Atmel AVR042 Istruzioni per l'installazione e il funzionamento

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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2521C-AVR-09/05

2 Power Supply

Two aspects should be considered when designing the power supply for the

discrete/digital elements of an 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.

2.1 Digital supply

Looking at the datasheet for an 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 2-1. Incorrect decoupling

Figure 2-1 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 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

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2521C-AVR-09/05

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.

Figure 2-2. Decoupling with series inductor.

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

(like the ATmega8535) will get better decoupling than devices with industry standard

pin-out (like the 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 capacitor of some µF to stabilize it.

2.2 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. The analog

ground, AGND, is also separated from the digital ground, - for the same reasons.

To be able to obtain good accuracy with the ADC the analog supply voltage must be

decoupled separately, in the same manor as the digital supply voltage. The analog

ground should be separated from the digital ground, - so that the analog and digital

ground are only connected at one point, - the origin of the GND i.e. at the power

supply GND.

3 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.

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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

11.5 – 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.

Different approaches can be used to connect the RESET pin so that unintentional

reset of the AVR is avoided. External brown-out or supervisory circuit can be used to

control the RESET pin. If an external brown-out circuit, like the ones described in

application note AVR180, is applied one would not need to consider how to connect

the RESET pin further. However, if the AVR device used in the application has built-in

brown-out circuit, the external brown-out can be saved and a more simple solution

can be chosen to control the state of the RESET pin.

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 not go low unintended.

The pull-up resistor can in theory be of any size, but if the AVR should be

programmed from e.g. STK500/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 4.7kOhm or larger when using STK500 for programming.

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. 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. 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.

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2521C-AVR-09/05

Figure 3-1. Recommended Reset Pin connection.

4 Connecting ISP lines

The In-System Programmable (ISP) lines are used for programming the Flash,

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

ATtiny28). This feature makes it possible to program the AVR on 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. 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.

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.

Figure 4-1. Standard ISP connectors on STK500, AVR ISP and STK200/STK300

Some ISP programmers are supplied by the target power supply. In this way they

easily adapt to the correct voltage level of the target board. Other ISP programmers,

like STK500, can alternatively power the target board via the VTG line. In that case it

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is important that the power supply on the target is not switched on. Read the User

Guide of the ISP programmer to find out what capabilities your programmer has and

what kind of physical interface it has.

The ISP lines are on most devices located at the same pins as the Peripheral Serial

Interface (SPI), or else on pins that can be used for other purposes. Consult the

device data sheet to find out which lines are used for ISP. In case other devices than

the AVR is 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 4-2, is the easiest way of doing 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 4-2. Connecting the SPI to the ISP interface.

Multiple AVRs in a single application can share the same ISP interface to make it

possible to program all of 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. In this way all AVRs can

be located “behind” the same protection resistors, since they all are held in RESET

while the ISP reset line is activated. The gating of the ISP clock can be accomplished

e.g. using jumpers. Alternatively, a solution is to have multiple ISP interfaces, one for

each device, all protected as shown in Figure 4-2.

5 Using crystal and ceramic resonators

Most 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 are not well understood. This section therefore treats the topic

of using crystals and ceramic resonators in relation to AVR MCUs. The description

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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. 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

3 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.

One should be aware that if selecting a different clock source than is actually applied,

the AVR might not run since different oscillator circuits are activated internally in the

AVR dependent on the selected clock option. Since the fuses are not cleared by a

memory erase, it can cause problems if incorrect settings are chosen.

5.2 General about crystals and ceramic resonators

The typical type of crystal used for the AVR is the AT-cut parallel resonant crystal.

The ceramic resonator is very similar to the AT-cut parallel resonant crystal, but is so

to say a low cost, low quality version of the crystal. The ceramic resonator has a

lower Q-value, which is both an advantage and disadvantage. Due to the lower Q-

value the oscillator frequency of the ceramic resonator can more easily be “tuned” to

a desired frequency, but is also more sensitive to temperature and load changes,

causing undesired frequency variations. The advantage of the ceramic resonators is

that it has a faster start-up than crystals.

In general there will not be distinguished between crystals and ceramic resonators in

this section and the term “resonator” will thus refer to both devices.

Table 5-1. Technical differences between ceramic resonators and quarts crystal

Ceramic resonator Quartz crystal

Aging ±3000 ppm ± 10 ppm

Frequency tolerance ± 2000-5000 ppm ± 20 ppm

Freq. temperature characteristics

± 20-50 ppm/˚C ± 0.5 ppm/˚C

Frequency pullability ± 100-350 ppm/pF ± 15 ppm/pF

Oscillator rise time 0.01-0.5 ms 1-10 ms

Quality factor (Qm) 100-5000 10

-5·10

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The parallel resonator is intended used in circuits, which contains reactive

components, usual capacitors. Such circuits depend on the combination of the

reactive components and the resonator to accomplish the phase shift necessary to

start and maintain the oscillation at a given frequency. Basic oscillator circuits used

for parallel resonators are illustrated in Figure 5-1. The part of the circuit above the

dashed line, represent the oscillator circuit internally in the AVR.

