Sunday, February 22, 2015

Using shift registers in Arduino projects

If  you are using an Arduino board(like Uno) you get about 12 digital pins for your projects .this number is fairly enough if your  project is a simple one that won’t consume no more than few pins but as the complexity of your projects increase soon you will run out of pins. There are few solutions for this.
  • You can buy a board like mega and expand your number of digital I/O pins.
  • You can use multiplexing/ demultiplexing chips to read and write to pins.
  • You can use shift register chips to expand the I/O pins.

It’s obvious that the first solution shouldn't be your first choice since it will cost you few bucks and it’s probably is not the best solution as well. Eventually you will run out the pins if your project needs more than 54 digital I/O pins .So keep that option ,optional!!!.

Today I’m going to discuss about the 3rd solution I mentioned. I will discuss the 2nd (mux and demux) solution in future when I get a chance. Shift registers are used to expand the I/O capabilities in an electronic circuit. In Arduino domain using only 3 pins of the board and a single chip you will be able to control up to 8 outputs. And by cascading the shift registers you will be able to control thousands of outputs only using 3 arduino pins. In order to do this I have used 74HC595N chip which has 8 outputs. For this instance I used 3 595 chips to control 24 LEDs.

Figure1 74HC595 chip


Figure2-Pin configuration














Q0-Q7   are the output pins that connected to LEDs anode.74HC595 is capable of sourcing current to               LEDs  but there are different chips that use sinking current(current is drawn into the chip)                  method to drive LED. So make sure you check the data sheet if you are not using this chip.

VCC/GND   +5v supply and ground.

DS   serial data input to the chip. This pin is connected to one of the digital output pins in Arduino              and if you cascade several of them, you have to connect the DS pin of the second chip to the                Q7’ pin of the first chip.

OE      output enable .the horizontal piece of line indicates that it’s active low. That means you                      have to make the pin low in order to make the output enabled. So connect that to the                           ground to make that pin low .

ST_CP   storage register clock input .this pin is also called latch pin since it’s the pin that is used to                  transfer the data stored in the chip to the output pins. The value loaded into the chip won’t                    get pass through unless this pin gets a LOW to HIGH transition.

SH_CP  shift register clock input. This pin is used to insert the serial data into the chip register.                         When the pin’s state gets a transition from LOW to HIGH it reads the value of the DS pin                  and sends the value to register.


MR         Master reset.(active low )this pin can reset the whole chip when activated. The values                         stored in the chip will be erased .So connect this pin to +5v supply to avoid resetting the                       chip.

Q7’         This pin is used to cascade several chips together and further expand the outputs.
               Please read the data sheet I provide carefully to understand the functionality and the                            autonomy of the chip better.   And also I have add some links to some youtube videos  that I               referred in order to learn about shift registers. I highly recommend you to watch them to get                 more understanding how this chip works.

Writing the program

There is a function called shiftOut() defined in Arduino to use in shift registers but I will present you another way of implementing the functionality and later I’ll give the code I wrote using shiftOut() function. As a first instance we’ll look how to implement a single shift register.  Use the above diagram to construct the circuit. If you want you may avoid the 1uF capacitor ,it is used to remove the flickering .

Figure3-Circuit Diagram 
















Now we’ll look at the code. I have attached the source file at the end of the post. So you  may have a  look at it or you can use it in your project.

Figure4-Initializing code












Initialising is done as usual in first place. In there I have assigned pins 8,9 and 10 to the serial data pin, clock pin and latch pin of the chip. And also I created an array consists of 24 elements in order to store the status of each LED. It’s type is Boolean ,therefore I can store a HIGH or  LOW  value in each element. time_const  is used to control the speed of the LED on and off. Writereg()  is a function which defined to shift the data into the chip.


Figure5-Writereg() function














At the beginning of the function the latch pin is set to low and at the end (when the data has been transferred to the chip) it is set to high. In that way we can give a LOW to HIGH transition and data has been sent to output pins. As I mentioned  above the clock pin is used to distinguish between data bits .First the clock pin set to low then the data bit is sent to a storage register in the chip .and again clock pin is set to high and this transition stores the bit in storage register this happens in all 3 chips. The first 8 bits stored in the chip which connected directly to the Arduino. When we moved the next  byte to the first chip the previous byte get shifted to the next shift register .Like that we can shift any amount of bytes as long as we have enough chips and the first byte that shifted will be stored in the last shift register that cascaded.  

Using shiftout() function in sketches 


Arduino community has provided a built in function to handle shift functions easily.

Syntax

Shiftout(Data pin,clock pin,bitorder,value)

Data pin -  serial data input  pin of the chip is assigned to this pin.
Clock pin – SHCP pin of the chip is assigned to the this pin.
Bitorder- For this you can assign two values. They are MSBFRIST or MSBLAST .

MSBFIRST 

setup the least significant bit of the value you pass to Q0 pin. For example if you send value 6       (110 in binary) to the shift register, it will be represented as follows,

Q0 – 0
Q1 – 1
Q2 –  1 

LSBFIRST 

This is the opposite of MSBFIRST ,it will assign the LSB bit to Q7 pin and  MSB bit to Q0.

 Most of the time we are only going to use MSBFIRST since it’s much convenient .So if something is not displayed as you wish when you are executing the code try replace MSB with LSB or LSB to MSB that might solve the problem. There is a tutorial in Arduino site that explains this function in more details. Please read that as well.
Shift registers are very useful way to expand the I/O capabilities of your microcontroller. I found about this technique when I was looking for a way to control a LCD display by Arduino without compromising 6 pins. I haven’t tried it yet but I hope to write another about that in some other time. Please use the links I provide below to get to know more about shift registers.

Links


following links are few tutorials I referred in order to learn Shift registers.