APPLICATION NOTE Atmel AT01607: XMEGA C Schematic Checklist 8-bit Atmel Microcontrollers Features • • • • • Power supplies Reset circuit Clocks and crystal oscillators PDI USB Introduction This application note describes a common checklist which should be used when starting and reviewing the schematics for an Atmel® AVR® XMEGA® C design. 8466A−AVR−06/2013 Table of Contents 1. Power supplies ................................................................................... 3 1.1 1.2 Power supply connections ................................................................................ 3 External analog reference connections ............................................................. 4 2. External reset circuit........................................................................... 4 3. Clocks and crystal oscillators ............................................................. 5 3.1 3.2 3.3 External clock source ........................................................................................ 5 Crystal oscillator ................................................................................................ 6 Real-time oscillator............................................................................................ 6 4. PDI interface ...................................................................................... 8 5. USB interface ..................................................................................... 9 6. Suggested reading ........................................................................... 10 6.1 Datasheets and manual .................................................................................. 10 7. Revision History ............................................................................... 11 Atmel AT01607: XMEGA C Schematic ChecklistAtmel AT01607: XMEGA C Schematic Checklist [APPLICATION NOTE] 8466A−AVR−06/2013 2 1. Power supplies 1.1 Power supply connections All power supply pins of the device must be connected to the microcontroller supply. Both VCC (digital) and AVCC (analog) must be connected to the same microcontroller positive supply, thus ensuring that they both share an identical supply profile. Likewise both ground pins must be connected to the same microcontroller ground reference supply. Figure 1-1. Power supply schematic. Close to device VCC 10μH 100nF 10μF GND POWER SUPPLY Close to power supply Close to device Close to device Ferrite Bead AVCC 10μF 100nF AGND Table 1-1. Power supply checklist. Signal name Recommended pin connection Description VCC 1.6V to 3.6V (1)(2) (1) Decoupling/filtering capacitors 100nF and 10µF (1)(3) Decoupling/filtering inductor 10µH Digital supply voltage AVCC 1.6V to 3.6V (1)(2) (1) Decoupling/filtering capacitors 100nF and 10µF (4) Ferrite bead prevents the VCC noise interfering the AVCC Analog supply voltage GND Notes: Ground 1. These values are given only as a typical example (that is, ceramic capacitors: 100nF, SMD 0402, X7R, 16V and 10µF, SMD1206, X5R, 6.3V) (that is, inductor: 10µH, 1.2A). 2. Decoupling capacitor should be placed close to the device for each supply pin pair in the signal group, low ESR caps should be used for better decoupling. 3. Wire wound inductor should be added between the external power and the VCC for power filtering. 4. Ferrite bead has better filtering performance than the common inductor at high frequency. It can be added between VCC and AVCC for preventing digital noise from entering the analog power. The BEAD should provide enough impedance (for example, 220Ω at 100MHz, rated current 200mA, that is, Murata BLM15BB221SN1D) for separating the digital power to the analog power. Atmel AT01607: XMEGA C Schematic ChecklistAtmel AT01607: XMEGA C Schematic Checklist [APPLICATION NOTE] 8466A−AVR−06/2013 3 1.2 External analog reference connections Atmel AVR XMEGA C proposes one ADC using internal references or an external analog reference (AREFA on PORTA). The following schematic checklist is only recommended if the design is using the external analog reference. If the internal reference is used, the circuit is not necessary. Figure 1-2. External VREF schematic. Close to power supply Close to device AREFA EXTERNAL REFERENCE 1 4.7μF 100nF AGND Table 1-2. External analog reference checklist. Signal name Recommended pin connection Description AREFA 1.0V to AVCC-0.6V for ADC (1)(2) (1) Decoupling/filtering capacitors 100nF and 4.7µF External reference from AREF pin on PORT A GND Notes: 2. Ground 1. These values are given only as a typical example. 