Figure 5-1. Basic inverter circuits equivivalent to the oscillator circuits in AVRs.

Clock out

Xtal

XTAL1 XTAL2

(A)

Clock out

Xtal

XTAL1/

TOSC1 XTAL2/

TOSC2

(B)

Notes: (A) Oscillator circuit for crystals and ceramic resonators faster than 400kHz

(B) Circuit for low frequency crystals (32.768kHz) (not on all AVRs)

Simplifying the description of the AVRs built-in oscillator circuits they can be

understood as the inverter based oscillator circuits illustrated in Figure 5-1. The circuit

used with resonators of frequencies above 400kHz is depicted in (A). When using this

circuit, capacitive load must be applied externally. The oscillator circuit seen in (B) is

the circuit used for low frequency crystals on some AVRs - to be more specific -

optimized for 32.768kHz crystals. This circuit provides the capacitive load required by

the crystal internally and further adds the resistor Rbto bias the crystal and limit the

drive current into the crystal. The resistor Rfis, when using CMOS inverters,

approximately 1Mohm, and provides a feedback to bias the inverter to operate it in its

linear region. Consult data sheet for the relevant device to see if is has internal

circuitry for low frequency crystals.

When using resonators with the AVR, it is necessary to apply (external) capacitors

according to the requirements of the facilitated resonator. A parallel resonator will not

be able to oscillate stable if the capacitive load applied is insufficient. If the capacitive

load is too high the oscillation may have problems starting due to drive level

dependency of the load. The trick is therefore to find an appropriate value for the

capacitive load. The value to look for in the data sheet of the crystal is CL, the

recommended capacitive load of the resonator (viewed from the terminals of the

resonator). The capacitive load, CL, of the oscillator circuit, including stray

capacitances and the capacitances of the XTAL pins of the AVR can be determined

empirically or it can be estimated by Equation 5-1.

Equation 5-1.

CC CC

C+

⋅

Where CL1 and CL2 referrers to the external capacitors seen in Figure 5-1 and CSis

the combined capacitive load of the XTAL pins of the AVR and stray capacitances of

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2521C-AVR-09/05

the PCB. CScan be estimated to be 5-10pF. If CL1= CL2 then the external capacitors

can be determined by Equation 5-2:

Equation 5-2.

)(2

21 SLLL

CCCC

−

⋅

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. Overload is in general not

causing start-up problems for ceramic resonators and therefore this section primarily

applies to the use of crystals. To verify the safety factor of the oscillator, the

resonance load (or oscillation allowance) must be determined. By inserting a resistor

(RQ) in series with the crystal (temporarily), as illustrated in Figure 5-2, the resonance

load is determined.

Figure 5-2. RQin series with the crystal to determine the resonance load.

Clock out

Xtal

XTAL1 XTAL2

The way to do the test is to vary the external capacitors from a low capacitive load to

a load where the oscillator stops oscillating. Make sure to mount the capacitors firmly,

- avoid any kind of sockets.

Start the AVR having CL1 and CL2 as low as 1-2pF and verify that the oscillator is

actually oscillating. If a current probe is available this can be used to investigate if the

crystal is oscillating, otherwise a small test program toggling a pin on one of the I/O

ports can be used to accomplish this. Increase CL1 and CL1 until the oscillator stops

oscillating. Determine the safety factor by Equation 5-3. It is recommended to use a

capacitive load that results in a Safety Factor of 3-5.

Equation 5-3.

FactorSafety =

Where the resonance load RLis determined by Equation 5-4:

Equation 5-4.

1⎟

⎟

⎠

⎞

⎜

⎝

⎛+⋅=

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2521C-AVR-09/05

Where R1and C0are obtained from the datasheet of the resonator and CLis the

applied capacitive load calculated using Equation 5-1.

In the optimal test the curvature of the safety factor as a function of the applied

capacitive load should be determined. If this is done one should be aware that it is not

recommended to be at the top-point of the safety factor curvature, but rather at a

point where the curve has started falling. Result of the test could look like the one

seen in Figure 5-3.

Figure 5-3. Safety factor curvature for a given value of RQ

/ C

[pF]

Safety factor

510 15 20 25 35 45 55

30 40 50 60 70

RQ=xx Ohm

In the example in Figure 5-3 the hatched area indicates the safe area. In general the

safety factor should be above 3 for the oscillator to be considered in the safe region of

operation. Other factors should though be considered as well: The minimum required

capacitive load, the recommended and the maximal drive level.

5.4 Recommended capacitor values

The recommendations here will work well in most applications, but there is no way to

provide general values for the external capacitors that can be guarantee to work with

all resonators.

When using the clock option “ext. crystal oscillator”, crystals with a nominal frequency

from 400 kHz and up can be used. For these standard “high” frequency crystals the

recommended capacitor value is in the range 22-33pF.

The clock option “ext. low frequency crystal” is intended for 32.768kHz crystals. When

selecting this clock source the internal oscillator circuit might provide the required

capacitive load. By programming the CKOPT Fuse, the user can enable internal

capacitors on XTAL1 and XTAL2. The value of the internal capacitors is typical 20pF,

but can vary. 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

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