With the 74LS164 8 bit parallel out serial shift registers, I'm confused about a few things. In the datasheet (page3) on the logic diagram why do they suggest using an active low inverter before the active low pin on the clear line? Is this for some sort of buffer?
My second question is if this is a 8-bit register why on the clear-shift-clear timing diagram (page 2) do they have 11 clock pulses? Would we be limited to 8 clock pulses before the registers are full and 11 clock pulses result in potential loss of data?
VHDL Code for shift register can be categorised in serial in serial out shift register, serial in parallel out shift register, parallel in parallel out shift register and parallel in serial out shift register.
I'm creating an n bit shift register. When the enable signal is high, I want the shift register to shift n times, irrespective of whether enable continues to be high or low. I've put a for loop to shift n times inside a process. My code is given below.
Shift Registers are used for data storage or for the movement of data and are therefore commonly used inside calculators or computers to store data such as two binary numbers before they are added together, or to convert the data from either a serial to parallel or parallel to serial format. The individual data latches that make up a single shift register are all driven by a common clock ( Clk ) signal making them synchronous devices.
Also, the directional movement of the data through a shift register can be either to the left, (left shifting) to the right, (right shifting) left-in but right-out, (rotation) or both left and right shifting within the same register thereby making it bidirectional. In this tutorial it is assumed that all the data shifts to the right, (right shifting).
The effect of each clock pulse is to shift the data contents of each stage one place to the right, and this is shown in the following table until the complete data value of 0-0-0-1 is stored in the register. This data value can now be read directly from the outputs of QA to QD.
This shift register is very similar to the SIPO above, except were before the data was read directly in a parallel form from the outputs QA to QD, this time the data is allowed to flow straight through the register and out of the other end. Since there is only one output, the DATA leaves the shift register one bit at a time in a serial pattern, hence the name Serial-in to Serial-Out Shift Register or SISO.
The SISO shift register is one of the simplest of the four configurations as it has only three connections, the serial input (SI) which determines what enters the left hand flip-flop, the serial output (SO) which is taken from the output of the right hand flip-flop and the sequencing clock signal (Clk). The logic circuit diagram below shows a generalized serial-in serial-out shift register.
The Parallel-in to Serial-out shift register acts in the opposite way to the serial-in to parallel-out one above. The data is loaded into the register in a parallel format in which all the data bits enter their inputs simultaneously, to the parallel input pins PA to PD of the register. The data is then read out sequentially in the normal shift-right mode from the register at Q representing the data present at PA to PD.
The final mode of operation is the Parallel-in to Parallel-out Shift Register. This type of shift register also acts as a temporary storage device or as a time delay device similar to the SISO configuration above. The data is presented in a parallel format to the parallel input pins PA to PD and then transferred together directly to their respective output pins QA to QD by the same clock pulse. Then one clock pulse loads and unloads the register. This arrangement for parallel loading and unloading is shown below.
The PIPO shift register is the simplest of the four configurations as it has only three connections, the parallel input (PI) which determines what enters the flip-flop, the parallel output (PO) and the sequencing clock signal (Clk).
Similar to the Serial-in to Serial-out shift register, this type of register also acts as a temporary storage device or as a time delay device, with the amount of time delay being varied by the frequency of the clock pulses. Also, in this type of register there are no interconnections between the individual flip-flops since no serial shifting of the data is required.
These universal shift registers can perform any combination of parallel and serial input to output operations but require additional inputs to specify desired function and to pre-load and reset the device. A commonly used universal shift register is the TTL 74LS194 as shown below.
Universal shift registers are very useful digital devices. They can be configured to respond to operations that require some form of temporary memory storage or for the delay of information such as the SISO or PIPO configuration modes or transfer data from one point to another in either a serial or parallel format. Universal shift registers are frequently used in arithmetic operations to shift data to the left or right for multiplication or division.
In the next tutorial about Sequential Logic Circuits, we will look at what happens when the output of the last flip-flop in a shift register is connected directly back to the input of the first flip-flop producing a closed loop circuit that constantly recirculates the data around the loop. This then produces another type of sequential logic circuit called a Ring Counter that are used as decade counters and dividers.
74LS164 is a high speed shift register with serial input of data and parallel output of data. It is an 8-bit IC. That means data goes into IC bit by bit serially and 8-bit data appears on the output pins. Data at serial input is fed through 1 input AND gate and it is synchronous with LOW to HIGH transition of the clock. In other words, the transition of data occurs on every positive edge of the input clock.
A shift register is a type of digital circuit using a cascade of flip-flops where the output of one flip-flop is connected to the input of the next. They share a single clock signal, which causes the data stored in the system to shift from one location to the next. By connecting the last flip-flop back to the first, the data can cycle within the shifters for extended periods, and in this form they were used as a form of computer memory. In this role they are very similar to the delay-line memory systems and were widely used in the late 1960s and early 1970s to replace that form of memory.
In most cases, several parallel shift registers would be used to build a larger memory pool known as a "bit array". Data was stored into the array and read back out in parallel, often as a computer word, while each bit was stored serially in the shift registers. There is an inherent trade-off in the design of bit arrays; putting more flip-flops in a row allows a single shifter to store more bits, but requires more clock cycles to push the data through all of the shifters before the data can be read back out again.
These are the simplest kind of shift registers. The data string is presented at "data in" and is shifted right one stage each time "data advance" is brought high. At each advance, the bit on the far left (i.e. "data in") is shifted into the first flip-flop's output. The bit on the far right (i.e. "data out") is shifted out and lost.
In cases where the parallel outputs should not change during the serial loading process, it's desirable to use a latched or buffered output. In a latched shift register (such as the 74595) the serial data is first loaded into an internal buffer register, then upon receipt of a load signal the state of the buffer register is copied into a set of output registers. In general, the practical application of the serial-in/parallel-out shift register is to convert data from serial format on a single wire to parallel format on multiple wires.
This configuration has the data input on lines D1 through D4 in parallel format, D1 being the most significant bit. To write the data to the register, the Write/Shift control line must be held LOW. To shift the data, the W/S control line is brought HIGH and the registers are clocked. The arrangement now acts as a PISO shift register, with D1 as the Data Input. However, as long as the number of clock cycles is not more than the length of the data-string, the Data Output, Q, will be the parallel data read off in order.
One of the most common uses of a shift register is to convert between serial and parallel interfaces. This is useful as many circuits work on groups of bits in parallel, but serial interfaces are simpler to construct. Shift registers can be used as simple delay circuits. Several bidirectional shift registers could also be connected in parallel for a hardware implementation of a stack.
Shift registers can also be used as pulse extenders. Compared to monostable multivibrators, the timing has no dependency on component values, however, it requires external clock, and the timing accuracy is limited by a granularity of this clock. Example: Ronja Twister, where five 74164 shift registers create the core of the timing logic this way (schematic).
In early computers, shift registers were used to handle data processing: two numbers to be added were stored in two shift registers and clocked out into an arithmetic and logic unit (ALU) with the result being fed back to the input of one of the shift registers (the accumulator), which was one bit longer, since binary addition can only result in an answer that has the same size or is one bit longer.
Very large serial-in serial-out shift registers (thousands of bits in size) were used in a similar manner to the earlier delay-line memory in some devices built in the early 1970s. Such memories were sometimes called "circulating memory". For example, the Datapoint 3300 terminal stored its display of 25 rows of 72 columns of 6-bit upper-case characters using 54 (arranged in 6 tracks of 9 packs) 200-bit shift registers, providing storage for 1800 characters. The shift register design meant that scrolling the terminal display could be accomplished by simply pausing the display output to skip one line of characters. 2b1af7f3a8