LOW POWER LINEAR FEEDBACK SHIFT REGISTERS

Abstract: A low power (LFSR) comprising an ordered set of register steps including memory means (1-7), wherein enabling means (F1-F7, N1-N7), enable a single current memory means (1-7) at every shift operation. Each register step comprises a low power memory means (7) consuming a minimum of power when it is disabled, feedback means (E7), an output terminal thereof being connected to an input terminal of the memory means (7), the feedback means (E7) having first and second input terminals connected to an output terminal (data7) of the memory means (7) and an output terminal (data2) of a second subsequent memory means (2), respectively, in the set. The output terminal (data1-data7) of each memory means (1-7) is connected to selection means (M1), selecting at every shift operation the output terminal (data1) of a first subsequent memory means (1) following the current memory means (7) being enabled at the current shift operation.

Description: TITLE: LOW POWER LINEAR FEEDBACK SHIFT REGISTERS Field of the Invention The present invention relates to shift registers and more particularly to low power linear feedback shift registers . Description of the Prior Art Linear feedback shift registers (LFSR) are used in many different electronic apparatuses such as computers, calculators, personal organisers mobile phones etc. A register is normaly used as a storage for a set of information, usually a binary word. Conventionally, an n- bit register is an ordered set of n flip-flops used to store an n-bit word. Each bit of the word is stored in a separate flip-flop. Registers where the words or data transferred to or from the register in parallel are usually designed with D latches or D flip-flops. A widely used register design organised to allow left- or right-shift operations of the stored information is called a shift register. An n-bit shift register may consist of n D flip- flops each of which is connected to its left and/or right neighbour. Data may be entered 1 bit at a time at one end of the register and may be read 1 bit at a time from the other end, which is called serial input-output. A right shift of a D flip-flop shift register is achieved by enabling the clock input of each flip-flop. A problem with the above mentioned prior art shift register designs is that when they are used in applications or apparatuses, such as mobile phones, where power consumption is critical, the power consumption in the flip-flops in the shift registers is a considerable portion of the total power consumption in the current apparatus . However, US-A-5 295 174 discloses a shifting circuit having lower power consumption and a shift register employing the same. The shift register comprises a plurality of latch circuits, a multiplexer for selecting outputs of the latch circuits in sequence, and a clock control circuit for generating clocks used for controlling selection timings of the multiplexer, wherein the timing for selecting the output of a certain latch circuit is delayed with respect to the latch timing of the certain latch circuit by a predetermined timing. Summary of the Invention An object of the present invention is to provide an improved linear feedback shift register (LFSR) reducing the power consumption problem. This is accomplished by a low power linear feedback shift register according to the invention, using memory means such as flip-flops not consuming power when they are disabled. The register does not shift any bits but still generates the same sequence as a conventional linear feedback shift register. The register comprises enabling means enabling a single current memory means at every shift operation, register steps, each comprising a low power memory means consuming a minimum of power when it is disabled, and feedback means of each step. Each memory means is connected to selection means, selecting at every shift operation the output terminal of a first subsequent memory me'ans following the current memory means being enabled at the current shift operation. Another object of the present invention is to provide said enabling means. Still another object of the invetion is to provide said selection means. An advantage of the low power linear feedback shift register according to the invention is the reduction in power consumption. Brief Description of the Drawings In order to explain the invention in more detail and the advantages and features of the invention preferred embodiments will be described in detail below, reference being made to the accompanying drawings, in which FIG 1 is a schematical block diagram of a prior art linear feedback shift register (LFSR) , a first flip-flop of which is updated with a feedback value, FIG 2 is a schematical block diagram of the shift register (LFSR) in FIG 1 in further detail, FIG 3 is a schematical block diagram of a first modification of the shift register (LFSR) in FIG 2, FIG 4 is a schematical block diagram of a second modification of the shift register (LFSR) in FIG 2, FIG 5 is a schematical block diagram of an embodiment of a shift register (LFSR) according to the invention, FIG 6 shows a low power one-hot coded FSM, and FIG 7 shows an embodiment of a multiplexer of the shift register in FIG 5 controlled by a one-hot coded FSM. Detailed Description of the Invention A conventional prior art 7-position shift register comprising seven flip-flops 1-7 connected in series is shown in FIG 1 and in further detail in FIG 2. "The shift register is provided with a simple feedback by means of an EXOR (Exclusive-OR) gate E having first and second input terminals connected to the output terminal of the second flip-flop 2 and seventh flip-flop 7, respectively. An output terminal of the EXOR gate E is connected to an input terminal of the first flip-flop 1, an output of which is connected to an output terminal or ul of the register. This kind of shift register is called a linear feedback shift register (LFSR) . In the following description, numerous specific details, such as the number of positions in