Rainbow Electronics ATmega128RFA1 - Manual (Page 494)

494
8266A-MCU Wireless-12/09
ATmega128RFA1
The crystal (XTAL), the two load capacitors (CX1, CX2), and the internal circuitry
connected to pins XTAL1 and XTAL2 form the 16MHz crystal oscillator for the 2.4GHz
transceiver. To achieve the best accuracy and stability of the reference frequency, large
parasitic capacitances must be avoided. Crystal lines should be routed as short as
possible and not in proximity of digital I/O signals. This is especially required for the
High Data Rate Modes.
The 32.768 kHz crystal connected to the internal low power (sub 1µA) crystal oscillator
provides a stable time reference for all low power modes including 32 Bit IEEE 802.15.4
Symbol Counter (
"MAC Symbol Counter" on page 133 ) and real time clock application
using the asynchronous timer T/C2 ( "8-bit Timer/Counter2 with PWM and
Asynchronous Operation" on page 309 ). Total capacitance including CX3, CX4 should
not exceed 15pF on both pins. The very low supply current of the oscillator requires
careful layout of the PCB and any leakage path must be avoided.
Crosstalk and radiation from switching digital signals to the crystal pins or the RF pins
can degrade the system performance. The programming of minimum drive strength
settings for the digital output signal is recommended (see
"DPDS0 – Port Driver
Strength Register 0" on page 174 ).
Table 32-1. Example Bill of Materials (BoM) for "Basic Application Schematic" on page 493
Designator Description Value Manufacturer Part Number Comment
B1 SMD balun
SMD balun / filter 2.4 GHz Wuerth
Johanson
Technology 748421245
2450FB15L0001
Filter included
CB1
CB3 LDO VREG
bypass capacitor 1 µF
(100nF minimum)
CB2
CB4 Power supply bypass
capacitor 1 µF
(100nF minimum)
AVX
Murata 0603YD105KAT2A
GRM188R61C105KA12D X5R
(0603) 10% 16V
CX1, CX2
Crystal load capacitor
12 pF AVX
Murata 06035A120JA
GRP1886C1H120JA01 COG
(0603) 5% 50V
C0G 5% C1, C2
RF coupling capacitor
22 pF Epcos
Epcos
AVX B37930
B37920
06035A220JAT2A
(0402 or 0603)
50V
C4 (optional) RF matching 0.47 pF Johnstech
R1 CLKM low-pass
filter resistor 680Ω Designed for f
CLKM
= 1 MHz
XTAL Crystal CX-4025 16 MHz
SX-4025 16 MHz ACAL Taitjen
Siward XWBBPL-F-1
A207-011
XTAL 32kHz Crystal Rs=100 kOhm
32.2 Extended Feature Set Application Schematic
The ATmega128RFA1 supports additional features like:
• Security Module (AES)
• High Data Rate Mode up to 2MBits/s
• Antenna Diversity using alternate pin function DIG1/2 at Port G and F
• RX/TX Indicator using alternate pin function DIG3/4 at Port G and F
An extended feature set application schematic illustrating the use of the
ATmega128RFA1 Extended Feature Set, is shown in
Figure 32-2 on page 495 .