Atmel AVR042 Installation and operating instructions

APPLICATION NOTE

AVR042: AVR Hardware Design Considerations

AVR 8-bit Microcontrollers

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

Description

This application note has been written to provide answers to some of the questions

and problems faced when starting designs involving Atmel®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, especially 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.

Atmel-2521M-AVR-Hardware-Design-Considerations_ApplicationNote_092014

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Table of Contents

1Power Supply............................................................................................................... 3

1.1 Digital Supply ........................................................................................................................................3

1.2 Analog Supply .......................................................................................................................................4

2Connection of RESET Pin on AVRs ........................................................................... 4

2.1 External RESET Switch.........................................................................................................................5

3Connecting ISP Lines.................................................................................................. 6

3.1 SPI Programming Interface ...................................................................................................................6

3.1.1 Shared Use of SPI Programming Lines....................................................................................7

3.2 JTAG Interface ......................................................................................................................................7

3.2.1 Shared Use of JTAG Lines.......................................................................................................8

3.3 PDI Interface .........................................................................................................................................9

3.3.1 External Reset Circuitry............................................................................................................9

3.4TPI Interface........................................................................................................................................10

4Using Crystal and Ceramic Resonators................................................................... 10

4.1 Selecting the Clock Source in the AVR ...............................................................................................10

4.2 General About Crystals and Ceramic Resonators...............................................................................11

4.3 Recommended Capacitor Values........................................................................................................12

4.4 Unbalanced External Capacitors.........................................................................................................13

4.5 RTC Crystals.......................................................................................................................................13

4.6 PCB Layout .........................................................................................................................................13

5Example Layout ......................................................................................................... 13

6Revision History ........................................................................................................ 16

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1 Power Supply

Two aspects should be considered when designing the power supply for the discrete/digital elements of an Atmel

AVR: ESD protection and noise emission. Both these topics are treated in details in the AVR040 application note,

and only a short summary is included in this document.

1.1 Digital Supply

Looking at the datasheet for an Atmel AVR microcontroller, one can be fooled to believe that power supply is not

critical. The device has a very wide voltage range, and draws only a few mA supply current. But as with all digital

circuits, the supply current is an average value. The current is drawn in very short spikes on the clock edges, and

if I/O lines are switching, the spikes will be even higher. The current pulses on the power supply lines can be

several hundred mA if all eight I/O lines of an I/O port changes value at the same time. If the I/O lines are not

loaded, the pulse will only be a few ns.

This kind of current spike cannot be delivered over long power supply lines; the main source is (or should be) the

decoupling capacitor.

Figure 1-1. Incorrect Decoupling

Figure 1-1 shows an example of insufficient decoupling. The capacitor is placed too far away from the

microcontroller, creating a large high current loop. The power and ground planes here are parts of the high

current loop. As a result of this, noise is spread more easily to other devices on the board, and radiated emission

from the board is increased even further. The whole ground plane will act as an antenna for the noise, instead of

only the high current loop. This will be the case if the power and ground pins are connected directly to the planes

(typical for hole-mounted components) and the decoupling capacitor is connected the same way. The same is

often seen for boards with surface-mount components if the integrated circuits are placed on one side of the

board and the decoupling capacitors are placed on the other.

Figure 1-2 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 power and ground planes otherwise will spread a lot of

noise. Further, the figure shows another improvement of the decoupling. A series ferrite bead is inserted to

reduce the switching noise on the power plane. The series resistance of the ferrite bead must of course be low

enough to ensure that there will be no significant DC voltage drop.

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Figure 1-2. Decoupling with Series Inductor

Generally, the Atmel AVR devices where power and ground lines are placed close together (like the Atmel

ATmega8535) will get better decoupling than devices with industry standard pin-out (like the Atmel

ATmega8515), where the power and ground pins are placed in opposite corners of the DIP package. This

disadvantage can be overcome by using for example a TQFP package, which allows decoupling capacitors to be

placed very close to the die. For devices with multiple pairs of power and ground pins, it is essential that every

pair of pins get its own decoupling capacitor.

The main supply should also have a tantalum or ceramic capacitor of some µF to stabilize it.

1.2 Analog Supply

The AVR devices that have built-in ADC have a separate analog supply voltage pin, AVcc. This separate voltage

supply is provided to make the analog circuits less prone to the digital noise originating from the switching of the

digital circuits.

To be able to obtain good accuracy with the ADC the analog supply voltage must be decoupled separately, in the

same manner as the digital supply voltage. AREF must also be decoupled; a typical value for the capacitor is

100nF. If a separate analog ground (AGND) is present, the analog ground should be separated from the digital

ground, - so that the analog and digital grounds are only connected at one point, - the origins of the GND i.e. at

the power supply GND.

2 Connection of RESET Pin on AVRs

The RESET pin on the AVR is active LOW, and setting the pin LOW externally will thus result in a reset of the

AVR. The RESET has two purposes:

1. To release all lines by tri-stating all pins (except XTAL pins), initialize all I/O registers and set program

counter to zero.

