I had a requirement where a 20-pin microcontroller had to drive one LED display plus other peripherals. To drive the LED display, required 8 pins that were not available, because they were already used by other peripherals. After a literature search, I found a cheap shift register which performs like the PCF8574 (which is not cheap at all!).

Using shift registers has been mentioned in various places including the Svet elektronike magazine, but none contained a practical example. In this chapter I will show you a practical way to implement this. For this purpose, PCBs were designed to drive four 7-segment LED displays as well as a 32 LED circular bar-graph display. The basic circuit is shown in Figure 1. The shift register is driven using the pins Strobe (ST), clock (CLK) and data (DATA).

Before we write the data (which will be shifted into the shift register), we must first set the strobe pin to logical 1 (note that the pin on the microcontroller is also labelled STR). Then on the data pin we set a data value (0 or 1) followed by setting the clock pin to logical 1. By setting the clock pin to a logical 1, the shift register shifts that data by one bit, into its 8-bit register. After that, we reset the clock pin to a logical 0, set a new data value, for the second bit, and raise the clock pin to a logical 1 again. This sequence is repeated until all 8 bits are shifted in. At the end of this sequence, we must reset the strobe pin to a logical 0, which transfers the register data to the output pins (Q1 – Q8). Let’s look at a Bascom program (Out_port.bas) which does this.

Dim I As Word , N As Byte , V As Byte , 

C As Byte , Z As Byte

Config Portd = Output

Portd = 0

Strobe Alias Portd.2              ‘to pin 1 CD4094

Dat Alias Portd.3                   ‘to pin 2 CD4094

Clk Alias Portd.4                   ‘to pin 3 CD4094

N = 255                                ‘all outputs are set to high

Gosub 4094

Do

 Input I

 N.i = Not N.i

 Gosub 4094

Loop

End

4094:

  Set Strobe                         ‘enable writing

  For C=7 To 0 Step-1           ‘each bit of variable

   
Dat = N.c
Set Clk
Reset Clk
Next

  Reset Strobe                   ‘shift register changes do not, 
                                       ‘affect output until strobe goes low 

Return

By setting N = 255 (binary = 11111111), we set all bits to a logical 1 and therefore no LED is lit. If we want to turn on one LED we have to pull down that particular bit. Many newbies don’t know that one can change a bit within a variable very easily. For example: if a variable (N) has a value 255 (binary 11111111) and you want to change only the least-significant bit to 0, (binary 11111110) you can do that very easily using the statement N.0 = 0. With this simple statement you can change single bits in your byte variable.

I/O Expansion

2012_AVR_UK_39

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