55:036 embedded systems and systems...

Hardware Considerations Slide 1 Embedded Systems and Software, 55:036. The University of Iowa, 2013

Embedded Systems and Software

Lecture 12 Some Hardware Considerations


Logic States

Digital signals may be in one of three states

State 1: High, or “1”. Using positive logic polarity, this corresponds to a voltage level closer to the power supply than to ground.

In negative logic polarity, voltages levels closes to the supply than to ground are “0”, and voltage levels closer to ground than to the power supply are “1”. This is used in some serial communication protocols.

Example. For an ATmega88PA and Vcc = 5 V, the data sheet indicates that an input voltage larger than 0.6×Vcc = 3 V is considered high, or logic “1”.

Example. For an ATmega88PA and Vcc = 5 V, the data sheet indicates that an input less than 0.3×Vcc = 1.5 V is treated as a logic low or “0”

State 2: Low, or “0”. Using positive logic polarity, this corresponds to a voltage level closer to ground than to the power supply. For example, for an ATmega88PA and Vcc = 5 V, the data sheet indicates that an input voltage larger than 0.6 Vcc = 3 V is considered logic high, or “1”.

State 3: Tristate or high-Z, or simply Z. In this case the digital line is in a high-impedance state or “floating” . External components, often a pull-up resistor can pull the bus high or low.


Driving an LED

Vcc = 5 V AV

R PB2

𝑅𝑅𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙

Use the LED’s V-I characteristics from its datasheet to determine the forward voltage for a given current. Then apply Ohm’s Law to determine 𝑅𝑅𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙𝑙


Driving a High Power Load

Solid State Relay (SSR)

Cost ~ $14

• SIP SSR • Ratings of 5A @ 660 VAC • SCR output for heavy industrial loads • AC or DC control • Zero-crossing (resistive loads) or random-fire (inductive loads) output

Control

Load


Driving a High-Power External Load

Vcc = 5 V AV

R

PB2

SSR Control 5-14 V

@ 15 mA

Opt

o-is

olat

ed

Solid

Sta

te R

elay

220 V, 5 A Load

300 Ω

Rlimit

Depending on the SSR, one may need an external resistor here to limit current out of microcontroller port


Driving a High-Power External Load

Vcc = 5 V AV

R

PB2

Opt

o-is

olat

ed

Solid

Sta

te R

elay

220 V, 5 A Load

V on =

1.2

V @

10

mA Rlimit

Example: The absolute max rating for I/O current is 40 mA, and the SSR has Von = 1.2 V @ 10 mA. Determine Rlimit


Well-defined Reset Pin

It is important that the RESET pin has a well-defined voltage in normal operation, otherwise it may randomly reset. Also, if a mechnical switch is used for reset, consider contact bounce

As a minimum, pull the line high and add a capacitor to filter out any glitches that may get to the RESET line


Well-defined Reset Pin

This supervisor chip makes sure the MCUs RESET line is well-behaved when the RST button is pushed and released

Reset circuitry for 28-pin development board


Power Supplies

Bridge Rectifier

Voltage Regulator


Power Supplies

Power supply section from 28-pin development board

Bridge rectifier converts ac to dc. Also allows us to use dc.


Power Supplies


Smoothing Capacitor


Power Supplies


Three Terminal Regulator


Power Supplies


Sets Output Voltage


Power Supplies


Jump for 3.3V


Power Supplies


Needed for Stability


Power Supplies

Reverse-Polarity Protection

Diodes should be Schottky rather than regular Si diodes. Why?

Bridge provides reverse-polarity protection

What voltage is supplied to embedded system if the input voltage is 5 V? Assume Si diodes.


Power Supplies

Linear Three-Terminal Regulator

Constant voltage: 3 V, 5 V, etc.

Varying input voltage, say 7-14 V. Must be larger than output voltage + some headroom, typically 1-2 V

Note that linear regulators cannot step up a voltage, but regulate down.


Power Supplies


LM78XX Three-terminal regulator pinout in TO220 package

Heat sink to cool down regulator


Power Supplies


These capacitors are easily-overlooked but a crucial for proper operation


Power Supplies


Quiescent current for LM78xx regulators are large, and can waste lots of energy not suitable for battery-operated equipment.


