Shift Register Counter

Question

I am working on a project with my MBED microcontroller and I'm running into some snags, because I'm new to it and getting myself confused with how exactly everything works.

I have my MBED hooked up to a 74HC595, which in turn is hooked up to a 7-segment display. I am currently just trying to test the seven segment by creating some sort of counter that goes from 0-9. However, I'm not sure how to do this. Below is my code and how I have it hooked up, I have a few extra declarations in place for the final project, so please just ignore them:

#include "mbed.h"

SPI spi(p11,p12, p13);
DigitalOut red(p25);
DigitalOut green(p26);
DigitalOut cs(p14);
DigitalIn button(p27); //enter
DigitalIn button2(p28); //down
DigitalIn button3(p29); //up

char switchWord;
char record;
int i;

int main() {
  spi.format(8,0);
  while(1) {
    switchWord=0x3F;
    if(button==1)
      switchWord=switchWord^0x06;
    if(button2==1)
      switchWord=switchWord^0x5B;

    cs=0;
    record=spi.write(switchWord);
    cs=1;
    wait(0.01);

    red=0;
    green=0;
    record=record&0x03;

    if(record==1)
      red=1;
    if(record==2)
      green=1;
    if(record==3){
      red=1;
      green=1;
    }
  }
}

Obviously this isn't working how I want it to, I have tried several different ways to get it to count up, and I have been playing with 2's compliment but it's been awhile since I learned about that so I am really fuzzy on it and I couldn't get it to work. Obviously counting up in this set of binary will be different since the 7-segment displays zero when 6/7 LEDs are set to 1, instead of all zeros, so I can't just add the next number or I get a nonsense display. I eventually want to be able to count from 0-9 by hitting the first button, and count down with the second button, but I can't even figure out how to get it to go from 0-1 so if I can get that I think I can get the ball rolling.

I hope this is all clear and I appreciate any help I can get! Thanks!

Alli

Answer 1

Unless I am missing some important information here regarding what you are trying to do, in order to get the 7-seg to "count" you are going to need to shift into the register the equivalents of each number you want to display in your sequence (0, 1, 2 ... )

Consider this schematic of a segment display. Although this is likely not exactly how yours is laid out, it will suffice for an explanation.

Doing of few experiments we find that if we connect the register to the display as follows (starting with the registers LSb):

• 1 -> a
• 2 -> b
• 3 -> c
• 4 -> d
• 5 -> e
• 6 -> f
• 7 -> g

Consider the event in which we shift out a character 5 (00110101), for example. The correct pattern on the display to create the symbol 5 would be:

• 1 -> a
• 0 -> b
• 1 -> c
• 1 -> d
• 0 -> e
• 1 -> f
• 1 -> g

As you can see, 00110101 -> 1101101 which could be called data conditioning, if you will.

The point of what I am trying to get at here is that you cannot shift out a number or a character, you must in fact manipulate the word based on what data you are trying to display; such work is most simply done with a look-up-table(LUT), as suggested earlier.

For reference, see: The ASCII Table

Answer 2

Are you expecting your 7-seg display to understand ASCII or to be able to interpret binary numbers into a symbol to display? That's not how 7-seg displays work.

You're driving each individual segment of the display, so you decide on what pattern of segments to light up. You choose a pattern for each digit you want to display.

When you say that it displays a zero when 6/7 LEDs are set to 1, this is perfectly normal & expected behavior. Count the number of segments which are lit up. - '1' requires 2 segments - '2' requires 5 segments - '3' also requires 5 segments, but not the same set as a '2' - etc ...

You could also add on a decoder IC - something like a 74HC4511 would work - to do that job for you. It takes a 4-bit BCD input and produces the correct combination of outputs to drive all of the segments and display recognisable digits.

Answer 3

the TDSR1360 is a common-anode display, so to make the segments light up they must be grounded through individual resistors while the display's anodes are connected to Vcc.

In order to do that, each segment must be connected to a 595 output, through a resistor, and will only light when that 595 output goes low.

One way to map the segments for the digits "0" through "9" to the 595's outputs is like you've shown, but with the display's anodes (pins 3 and 8) connected to Vcc instead of to ground.

Then, in order to make the display count up, what you must do is serially shift a byte which corresponds to, say, "0" into the 595, and then assert RCK. That'll broadside load the byte into the 595's parallel outputs and light up the segments which correspond to "0".

Next, you'll shift in a byte which corresponds to "1", assert and deassert RCK, and the display will now read "1".

Next, you'll shift in the byte which corresponds to "2", assert and deassert RCK, and the display will now read "2".

Continuing on with the bytes corresponding to 3 through 9 will make it seem like the 595 is counting up, when all it's really doing is sequentially displaying canned bit sequences arranged to display digits.

Answer 4

Perhaps I'm blind, but I don't see the translation table in your code that translates between the integer holding your counter and the pixel-pattern of the glyph that represents that digit.

// ...
int counter=0;
char translation_table[] = {
    0b11110111; // 0
    0b00001100; // 1
    // ... fill in details here ...
};
while(1) {
    counter=counter+1;
    // FIXME: do something about overflow
    char LED_pattern = translation_table[counter];
    cs=0;
    record=spi.write( LED_pattern );
    cs=1;
    wait(0.5);
    // ...
}

The "translation_table" mentioned in the above code is also called the

• the sevenSegmentLookUp[] array in "Principles of Embedded Systems: Display Devices"
• the "n[]" array in "Parallax Seven-Segment Display"
• the "b7SegmentTable[]" array in ST application note AN1781: STR71x GPIO driving four 7-segment display
• the "characterArray[]" array in the Arduino library SevSeg used in the Sparkfun Serial7SegmentDisplay
• the "seven_seg_digits[]" array in Arduino and 7 Segment LED
• what Majenko calls a "mapping table" or "look-up table" (LUT)