A SHIFT register is the basic building block in a VLSI circuit. Shift registers are commonly used in many applications, such as digital filters, communication receivers, and image processing ICs . Recently, as the size of the image data continues to increase due to the high demand for high quality image data, the word length of the shifter register increases to process large image data in image processing ICs. An image-extraction and vector generation VLSI chip uses a 4K-bit shift register. A 10-bit 208 channel output LCD column driver IC uses a 2K-bit shift register. A 16-megapixel CMOS image sensor uses a 45K-bit shift register. As the word length of the shifter register increases, the area and power consumption of the shift register become important design considerations.
The architecture of a shift register is quite simple. An N-bit shift register is composed of series connected N data flip-flops. The speed of the flip-flop is less important than the area and power consumption because there is no circuit between flip-flips in the shift register. The smallest flip-flop is suitable for the shift register to reduce the area and power consumption. Recently, pulsed latches have replaced flip-flops in many applications, because a pulsed latch is much smaller than a flip-flop. But the pulsed latch cannot be used in a shift register due to the timing problem between pulsed latches.
- Power consumption is high
All pulsed latches share the pulse generation circuit for the pulsed clock signal. As a result, the area and power consumption of the pulsed latch become almost half of those of the master-slave flip-flop. The pulsed latch is an attractive solution for small area and low power consumption. The pulsed latch cannot be used in shift registers due to the timing problem, as shown in Fig. 6. The shift register in Fig. 6(a) consists of several latches and a pulsed clock signal (CLK_pulse). The operation waveforms in Fig. 6(b) show the timing problem in the shifter register. The output signal of the first latch (Q1) changes correctly because the input signal of the first latch (IN) is constant during the clock pulse width . But the second latch has an uncertain output signal (Q2) because its input signal (Q1) changes during the clock pulse width.
One solution for the timing problem is to add delay circuits between latches, as shown in Fig. 7(a). The output signal of the latch is delayed and reaches the next latch after the clock pulse. As shown in Fig. 7(b) the output signals of the first and second latches (Q1 and Q2) change during the clock pulse width , but the input signals of the second and third latches (D2 and D3) become the same as the output signals of the first and second latches (Q1 and Q2) after the clock pulse. As a result, all latches have constant input signals during the clock pulse and no timing problem occurs between the latches. However, the delay circuits cause large area and power overheads.
The proposed shift register reduces the number of delayed pulsed clock signals significantly, but it increases the number of latches because of the additional temporary storage latches. As shown in Fig. 6 each pulsed clock signal is generated in a clock-pulse circuit consisting a delay circuit and an AND gate.
The maximum clock frequency in the conventional shift register is limited to only the delay of flip-flops because there is no delay between flip-flips. Therefore, the area and power consumption are more important than the speed for selecting the flip-flop. The proposed shift register uses latches instead of flip- flops to reduce the area and power consumption. In chip implementation, the SSASPL (static differential sense amp shared pulse latch) in Fig. 8, which is the smallest latch, is selected. The original SSASPL with 9 transistors is modified to the SSASPL with 7 transistors in Fig. 10 by removing an inverter to generate the complementary data input (Db) from the data input (D). In the proposed shift register, the differential data inputs (D and Db) of the latch come from the differential data outputs (Q and Qb) of the previous latch. The SSASPL uses the smallest number of transistors (7 transistors) and it consumes the lowest clock power because it has a single transistor driven by the pulsed clock signal.
- Reduce the power level
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