2. Decoupling capacitor should be placed close to the device. External reset circuit The external reset circuit is connected to /RESET pin only if the external reset function is used. Figure 2-1. External reset circuit example schematic. VCC 10 kΩ RESET Reset Switch 100nF 100 Ω Atmel AT01607: XMEGA C Schematic ChecklistAtmel AT01607: XMEGA C Schematic Checklist [APPLICATION NOTE] 8466A−AVR−06/2013 4 Table 2-1. Reset circuit checklist. Signal name Recommended pin connection Description ______ RESET Reset low level threshold voltage VCC = 2.7 - 3.6V: Below 0.45 × VCC VCC = 1.6 - 2.7V: Below 0.42 × VCC (VCC = 2.7V included) Reset pin Notes: The pull-up resistor makes sure that reset does not go low unintended. When the PDI programming and debugging is used, the reset line is used as clock. The reset pull-up should be 10kΩ or weaker, or be removed. The pull-down resistor prevents from overvoltage on the RESET pin when the switch is pressed. Any reset capacitors should be removed if PDI programming and debugging is used. Other external reset sources should be disconnected 3. Clocks and crystal oscillators 3.1 External clock source Figure 3-1. External clock source example schematic. External Clock XTAL1 N.C. Table 3-1. XTAL2 External clock source checklist. Signal name Recommended pin connection Description XTAL1 XTAL1 is used as input for an external clock signal Input for inverting oscillator pin 1 XTAL2 Can be left unconnected or used as GPIO Atmel AT01607: XMEGA C Schematic ChecklistAtmel AT01607: XMEGA C Schematic Checklist [APPLICATION NOTE] 8466A−AVR−06/2013 5 3.2 Crystal oscillator Figure 3-2. Crystal oscillator example schematic. 15pF XTAL1 Up to 16 MHz XTAL2 15pF Table 3-2. Crystal oscillator checklist. Signal name XTAL1 Load capacitor 15pF (1)(2) XTAL2 Load capacitor 15pF (1)(2) Notes: 3.3 Recommended pin connection Description External crystal between 0.4MHz to 16MHz 1. These values are given only as a typical example. Please refer to the crystal datasheet to determine the capacitor value for the crystal used or refer to the application note “AVR1003: Using the XMEGA Clock System”. 2. Load capacitors should be placed close to the device and crystal pins. Real-time oscillator The low-frequency crystal oscillator is optimized for use with a 32.768kHz watch crystal. When selecting crystals, load capacitance and crystal’s equivalent series resistance, ESR must be taken into consideration. Both values are specified by the crystal vendor. The Atmel AVR XMEGA C oscillator is optimized for very low power consumption, and thus when selecting crystals, see Table 3-3 for maximum ESR recommendations on 9pF and 12.5pF crystals. Table 3-3. Maximum ESR recommendation for 32.768kHz watch crystal. Crystal CL [pF] Notes: 1. Maximum ESR [kΩ] 9.0 65 12.5 30 (1) Maximum ESR is typical value based on characterization. The low-frequency crystal oscillator provides an internal load capacitance of typical 3.0pF. Crystals with recommended 3.0pF load capacitance can be without external capacitors as shown in Figure 3-3. Atmel AT01607: XMEGA C Schematic ChecklistAtmel AT01607: XMEGA C Schematic Checklist [APPLICATION NOTE] 8466A−AVR−06/2013 6 Figure 3-3. Real-time oscillator without load capacitor. TOSC1 32.768kHz TOSC2 Crystals specifying load capacitance (CL) higher than 3.0pF, require external capacitors applied as described in Figure 3-4. Figure 3-4. Real-time oscillator with load capacitor. 18pF TOSC1 32.768kHz TOSC2 18pF To find suitable load capacitance for a 32.768kHz crystal, please consult the crystal datasheet. Table 3-4. External real-time oscillator checklist. Signal name Recommended pin connection TOSC1 Load capacitor 18pF (1)(2) TOSC2 Load capacitor 18pF (1)(2) Notes: Description Timer oscillator pin 1 Timer oscillator pin 2 1. These values are given only as a typical example. Please refer to the crystal datasheet to determine the capacitor value for the crystal used or refer to the application notes “AVR1003: Using the XMEGA Clock System” and “AVR4100: Selecting and testing 32kHz crystal oscillators for Atmel AVR microcontrollers”. 