10
8393CS-MCU Wireless-09/14
ATmega256/128/64RFR2
Capacitors should be placed as close as possible to the pins and should have a low-
resistance and low-inductance connection to ground to achieve the best performance.
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 1
µ
A) crystal oscillator
provides a stable time reference for all low power modes including 32 Bit IEEE 802.15.4
Symbol Counter (
"MAC Symbol Counter"
) and real time clock application using the
asynchronous timer T/C2 (
"Timer/Counter2 with PWM and Asynchronous Operation"
).
Total shunt capacitance including CX3, CX4 should not exceed 15pF across 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"
).
Table 4-1.
Bill of Materials (BoM)
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
µ
F
(100nF minimum)
AVX
Murata
0603YD105KAT2A
GRM188R61C105KA12D
X5R
(0603)
10%
16V
CB2
CB4
Power supply bypass
capacitor
1
µ
F
(100nF minimum)
CX1, CX2
16MHz crystal load
capacitor
12 pF
AVX
Murata
06035A120JA
GRP1886C1H120JA01
COG
(0603)
5%
50V
CX3, CX4
32.768kHz crystal load
capacitor
12 … 25 pF
C1, C2
RF coupling capacitor
22 pF
Epcos
Epcos
AVX
B37930
B37920
06035A220JAT2A
C0G
5%
50V
(0402 or 0603)
C4 (optional) RF matching
0.47 pF
Johnstech
XTAL
Crystal
CX-4025 16 MHz
SX-4025 16 MHz
ACAL Taitjen
Siward
XWBBPL-F-1
A207-011
XTAL 32kHz Crystal
Rs=100 kOhm
4.2 Extended Feature Set Application Schematic
The ATmega256/128/64RFR2 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