Power Supplies

MAX604 Linear Regulator

Quiescent current for MAX604 a few micro-amps, thus much more suitable for battery-operated equipment.


Power Supplies

MAX724 DC/DC switching Regulator

Very wide input range, efficient conversion to output voltage

Note that switching regulator can step up voltages


Decoupling Capacitors Consider a microntroller that runs on a 10 MHz clock. This means the clock period is 100 ns. The rise and fall time of the clock is thus on the order of 10 ns or shorter.

The microcontroller’s internal electronics (logic gates) create significant switching noise that appears on the power supply lines.

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.

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

Decoupling capacitor ~ 0.1 μF

The decoupling capacitor’s (local) energy provides local storage to supply to switching circuits.

Used properly, a decoupling capacitor can greatly reduces switching noise.

Sometimes called a bypass capacitor


Power Supply Decoupling

Wrong way to decouple power supply

Decoupling capacitor is ~ 0.1 μF

Note: high loop current flows through the ground and can contaminate other circuits


Power Supply Decoupling

Better way to decouple power supply

Decoupling capacitor is very close to the controller, across its GND and Vcc pins. Large current spikes never flow through the circuit ground.

Inductor blocks residual current spikes from entering power supply. Typical value is 47 nH.


External Clock Sources

For crystals with frequencies larger than 400 kHz

For crystals with frequencies less than 400 kHz, typically 32.768kHz watch crystal. Not supported on all AVRs



For simplicity, the oscillator capacitors are not always indicated, but they are crucial for proper operation, and in most cases the oscillator will not oscillate without these capacitors

Capacitor values are ~ 20 pF, which are small.

Crystals



10 MHz ceramic resonator. Cost ~ $0.25

Ceramic Resonators

Lower Q than crystals. This means less stable, but also faster start-up time

Are available with built-in capacitors

Equivalent circuit

Connect to uController



Cost ~ $0.25 Cost ~ $0.75

Question: What does “ppm” mean?

Answer: Parts per million

To convert from ppm to %, divide the number by 10,000.

To convert from % to ppm, multiply by 10,000


Board Layout


Question: what type of diodes should D1, D2, be, and why?

Answer: Schottky diodes, because they have lower turn-on voltages than Si diodes. Thus, these diodes dissipate less power.

In normal operation, the 5 V linear regulator provides clean 5 V. D1 drops VD(on) . This is greater than (3 V + VD(on) ), so D2 is reverse-biased and open. Without main power, D2 is forward biased turn on, and powers the controller.


Recall Earlier Slide

sbi DDRB,1 ; PB1 is now output sbi DDRB,2 ; PB2 is now output

Example. Turning LEDs on and off

Connecting LEDs

sbi PORTB,1 ; LED at PB1 off

Configuring port pins for output

Turn LED at PB1 OFF

Turn LED at PB1 ON cbi PORTB,2 ; LED at PB2 on

Note LEDs will light up when ports pins

are pulled LOW

Data Direction Register


DDRB, PORTB, PINB,…

Simplified I/O equivalent schematic

I/O ports on ATmege88P: PORTB, PORTC, and PORTD

Individual pins of the ports are PB1, PD4, etc.

Complex circuitry sits behind each port pin.

Pins can be digital input, digital output, and analog Input

Three registers located in I/O space

DDRB – Data Direction Register for PORTB

PORTB– Digital Output Register for PORTB

PINB– Input Register for PORTB



Protection Diodes

All I/O pins have protection diodes to both VCC and Ground

All AVR ports have true Read-Modify-Write functionality when used as general digital I/O ports. This means that the direction of one port pin can be changed without unintentionally changing the direction of any other pin with the SBI and CBI instructions.

The same applies when changing drive value (if configured as output) or enabling/disabling of pull-up resistors (if configured as input). Each output buffer has symmetrical drive characteristics with both high sink and source capability.

The pin driver is strong enough to drive LED displays directly. All port pins have individually selectable pull-up resistors with a supply-voltage invariant resistance.

