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Chapter 6 - Samples

Shift registers

There are two types of shift registers: input and output. Input shift registers receive data in parallel, through 8 lines and then send it serially through two lines to a microcontroller. Output shift registers work in the opposite direction; they receive serial data and on a "latch" line signal, they turn it into parallel data. Shift registers are generally used to increase the number of input-output lines of a microcontroller. They are not so much in use any more though, because most modern microcontrollers have a large number of input/output lines. However, their use with microcontrollers such as PIC16F84 can be justified.

Input shift register 74HC597

Input shift registers transform parallel data into serial data and transfer it to a microcontroller. Their working is quite simple. There are four lines for the transfer of data: clock, latch, load and data. Data is first read from the input pins by an internal register through a 'latch' signal. Then, with a 'load' signal, data is transferred from the input latch register to the shift register, and from there it is serially transferred to a microcontroller via 'data' and 'clock' lines.

An outline of the connection of the shift register 74HC597 to a micro, is shown below.

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Chapter 6 - Samples

How to connect an input shift register to a microcontroller

In order to simplify the main program, a macro can be used for the input shift register. Macro HC597 has two arguments:

HC597 macro Var, Var1

Var variable where data from input pins is transferred

Var1 loop counter

Example: HC597 data, counter

Data from the input pins of the shift register is stored in data variable. Timer/counter variable is used as a loop counter.

Macro listing:

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Chapter 6 - Samples

Example of how to use the HC597 macro is given in the following program. Program receives data from a parallel input of the shift register and moves it serially into the RX variable of the microcontroller. LEDs connected to port B will indicate the result of the data input.

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Chapter 6 - Samples

Output shift register

Output shift registers transform serial data into parallel data. On every rising edge of the clock, the shift register reads the value from data line, stores it in temporary register, and then repeats this cycle 8 times. On a signal from 'latch' line, data is copied from the shift register to input register, thus data is transformed from serial into parallel data.

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Chapter 6 - Samples

Example: HC595 Data, counter

The data we want to transfer is stored in data variable, and counter variable is used as a loop counter.

An example of how to use the HC595 macro is given in the following program. Data from variable TX is serially transferred to shift register. LEDs connected to the parallel output of the shift register will indicate the state of the lines. In this example value 0xCB (1100 1011) is sent so that the eighth, seventh, fourth, second and first LEDs are illuminated.

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Chapter 6 - Samples

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Chapter 6 - Samples

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Chapter 6 - Samples

Seven-Segment Display (multiplexing)

The segments in a 7-segment display are arranged to form a single digit from 0 to 9 as shown in the animation:

We can display a multi-digit number by connecting additional displays. Even though LCD displays are more comfortable to work with, 7-segment displays are still standard in the industry. This is due to their temperature robustness, visibility and wide viewing angle. Segments are marked with non-capital letters: a, b, c, d, e, f, g and dp, where dp is the decimal period.

The 8 LEDs inside each display can be arranged with a common cathode or common anode. With a common cathode display, the common cathode must be connected to the 0v rail and the LEDs are turned on with a logic one. Common anode displays must have the common anode connected to the +5v rail. The segments are turned on with a logic zero.

The size of a display is measured in millimeters, the height of the digit itself (not the housing, but the digit!). Displays are available with a digit height of 7,10, 13.5, 20, or 25 millimeters. They come in different colors, including: red, orange, and green.

The simplest way to drive a display is via a display driver. These are available for up to 4 displays.

Alternatively displays can be driven by a microcontroller and if more than one display is required, the method of driving them is called "multiplexing."

The main difference between the two methods is the number of "drive lines." A special driver may need only a single "clock" line and the driver chip will access all the segments and increment the display.

If a single display is to be driven from a microcontroller, 7 lines will be needed plus one for the decimal point. For each additional display, only one extra line is needed.

To produce a 4, 5 or 6 digit display, all the 7-segment displays are connected in parallel.

The common line (the common-cathode line) is taken out separately and this line is taken low for a short period of time to turn on the display.

Each display is turned on in turn and if this is repeated at a rate above 100 times per second, it will appear that all the displays are on at the same time.

As each display is turned on, the appropriate information must be delivered to it so that it will give the correct reading.

Up to 6 displays can be accessed like this without the brightness of each display being affected. Each display is turned on very hard for one-sixth the time and the POV (persistence of vision) of

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Chapter 6 - Samples

our eye thinks the display is turned on the whole time.

All the timing signals for the display are produced by the program, the advantage of a microcontroller driving the display is flexibility.

The display can be configured as an up-counter, down-counter, and can produce a number of messages using letters of the alphabet that can be readily displayed.

The example below shows how to dive two displays.

Connecting a microcontroller to 7-segment displays in multiplex mode

File Led.inc contains two macros: LED_Init and LED_Disp2. The first macro is used for display initialization. That is where display refreshment period is defined as well as microcontroller pins used for connecting the displays. The second macro is used for displaying numbers from 0 to 99 on two displays.

Macro LED_Disp2 has one argument:

LED_Disp2 first macro

first is the number from 0 to 99 to be displayed on Msd and Lsd digit.

Example: LED_Disp12 0x34

Number 34 will be shown on the display

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Chapter 6 - Samples

Realization of a macro is given in the following listing.

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Chapter 6 - Samples

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Chapter 6 - Samples

The following example shows the use of macros in a program. Program displays number '21' in two 7-segment digits.

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Chapter 6 - Samples

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