55:036 embedded systems and systems...

Lecture 2-4, Slide 1 Embedded Systems and Software, ECE:3360. The University of Iowa, 2016 A. Kruger

Embedded Systems and Software

Embedded Systems in Vehicles

Notes on Lab 2


Lab 02

Pushbutton Switch

ATtiny45

LED Driver

In this lab students implement an interval timer using a pushbutton switch, ATtiny45, an LED driver, and two seven-segment displays

When the user presses the button, the microcontroller start counting from 0 to 9.9 in 100 ms intervals. When the user presses the button again, the counter stops.

If there is an overflow (user wait longer than 9.9 s), the display should show “9.9” and flash once per second.


Seven Segment Displays

Each of the seven segments and the decimal point is a LED

Segments

By turning a subset of the LEDs on, one can display the numbers 0 – 9 and some letters as well

Decimal Point

These displays are sold in single- or multiple character units. They come in all shapes and sizes

Other variations include multiple decimal points, colons, + and – signs etc.


Common Anode, Common Cathode

Most of these displays connect either all the cathodes or all the anodes together.

They are called Common Anode or Common Cathode displays

Each segments and the decimal point is an LED


Driving a Seven Segment Display

Common Anode

Pulling a line low turns the corresponding segment on



Common Anode



Common Anode

Turn on multiple LEDs to create the desired number


Issues #1 – Too manly Lines

Common Anode

Turn on multiple LEDs to create the desired number

To drive the display directly from a microcontroller means we need 8 lines per seven segment module

This will quickly deplete the number I/O pins on a microcontroller


Issues #2 –LED Temperature and Aging Effects

Common Anode

An LED’s forward bias voltage is a function of ambient temperature

With resistors, this means the current and therefore the brightness depends on temperature


LED Driver ICs

To address the issues with the simplistic driving of displays and LEDs in general, several companies market special LED drivers ICs.

A bit pattern is shifted in on the

DATA line

SDI

Control lines latch the data and turn on the

current sources

The data is also shifted out so that one can put several IC in series

Current sources/sinks ensure LED current does not change with temperature

CLK

OE


Adding More Displays

SDI

CLK

OE1

SDI

CLK

OE2

To add another display, we need just one more line

In operation, shift the desired 16-bit pattern in, and turn on the drivers. We control individual modules with their Output Enable (OE) lines


Adding More Displays - Alternative Method

SDI

CLK

OE1

SDI

CLK

OE2

All displays use the same control lines

In operation, shift the desired 16-bit pattern in, and turn on the drivers. We control individual modules with by shifting in the proper bit pattern


The TLC5916IN LED Driver Overview To use the IC successfully, you have to read the data sheet. In the next few slides we give an overview.


The TLC5916IN LED Driver Overview

TLC5916IN

Serial Data In

Latch Enable

Clock

~OE

Serial Data Out

𝐼𝐼𝑂𝑂𝑂𝑂𝑂𝑂 = 151.25𝑅𝑅𝐸𝐸𝐸𝐸𝑂𝑂

𝑉𝑉𝐶𝐶𝐶𝐶 (5 V)

This notation is the same as OE

𝑽𝑽𝑪𝑪𝑪𝑪


Notation

A bar above a pin means that the pin is active low

For an output pin, then means the pin will be pulled low when it is activated

For an input pin, it means it must be pulled to perform its function

LEDs turn on when output pins are pulled low

Output drivers are turned on when OE is pulled low

Sometimes this is written as ~OE

𝑽𝑽𝑪𝑪𝑪𝑪


Cascading TLC5916IN LED

For this lab, cascade two drivers is this fashion. This means you will need 4 output lines and 1 input line.

Design 𝑅𝑅 for 10 mA


The LDS-A514RI Display

Pin 5 Pin1 Pin 5

Pin 10

Pin1

Pin 6 Pin 10



Mapping from pin numbers to segments



Be sure not to exceed maximum ratings. For the lab, design the LED driver so that each segment gets 10 mA


Assigning ATtiny45 Pins

These are used by the ISP programmer when it downloads code

𝑉𝑉𝐶𝐶𝐶𝐶 (5 V) Reset Line

We need 4 control lines and one input line. However, here is the status of the development board:

Seems like we don’t have enough pins. This is a typical constraint in embedded systems

The game plan is to disable disconnect the crystal and that frees up two pins. If we are careful, we can also use the ISP lines.