2. To enter programming mode (for some parts also the PEN line is used to enter programming mode).

Furthermore it is possible to enter high-voltage/parallel programming mode by drawing the RESET pin

“very” high, where very high means 11.5V – 12.5V (refer to the datasheet of the device for more

information).

The reset line has an internal pull-up resistor, but if the environment is noisy it can be insufficient and reset can

therefore occur sporadically. Refer to datasheet for value of pull-up resistor on specific devices.

Connecting the RESET so that it is possible to enter both high-voltage programming and ordinary low level reset

can be achieved by applying a pull-up resistor to the RESET line. This pull-up resistor makes sure that reset does

AVR Hardware Design Considerations [APPLICATION NOTE]

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not go low unintended. The pull-up resistor can in theory be of any size, but if the Atmel AVR should be

programmed from e.g. STK®500/AVRISP the pull-up should not be so strong that the programmer cannot

activate RESET by draw the RESET line low. The recommended pull-up resistor is 4.7kΩ or larger when using

STK500 for programming. For DebugWIRE to function properly, the pull-up must not be smaller than 10kΩ.

To protect the RESET line further from noise, it is an advantage to connect a capacitor from the RESET pin to

ground. This is not directly required since the AVR internally have a low-pass filter to eliminate spikes and noise

that could cause reset. Applying an extra capacitor is thus an additional protection. However, note that this

capacitor cannot be present if DebugWIRE or PDI is used.

If not using High Voltage 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. Alternatively, or in addition, a Zener diode

can be used to limit the RESET voltage relative to GND. The Zener diode is highly recommended in noisy

environments. The components should be located physically close to the RESET pin of the AVR. Figure 2-1

shows the recommended circuit on the RESET line.

2.1 External RESET Switch

If an external switch is connected to the RESET pin it is important to add a series resistance. Whenever the

switch is pressed it will short the capacitor, the current through the switch can have high peak values. This will

cause the switch to bounce and give steep spikes in 2ms - 10ms periods until the capacitor is discharged. The

PCB tracks and the switch metal will introduce a small inductance, this inductance can generate high voltages

according to Vi = L * di/dt. These voltages are most likely outside the specification of the RESET pin. By adding a

series resistor between the switch and the capacitor the peak currents generated will be significantly reduced,

these currents will not be large enough to generate high voltages at the RESET pin. An example connection is

shown in Figure 2-2.

Figure 2-1. Recommended Reset Pin Connection

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Figure 2-2. Switch Connection for Reset Pin

3 Connecting ISP Lines

Atmel AVR microcontrollers feature one or more interfaces for In-System Programming (ISP). These are used for

programming the Flash, EEPROM, Lock-bits, and most Fuse-bits in all AVRs (except the Atmel ATtiny11 and

ATtiny28). This feature makes it possible to program the AVR in the last stage of production of a target

application board, reprogram if SW bugs are identified late in the process, or even update the AVR in the field if

required. Some of the ISP interfaces may also be used for on-chip debugging. It is therefore highly

recommended to always design the target application board so that the ISP connectors can be accessed in some

way — just in case.

The different programming interfaces are introduced in the following subsections. Refer to the device datasheet

and Atmel AVR Tools help for further information on which interfaces are supported by the device, and how to

connect the programming tool.

3.1 SPI Programming Interface

On devices that use a Serial Peripheral Interface (SPI) for ISP, these lines are usually located on the same pins

as regular SPI, or else on pins that can be used for other purposes. Consult the device data sheet to find out

which pins are used for ISP.

Two standard SPI connectors are provided by the Atmel ISP programmers; a 6-pin and a 10-pin connector.

These are seen in Figure 3-1. 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.

Figure 3-1. Standard ISP Connectors on STK500, AVR ISP, and STK200/STK300

Some ISP programmers are powered by the target power supply. In this way they easily adapt to the correct

voltage level of the target board. Other ISP programmers, like Atmel STK500, can alternatively power the target

board via the VTG line. In that case it is important that the power supply on the target is not switched on. Read the

user guide for your ISP programmer for details on its capabilities and physical interface.

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3.1.1 Shared Use of SPI Programming Lines

If additional devices are connected to the ISP lines, the programmer must be protected from any device, other

than the AVR, that may try to drive the lines. This is especially important with the SPI bus, as it is similar to the

ISP interface. Applying series resistors on the SPI lines, as depicted in Figure 3-2, is the easiest way to achieve

this.

The AVR will never drive the SPI lines in a programming situation, since the AVR is held in RESET to

enter programming mode – and RESET’ing the AVR tri-states all pins.

Figure 3-2. Connecting the SPI Lines to the ISP Interface

Multiple Atmel AVR in a single application can share the same ISP interface to allow for programming of all the

devices through a minimal interface. However, the AVR devices will all respond to the ISP instructions if special

design considerations are not made.

If it is desired to have only one ISP interface on the target board, the ISP programming can be designed so that

only one of the AVR devices is provided with a SPI clock at a time. All other SPI lines can then be shared. This

way, several AVRs can be located “behind” the same protection resistors, since they are all held in RESET while

the ISP reset line is activated. The gating of the ISP clock can be accomplished by use of, e.g., jumpers or DIP

switches. An alternate solution is to have multiple ISP interfaces, one for each device, all protected as shown in

Figure 3-2.

