and tuning structure.
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 REB231ED, the environment differs considerably because the FR4 height is 1.
AVR2043 Figure 5-10. Antenna feed line short for extended length calibration. 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 5-11. 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 1 Xc = * 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 5-12, page 21.
AVR2043 Figure 5-12. 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 5-13. Final tuning.
Figure 5-13, page 21, and Figure 5-14 show the final result as a diagram and on the board. Figure 5-14. 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.
AVR2043 6 Mechanical description The REB231ED 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. Figure 6-1. Mechanical outline (dimensions in mm). Table 6-1. REB231ED mechanical dimensions.
7 Electrical characteristics 7.1 Absolute maximum ratings Stresses beyond those listed under “Absolute Maximum Ratings” 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.
AVR2043 No. Parameter Condition Minimum Typical Maximum Units 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.
Appendix A - PCB design data A.
AVR2043 A.2 Assembly drawing Figure 8-1.
A.3 Bill of materials Table 8-1. Bill of materials. Qty. Designator Description Footprint Manuf. Part# Manufacturer Comment 2 X2, X3 SMA - Buchse SMA_edge_1.5 142-0711-821 Johnson Phoenix 522148603G 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.
AVR2043 A.4 – Radio certification The REB231ED, mounted on a REB controller base board (REB-CBB), has received regulatory approvals for modular devices in the United States and European countries. A.4.1 United States (FCC) Compliance Statement (Part 15.19) The device complies with Part 15 of the FCC rules.
A.4.2 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.
AVR2043 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 ........................................................................................... 2 4 Functional description................................................
Atmel Corporation 2325 Orchard Parkway San Jose, CA 95131 USA Tel: (+1)(408) 441-0311 Fax: (+1)(408) 487-2600 www.atmel.com Atmel Asia Limited Unit 01-5 & 16, 19F BEA Tower, Milennium City 5 418 Kwun Tong Road Kwun Tong, Kowloon HONG KONG Tel: (+852) 2245-6100 Fax: (+852) 2722-1369 Atmel Munich GmbH Business Campus Parkring 4 D-85748 Garching b. Munich GERMANY Tel: (+49) 89-31970-0 Fax: (+49) 89-3194621 Atmel Japan 9F, Tonetsu Shinkawa Bldg.
