APPLICATION NOTE Atmel AT01080: XMEGA E Schematic Checklist Atmel AVR XMEGA E Features • • • • • Power supplies Reset circuit Clocks and crystal oscillators PDI TWI Introduction This application note describes a common checklist which should be used when starting and reviewing the schematics for an Atmel® AVR® XMEGA® E design. 42087A−AVR−04/2013 Table of Contents 1. Power Supplies .................................................................................... 3 1.1 Power supply connections ................................................................................ 3 1.2 External analog reference connections ............................................................. 4 2. External Reset Circuit .......................................................................... 4 3. Clocks and Crystal Oscillators ............................................................. 5 3.1 External clock source ........................................................................................ 5 3.2 Crystal oscillator ................................................................................................ 6 3.3 Real-time oscillator............................................................................................ 6 4. PDI Interface ........................................................................................ 8 5. TWI Interface ....................................................................................... 9 6. Suggested Reading ............................................................................. 9 6.1 Datasheets and manual .................................................................................... 9 7. Revision History ................................................................................. 10 Atmel AT01080: XMEGA E Schematic Checklist [APPLICATION NOTE] 42087A−AVR−04/2013 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. Table 1-1. Power supply checklist. Signal name Recommended pin connection Description VCC 1.6V to 3.6V Decoupling/filtering capacitors 100nF (1)(2) and 10µF (1) Decoupling/filtering inductor 10µH (1)(3) Digital supply voltage AVCC 1.6V to 3.6V Decoupling/filtering capacitors 100nF (1)(2) and 10µF (1) Ferrite bead (4) 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 AT01080: XMEGA E Schematic Checklist [APPLICATION NOTE] 42087A−AVR−04/2013 3 1.2 External analog reference connections Atmel AVR XMEGA E proposes one ADC using internal references or an external analog reference (AREFA on PORTA or AREFD on PORTD). 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. Table 1-2. External analog reference checklist. Signal name Recommended pin connection Description AREFA 1.0V to AVCC-0.6V for ADC Decoupling/filtering capacitors 100nF (1)(2) and 4.7µF (1) External reference from AREF pin on PORT A AREFD 1.0V to AVCC-0.6V for ADC Decoupling/filtering capacitors 100nF (1)(2) and 4.7µF (1) External reference from AREF pin on PORT D 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. Atmel AT01080: XMEGA E Schematic Checklist [APPLICATION NOTE] 42087A−AVR−04/2013 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. Table 3-1. External clock source checklist. Signal name Recommended pin connection Description XTAL1 XTAL1 is used as input for an external clock signal EXTCLK: input for external clock signal on PORT R pin 1 XTAL2 Can be left unconnected or used as GPIO PC4 PC4 is used as input for an external clock signal EXTCLK: input for external clock signal on PORT C pin 4 Atmel AT01080: XMEGA E Schematic Checklist [APPLICATION NOTE] 42087A−AVR−04/2013 5 3.2 Crystal oscillator Figure 3-2. Crystal oscillator example schematic. Table 3-2. Crystal oscillator checklist. Signal name Description (1)(2) XTAL1 Load capacitor 15pF XTAL2 Load capacitor 15pF (1)(2) Notes: 3.3 Recommended pin connection 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 E 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. Note: Maximum ESR recommendation for 32.768kHz watch crystal. Crystal CL [pF] Maximum ESR [kΩ] (1) 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 AT01080: XMEGA E Schematic Checklist [APPLICATION NOTE] 42087A−AVR−04/2013 6 Figure 3-3. Real-time oscillator without load capacitor. 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 TOSC1 TOSC2 Notes: Recommended pin connection Description Load capacitor 18pF (1)(2) Timer oscillator pin 1 Load capacitor 18pF (1)(2) 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 AT01080: XMEGA E Schematic Checklist [APPLICATION NOTE] 42087A−AVR−04/2013 7 4. PDI Interface Figure 4-1. PDI interface example schematic. The connector pinout that is shown in Figure 4-1 mates with Atmel tools like the Atmel AVR JTAGICE3 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 AT01080: XMEGA E Schematic Checklist [APPLICATION NOTE] 42087A−AVR−04/2013 8 5. TWI Interface Figure 5-1. TWI interface example schematic. The TWI module in XMEGA devices follows the electrical specifications and timing of I2C bus and SMBus. The two lines are open-collector lines (wired-AND), and pull-up resistors (Rp) are the only external components needed to drive the bus. Table 5-1. TWI interface checklist. Signal name Recommended pin connection Description SCL This pull-up resistor is mandatory on the TWI bus topology since this line is open-collector line. The value of pull-up resistor Rp will depend on the SCL frequency: (fSCL ≤100kHz, fSCL ≤400kHz, fSCL ≤1MHz). Refer to chapter Two-wire interface characteristics in the datasheet for choosing the right value. SCL serial clock line SDA This pull-up resistor is mandatory on the TWI bus topology since this line is open-collector line. The value of pull-up resistor Rp will depend on the SCL frequency: (fSCL ≤100kHz, fSCL ≤400kHz, fSCL ≤1MHz). Refer to chapter Two-wire interface characteristics in the datasheet for choosing the right value. SDA serial data line 6. Suggested Reading 6.1 Datasheets and manual 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 and Manuals section. Atmel AT01080: XMEGA E Schematic Checklist [APPLICATION NOTE] 42087A−AVR−04/2013 9 7. Revision History Doc. Rev. Date Comments 42087A 04/2013 Initial document release Atmel AT01080: XMEGA E Schematic Checklist [APPLICATION NOTE] 42087A−AVR−04/2013 10 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. 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