Question - what is ISP?


Use ATtiny45’s Internal RC Oscillator

These jumpers connect the crystal to the microcontroller

We want to use the internal RC oscillator so that can use PB3 and PB4 for controlling the LED drivers

Thus, we must first configure the Attiny45 to use the internal oscillator and then remove these jumpers.

If you remove the jumpers first then ATtiny45 has no clock and will not run…


Microcontroller Configuration/Fuses

Set this to the internal RC oscillator


Using the ISP Lines

These are used by the ISP programmer when it downloads code



Insert 4.7K resistors to avoid your hardware conflicts with programming. Note that this may not be needed if the pins are uses as output only.


Testing Status of an I/O PIN

... cbi DDRB,3 ; Make PB3 input chk: sbis PINB,3 ; See if PB3 is set (high) rjmp down ; No, PB3 is low => switch down => continue rjmp chk ; Yes, PB3 is high, wait some more down: ...

This instruction tests a single bit in an I/O register and skips the next instruction if the bit is set.


Pullup resistor. Value not critical, 4.7K 100K


Testing Status of an I/O PIN

... cbi DDRB,3 ... sbis PINB,3 ...

For input use PINB, not PORTB

... cbi DDRB,3 ... sbic PINB,3 ...

Also, there is a companion instruction SBIC (Skip next instruction if bit in I/O is Clear)

You could use either SBIS or SBIC in a loop to test if the pushbutton switch has been pressed



Switch Bounce and Debouncing

Bounce

~ 5 ms


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 discharges through Rb


Software Debounce

char isPressed(void) { char ones=0, zeroes=0, i; for(i=0;i<=10-1;i++){ if(PINA&0x01){ // read pin == 1 ones++; } else { // read pin == 0 zeroes++; } _delay_ms(10); } return (ones > zeroes); }

One idea. Sample n times at regular intervals, say 10 ms apart. Count how many times the switch is zero . In this is larger than the number of times the switch is high, consider the switch pressed.

We will use software debounce later in the course. For now, use hardware debounce


Which PIN to Use?

These are used by the programmer when it downloads code, so adding a debounce capacitor may interfere. You may have to experiment which PIN works best as an input pin. Consider using PB3 for the pushbutton switch.

There are 3 available PINs for use as switch input: PINB0, PINB1, PINB2. Which one should you use?


More on TLC5916 Operation

Diagram extracted from TLC5916 data sheet




Hold OE (drivers are off)




Latch Enable (LE) is low



1 0 1 1 1 0 0 1



Place bit on SDI line

Make CLK go low high to clock data in



1 0 1 1 1 0 0 1





Once the number of bits are clocked in, pulse LE

Latch data



1 0 1 1 1 0 0 1



Finally, pull OE low. While OE is low output drivers are on and LEDs light up




~OE





Finally, pull OE low. While OE is low output drivers are on and LEDs light up




1 0 1 1 1 0 0 1

1

0

1

1



1 0 1 1 1 0 0 1

1

0

1

1

Diagram extracted from TLC5916 data sheet

Latch data

~OE



Timing diagram from data sheet

With a 8-MHz clock, a clock cycle is

125 ns


𝑡𝑡𝑊𝑊 CLK Clock Pulse Duration (𝑡𝑡𝑤𝑤 CLK )

𝑡𝑡𝑊𝑊 CLK < 1 clock @ 8 MHz


𝑡𝑡𝑊𝑊 CLK

𝑡𝑡𝑆𝑆𝑂𝑂 D

SDI Setup Time (𝑡𝑡𝑠𝑠𝑠𝑠 D )

𝑡𝑡𝑠𝑠𝑠𝑠 D < 1 clock @ 8 MHz


𝑡𝑡𝑊𝑊 CLK

𝑡𝑡ℎ D

SDI Hold Time (𝑡𝑡ℎ D )

𝑡𝑡ℎ D < 1 clock @ 8 MHz


𝑡𝑡ℎ D

~OE Pulse Width (𝑡𝑡𝑊𝑊 OE )

𝒕𝒕𝒘𝒘 𝐎𝐎𝐎𝐎 > 1 clock @ 8 MHz

𝑡𝑡𝑊𝑊 OE


Note that, in general, timing values depend on power supply values, and often on temperature as well

55:036 embedded systems and systems...

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