Pull-up resistor ensures that is at expected logic levels if external devices are disconnected. The idea is that is that it weakly "pulls" the pin to Vcc. The resistor is high-resistance enough that, if something else strongly pulls the wire toward 0V, the pin will go to 0V.



Configuring a PIN

DDRB – Data Direction Register for PORTB

PORTB– Digital Output Register for PORTB

PINB– Input Register for PORTB


Unconnected Pins

The simplest method to ensure a defined level of an unused pin, is to enable the internal pull-up.

If some pins are unused, it is recommended to ensure that these pins have a defined level. Even though most of the digital inputs are disabled in the deep sleep modes as described above, floating inputs should be avoided to reduce current consumption in all other modes where the digital inputs are enabled (Reset, Active mode and Idle mode).

In this case, the pull-up will be disabled during reset. If low power consumption during reset is important, it is recommended to use an external pull-up or pull-down

Connecting unused pins directly to VCC or GND is not recommended, since this may cause excessive currents if the pin is accidentally configured as an output.


Example. Consider the table below, extracted from the “DC Characteristics” section of the ATtiny45/V microcontroller. Assume the power supply voltage is 2 V, and that PB0 is configured as digital input. Determine the threshold voltages for logic 1 and 0 on input.

Solution. PB0 is not XTAL or RESET, so VIL and VIH apply.

VI0 = 0.2Vcc = 0.2×2 = 0.4 V, and VI1 = 0.7Vcc = 0.7×2 = 1.4 V


Pull-Ups Revisited

The Pull-up Disable – PUD bit in MCUCR disables the pull-up function for all pins in all ports when set.

If PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is activated.

To switch the pull-up resistor off, PORTxn has to be written logic zero or the pin has to be configured as an output pin.

Self-Study

Give a code snippet to configure PB1 on the ATmega88PA as an input with the internal pull-up resistor activated. Comment your code.


Pull-Ups Revisited

Pull-up resistors can have a significant spread between parts, and even individual pins on a port, so beware…


Microcontroller Fuses

• Except for GND and VCC all other pins can perform at least 4 functions • PB5 can be hardware RESET or an ADC input • PB3 & PB4 can be ADC inputs or where crystal for external oscillator

goes • How does one configure controller for the external environment?

AVR ATtiny45 ® 8-Pin Microcontroller


Electrical Connections

Pullup resistors

Low-pass filter Low-pass filter

Test circuit, from the data sheet

Remember contact bounce?


Switch Bounce and Debouncing

Bounce


Switch Bounce and Debouncing

t

Volta

ge Question: what is the fall time?

Make sure you can calculate this.

When the switch closes, the capacitor discharger through Rb


Wired-Or Application: Bus

Bus is normally high

Any microcontroller can pull the bus low

Internal Pull-up Resistor OFF

Internal Pull-up Resistor OFF


Reading a Simple (3×4) Keypad

Note the weak pull-ups on PDB-PD7. These must be enabled in software



Step 1: Detecting a key press a) Drive PC4, PC5, PC6 low. b) READ Port D and look for zero on any of PD4-PD7 c) If so, a key has been pressed 0

0

0



Step 2: Debounce. Wait an appropriate period for the key switch to stabilize (10 ms is probably a good waiting time)



0

1

1

Step 3: Search for the pressed key a) Drive PC4 Low, PC5,PC6 High b) Read Port D PD4 = 0 “1” pressed PD5 = 0 “4” pressed PD6 = 0 “7” pressed PD7 = 0 “*” pressed c) If pressed key is not found in column 1, repeat the process as necessary for the other two columns



1

0

1

Step 4: Search for the pressed key a) Drive PC5 Low, PC4,PC6 High b) Read Port D PD4 = 0 “2” pressed PD5 = 0 “5” pressed PD6 = 0 “8” pressed PD7 = 0 “0” pressed c) If pressed key is not found in column 2, repeat the process as necessary for the other two columns



1

1

0

Step 5: Search for the pressed key a) Drive PC6 Low, PC4,PC5 High b) Read Port D PD4 = 0 “3” pressed PD5 = 0 “6” pressed PD6 = 0 “9” pressed PD7 = 0 “#” pressed

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