2. Load capacitors should be placed close to the crystal, GND and device oscillator pins. Atmel AT01607: XMEGA C Schematic ChecklistAtmel AT01607: XMEGA C Schematic Checklist [APPLICATION NOTE] 8466A−AVR−06/2013 7 4. PDI interface Figure 4-1. PDI interface example schematic. PDI_DATA VCC 1 3 5 DATA VCC N.C. N.C. CLK GND 2 4 6 10 kΩ 2x3 header PDI_CLK RESET The connector pinout that is shown in Figure 4-1 mates with Atmel tools like the Atmel AVR JTAGICE 3 and Atmel AVR ONE! Table 4-1. PDI port interface checklist. Signal name Recommended pin connection Description PDI_CLK This pull-up resistor makes sure that reset does not go low unintended. When the PDI programming and debugging is used, the reset line is used as clock. The reset pull-up should be 10kΩ or weaker, or be removed. Any reset capacitors should be removed if PDI programming and debugging is used. Other external reset sources should be disconnected. PDI clock input / reset pin PDI_DATA PDI_DATA: PDI data input / output Atmel AT01607: XMEGA C Schematic ChecklistAtmel AT01607: XMEGA C Schematic Checklist [APPLICATION NOTE] 8466A−AVR−06/2013 8 USB interface The impedance of the USB differential data line pair is 90Ω to each other and 45Ω to ground. The termination of the line is included within the Atmel AVR XMEGA C device as serial resistors. To ensure proper signal integrity, the two D+/Dsignals must be closely routed on the PCB (Refer to “AVR1017: XMEGA - USB Hardware Design Recommendations” application note). Figure 5-1. Low-cost USB interface example schematic. USB Connector VBUS USB Differential Data Line Pair VBUS D+ DGND USB_D+ USB_D- Shield GND (Board) Figure 5-2. Protected USB interface example schematic. VBUS USB Transient protection USB Connector USB Differential Data Line Pair VBUS D+ DGND Shield RC Filter (GND/Shield Connection) USB_D+ USB_D- 4.5nF 1MΩ 5. GND (Board) Atmel AT01607: XMEGA C Schematic ChecklistAtmel AT01607: XMEGA C Schematic Checklist [APPLICATION NOTE] 8466A−AVR−06/2013 9 Table 5-1. Signal name D+ USB interface checklist. Recommended pin connection • The impedance of the pair should be matched on the PCB to minimize reflections • USB differential tracks should be routed with the same characteristics (length, width, number of vias, etc.) D- • Signals should be routed as parallel as possible, with a minimum number of angles and vias 6. Suggested reading 6.1 Datasheets and manual Description USB full speed / low speed positive data upstream pin USB full speed / low speed negative data upstream pin The datasheet and the manual contain block diagrams of the peripherals and details about implementing firmware for the device. The datasheet and the manual are available on http://www.atmel.com/AVR in the Datasheets & Manuals section. Atmel AT01607: XMEGA C Schematic ChecklistAtmel AT01607: XMEGA C Schematic Checklist [APPLICATION NOTE] 8466A−AVR−06/2013 10 7. Revision History Doc. Rev. Date Comments 8466A 06/2013 Initial document release Atmel AT01607: XMEGA C Schematic ChecklistAtmel AT01607: XMEGA C Schematic Checklist [APPLICATION NOTE] 8466A−AVR−06/2013 11 Atmel Corporation Atmel Asia Limited Atmel Munich GmbH Atmel Japan G.K. 1600 Technology Drive Unit 01-5 & 16, 19F Business Campus 16F Shin-Osaki Kangyo Bldg. San Jose, CA 95110 BEA Tower, Millennium City 5 Parkring 4 1-6-4 Osaki, Shinagawa-ku USA 418 Kwun Tong Road D-85748 Garching b. Munich Tokyo 141-0032 Tel: (+1)(408) 441-0311 Kwun Tong, Kowloon GERMANY JAPAN Fax: (+1)(408) 487-2600 HONG KONG Tel: (+49) 89-31970-0 Tel: (+81)(3) 6417-0300 www.atmel.com Tel: (+852) 2245-6100 Fax: (+49) 89-3194621 Fax: (+81)(3) 6417-0370 Fax: (+852) 2722-1369 © 2013 Atmel Corporation. All rights reserved. / Rev.: 8466A−AVR−06/2013 Atmel®, Atmel logo and combinations thereof, AVR®, Enabling Unlimited Possibilities®, XMEGA®, and others are registered trademarks or trademarks of Atmel Corporation or its subsidiaries. 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