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AVR042: AVR Hardware Design Considerations
Features
• Providing robust supply voltage, digital and analog.
• Connection of RESET line.
• SPI interface for In-System Programming.
• Using external crystal or ceramic resonator oscillators.
1 Introduction
This application note has been written to provide answers to some of the questions
and problems faced when starting designs involving AVR microcontrollers. The
application note treats topics that are known to cause problems. The scope is to
provide an introduction to potential design problems rather than being an
exhaustive walk-through of how to design applications using the AVR
microcontrollers. This document is thus a collection of information from existing
Atmel AVR documents, combined with information that is not previously
documented.
It is highly recommended to read the application note AVR040 - "EMC Design
Considerations" – before initiating a new design, specially if the aim of the design is
a commercial application that needs to meet the requirements of the EMC directive
(or similar directives in countries outside Europe). The application note is available
from the AVR section of the official Atmel website http://www.atmel.com.
8-bit
Microcontrollers
Application Note
Rev.2521C-AVR-09/05
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Summary of Contents for Atmel AVR042
- Page 1 AVR microcontrollers. This document is thus a collection of information from existing Atmel AVR documents, combined with information that is not previously documented. It is highly recommended to read the application note AVR040 - “EMC Design Considerations”...
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Page 2: Power Supply
Figure 2-2 shows a better placement of the capacitor. The lines that are part of the high current loop are not part of the power or ground planes. This is important, as the AVR042 2521C-AVR-09/05... - Page 3 AVR042 power and ground planes otherwise will spread a lot of noise. Further, the figure shows another improvement of the decoupling. A series inductor is inserted to reduce the switching noise on the power plane. The series resistance of the inductor must of course be low enough to ensure that there will be no significant DC voltage drop.
- Page 4 Programming it is recommended to add an ESD protecting diode from RESET to Vcc, since this is not internally provided due to High Voltage Programming. The components should be located physically close to the RESET pin of the AVR. Figure 3-1 shows the recommended circuit on the RESET line. AVR042 2521C-AVR-09/05...
- Page 5 - just in case. Two standard connectors are provided by the Atmel ISP programmers; A 6 pin and a 10 pin connector. These are seen in Figure 4-1. In addition to the data lines (MOSI and MISO) and the bus clock (SCK), target voltage VTG, GND and RESET (RST) are provided through these connectors.
- Page 6 This section therefore treats the topic of using crystals and ceramic resonators in relation to AVR MCUs. The description AVR042 2521C-AVR-09/05...
- Page 7 AVR042 focus on features and parameters relevant for designing applications where crystals or ceramic resonators are used rather than trying to be a complete description of the theory related to the topic. 5.1 Selecting the clock source in the AVR The clock source used by the AVR is selected through the fuses.
- Page 8 Equation 5-1. Equation 5-1. ⋅ Where C and C referrers to the external capacitors seen in Figure 5-1 and C the combined capacitive load of the XTAL pins of the AVR and stray capacitances of AVR042 2521C-AVR-09/05...
- Page 9 AVR042 the PCB. C can be estimated to be 5-10pF. If C then the external capacitors can be determined by Equation 5-2: Equation 5-2. ⋅ − 5.3 Safety factor To avoid overloading the crystal, so that it does not start up, the safety factor of the oscillator (including the specific crystal) should be verified.
- Page 10 If using a 32.768kHz crystal that does not require more load than this, external capacitors can be left out. Otherwise external capacitive load must be added. In this case the capacitive load value, given by the manufacturer of the crystal, should AVR042 2521C-AVR-09/05...
- Page 11 AVR042 be used. Then the value of the external capacitors can be determined by Equation 2. The CKOPT Fuse should be unprogrammed when using external capacitors. Please reefer to the datasheet to assure whether the device has internal capacitors or not.
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Page 12: Pcb Layout
Figure 6-1 (A-D) shows a schematic and PCB layout using a crystal oscillator for ATmega8515, decoupling digital supply from VCC is seen. Note the ground plane surrounding the crystal and the very short distance between the crystal and the ATmega8515. AVR042 2521C-AVR-09/05... - Page 13 AVR042 Figure 6-1. ATmega8515 PB0/T0 AD0/PA AD1/PA PB1/T1 PB2/AIN0 AD2/PA AD3/PA PB3/AIN1 PB4/SS AD4/PA AD5/PA PB5/MOSI PB6/MISO AD6/PA AD7/PA PB7/SCK PD0/RXD A8/PC0 PD1/TXD A9/PC1 PD2/INT0 A10/PC2 PD3/INT1 A11/PC3 Inductor improves A12/PC4 power filtering PD5/OC1A A13/PC5 4.7k A14/PC6 PD6/WR PD7/RD A15/PC7...
- Page 14 ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. Atmel makes no representations or warranties with respect to the accuracy or completeness of the contents of this document and reserves the right to make changes to specifications and product descriptions at any time without notice. Atmel does not make any commitment to update the information contained herein.
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