3.2 JTAG Interface

Some devices have a JTAG interface, which can be used for both programming and debugging. The JTAG lines

are shared with analog input and should be connected so that the JTAG programmer can get control of the lines.

Though JTAG programming tools such as the AVR JTAGICE mkII can drive a resistive load (refer to AVR Studio®

help for details), capacitive load should in general be avoided.

Figure 3-3 shows the standard JTAG connector supplied with Atmel ISP programmers. Just as for the SPI

programming connector, the target’s voltage supply is made available to allow for powering of the device or

ensuring correct signal levels when programming.

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Figure 3-3. Standard JTAG Connector Used on e.g. AVR JTAGICE mkll and AVR ONE!

3.2.1 Shared Use of JTAG Lines

By creating a JTAG daisy-chain, a single JTAG connector can serve as an ISP interface for several devices. This

principle is shown in Figure 3-4 and is explained in further detail in the AVR Tools help file.

Figure 3-4. JTAG Daisy-Chain

Protection resistors, as shown in Figure 3-2, are necessary if the JTAG lines are also used for other purposes.

For example, if they are used for analog input to the Atmel AVR, there are often analog filters on the lines. If such

filters are present on the JTAG lines, it may be required to eliminate the filter capacitor during programming to

ensure that the load is mainly resistive. Figure 3-5 illustrates how this can be done.

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Figure 3-5. Filters on JTAG Pins – Correct and Incorrect Ways to Connect

3.3 PDI Interface

The Program and Debug Interface (PDI) is an Atmel proprietary two-line interface that was introduced with the

Atmel AVR XMEGA®microcontroller family. As the name implies, this interface can be used for both In-System

Programming and on-chip debugging of devices.

Figure 3-6 shows the standard PDI connector supplied with Atmel programmers. Only two pins on the device are

needed for use of this interface: RESET, also called PDI_CLK, and the dedicated PDI_DATA pin. The target’s

voltage supply is made available to allow for powering of the device or ensuring correct signal levels during

programming.

Figure 3-6. Standard PDI Connector Used on e.g. Atmel STK600 and AVR JTAGICE mkll

3.3.1 External Reset Circuitry

Since the RESET line is used for clocking the PDI, it is important to bypass or avoid any circuitry that can distort

the clock signal during programming or debugging, such as capacitors and external reset sources. During normal

operation the RESET pin has an internal filter to prevent unintentional resets, e.g. caused by short spikes on the

AVR Hardware Design Considerations [APPLICATION NOTE]

Atmel-2521M-AVR-Hardware-Design-Considerations_ApplicationNote_092014

reset line. Despite the fact that the clock signal is deformed capacitive loads up to 1nF have been tested to work

with the Atmel STK600, AVRISP mkII and AVR Dragon™ during programming and the JTAGICE mkII and the

Atmel AVR ONE! during programming and debugging.

Pull-up resistors should be at least 10kΩ, or be removed from the RESET line if an Atmel programmer is used.

3.4 TPI Interface

The Tiny Programming Interface is featured on the Atmel tinyAVR®devices with the lowest pin count.

Figure 3-7 shows the standard TPI connector supplied with Atmel programmers. Only three pins on the device

are needed for use of this interface: RESET, TPICLK, and TPIDATA. The two latter pins are multiplexed with

regular I/O pins.

Figure 3-7. Disabling RESET Pin Functionality

The RESET pin can be reconfigured as an I/O pin by programming the RSTDISBL fuse of the device. This

disables the reset functionality, and requires +12V to be applied to RESET for programming to work. Not all

programming tools are capable of generating this voltage.

4 Using Crystal and Ceramic Resonators

Most Atmel AVR MCUs can use different clock sources. The optional external clock sources are Clock, RC

oscillator, crystal or ceramic resonator. The use of crystals and ceramic resonators are in some designs causing

problems due to the fact that the use of these clock sources is not well understood. This section therefore treats

the topic of using crystals and ceramic resonators in relation to Atmel AVR MCUs. The description 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. For more information and theory

regarding crystals, refer to application note “AVR4100: Selecting and testing 32kHz crystal oscillators for AVR

microcontrollers”.

4.1 Selecting the Clock Source in the AVR

The clock source used by the AVR is selected through the fuses for all parts except the Atmel AVR XMEGA

family where the clock source is selected by software. Most ISP and parallel programmers can program the fuses

that are related to selecting a clock source. The fuses are not erased when the AVR memory is erased and the

fuses therefore only require to be programmed if the fuse settings should be altered. Programming the fuses

each time the device is erased and reprogrammed is thus not necessary. The clock options that are relevant for

this document are:

•“Ext. low-frequency crystal”

•“Ext. crystal oscillator”

•“Ext. ceramic resonator”

Several sub-settings relating to the start-up time of the AVR can be selected, but the three clock options

mentioned are the fundamental settings that should be focused on. The clock options available can vary between

AVR devices, - not all devices have the ability to run on various external oscillators. Check the datasheet for the

relevant device to determine the clock options.

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