Vhdl shift left1/16/2024 The second VHDL shift register, shown below, works in exactly the same way as the first shift register except that the shifting process is simplified. bit 7): shift_reg(7) <= D Second Shift Register The D input is fed to the MSB of the shift register (i.e. The source of the data being shifted is from the D input which is connected to a switch on the board. shift_reg(6) <= shift_reg(7) moves bit 7 to bit 6, and so on for each bit. The shifting is done by moving each bit to the bit position to its right, e.g. On every rising edge of the clock pulse (the divided clock pulse), the data in the shift register is shifted one bit to the right – from the MSB to the LSB. The shifting inside the shift register takes place in a VHDL process. Tutorial 6 (Clock Divider in VHDL) in this course shows how to set up the input clock for the home made CPLD board. Clock DividerĪ clock divider is used to slow down the input clock so that the contents of the shift register will be visible on the LEDs. The LED outputs connect each bit in the shift register to its own LED on the CPLD board. Data is shifted in the shift register on each rising edge of the clock pulse. The CLK input of the shift register is connected to a clock source. In the video, this input is connected to the right switch of the switch bank and feeds data into the shift register. The shift register has a D input for serial data. hook up the shift register bits to the LEDs If (clock_div(4)'event and clock_div(4) = '1') then Signal shift_reg : STD_LOGIC_VECTOR(7 downto 0) := X"00" Signal clock_div : STD_LOGIC_VECTOR(4 downto 0) On the home built CPLD board, the LEDs will all initially be switched on because of the wiring of the LEDs to the CPLD which effectively inverts the logic level on the CPLD pin.Īrchitecture Behavioral of shift_register_top is The shift_reg register is 8 bits wide and the VHDL code connects each bit in the register to an LED, so that 8 LEDs show the value in each bit of the register. This register is initialized with the value of 00h so that when power is switched on to the CPLD board, the register will be cleared. This example creates a shift register using a VHDL signal called shift_reg shown in the code listing below. Both VHDL code examples of the shift register behave in exactly the same way when implemented on the CPLD.īooks that may interest you: First Shift Register This video shows the VHDL shift register in action. The two different examples create the same shift register using slightly different VHDL code. There are two examples of a shift register written in VHDL below. It is also possible to shift data from right to left and to use the LSB as an input for serial data. The Shift Register as Created in VHDL Code Data is shifted from left to right – from Most Significant Bit (MSB) to Least Significant Bit (LSB). The image below shows an eight bit shift register that is created in VHDL code in this tutorial. A Shift Register is Made from D-type Flip-flops The data in each flip-flop will be shifted to the flip-flop on its right when the rising edge of the clock pulse occurs. Whatever the state of the data input when the rising edge of the clock pulse occurs will be the logic level that is shifted into the first flip-flop. This data can be either a 0 or a 1 and will be shifted to the right on each rising edge of the clock pulse. This shift register is configured to shift data from the left to the right.ĭata is fed into the D input of the first flip-flop on the left. This is a four bit shift register and therefore consists of four D flip-flops. Shift registers consist of D flip-flops as shown in the figure below. A shift register has the capability of shifting the data stored in the register from left to right or right to left.
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