the register or the feedback polynomial, are provided in order to give a more thorough description of the present invention. It will be obvious for those skilled in the art that the present invention may be practiced without these specific details. Some well-known features are not described in detail so as not to make the present invention unclear. Considering a steady state for the register in FIG 2, when the register is to be updated, each flip-flop 2-7 is updated with the value on the output terminal of the flip- flop in the preceding position. However, in the first position the flip-flop 1 is updated with the feedback value from the output terminal of the EXOR gate E. A low power LFSR according to the invention is accomplished by a conversion of the conventional prior art LFSR illustrated in FIG 2. A first step of the conversion is illustrated in FIG 3 , in which the feedback is arranged one step to the left and, hence, an output terminal of an EXOR gate E7 is connected to an input terminal of the seventh flip-flop 7. Further, the EXOR gate E7 has first and second input terminals connected to the output of the flip-flop 2 and the flip-flop 7, as described. Consequently, the output terminal of the flip-flop 7 is connected to the output terminal ul of the register. The output value from the output terminal ul in the modified LFSR shown in FIG 3 corresponds to the output value of the output terminal ul in the conventional prior art LFSR illustrated in FIG 2 after a one bit right shift operation. Additionally, each flip-flop 1-7 is provided with an enable input connection enl-en7, respectively, in order to control which flip-flop to be updated. Additionally, each flip-flop 1-7 has a clock input connection, not shown in the drawings. In a second step of the conversion illustrated in FIG 4, an output terminal of another EXOR gate E6 is connected to an input terminal of the flip-flop 6, the output terminal thereof being connected to the output terminal ul of the register, and the EXOR gate E6 has first and second input terminals connected to the output of the flip-flop 1 and the flip-flop 6, respectively. This second feedback is necessary since the bit values in the register are not really shifted. The output value from the output terminal ul in the modified LFSR shown in FIG 4 is the output value from flip-flop 6 and corresponds to the output value of the output terminal ul in the conventional prior art LFSR illustrated in FIG 2 after a another right bit shift operation. However, in order to choose the correct output terminal, i.e the output terminal of the sixth flip-flop 6, two input terminals of a selection means such as a MUX (Multiplexer) Ml is connected to the output terminals of the two flip-flops 6 and 7. In order to provide a correct output value after a first "shift" operation the flip-flop 6 is enabled and the output value on the flip-flop 7 is allowed to pass the MUX Ml, and after a second "shift" operation the output of the flip-flop 6 is allowed to pass the MUX Ml. A completely converted conventional LFSR, i.e. an embodiment of a low power 7-step LFSR according to the present invention is shown in FIG 5. The LFSR comprises seven flip-flops 1-7, seven EXOR gates E1-E7 for the feedback arrangements, and a MUX Ml provided with seven input terminals datal-data7 from the flip-flops and an output terminal ul . During a first shift operation the enable input of flip-flop 1 is activated via the enable input el and the MUX Ml allows the output data2 from the flip-flop 2 to pass as the current output value ul . A second shift operation involves an activation of the enable input e2 of the flip-flop 2 and the MUX Ml allows the output data3 from the flip-flop 3 to pass as the current output value ul and so on for every step. Thus, only one of the flip-flops in the LFSR register according to the invention is enabled or activated during each shift operation, and as a result the low power LFSR according to the invention generates the same output bit sequence as a conventional LFSR. This lower flip-flop activity causes less power loss and, further, the longer the shift register and the smaller the feedback the more efficient is the low power LFSR according to the invention. In a register of an arbitrary length n having a feed- back polynomial of the same degree as the present feedback, the LFSR comprises n flip-flops 1-n, n EXOR gates El-En for the feedback arrangements, and a MUX Ml provided with n input terminals datal-datan and an output terminal ul . If i=n-l..l, during a first shift operation the enable input of flip-flop i is activated via the enable input and the MUX Ml allows the output data(i+l) from the flip-flop i+1 to pass as the current output value ul . However, if i=n a shift operation involves an activation of the enable input of the flip-flop n and the MUX Ml allows the output datal from the flip-flop 1 to pass as the current output value ul. In order to generate the enable signals to the flip- flops, one after the other, enabling means such as a low power one-hot coded finite state machine (fsm) with n steps is used. An n-step fsm comprises n flip-flops Fl-Fn and n NOR gates Nl-Nn. Each flip-flop in the fsm has a clock input connection, not shown in the drawings, an enable input connection en (activated by a 0 value) , a data input connection d, and a data output connection q. If i=l..n, the data output connection d of a flip-flop Fi is connected to a first input terminal of a NOR gate Ni, an output terminal thereof being connected to the enable input connection en of the flip-flop Fi, the data input terminal of the flip-flop F(i- 1) (Fn if i=l) , and a second input of the NOR gate N(i-l) (Nn if i=l) . The output terminal of the flip-flop Fi is also connected to the enable input eni (enl-en7) of the flip-flop i in the low power LFSR according to the invention. An embodiment of a 7-step fsm for connection to and control of the low-power LFSR illustrated in FIG 5 is shown in FIG 6. The 7-step fsm comprises seven flip-flops F1-F7 and seven NOR gates N1-N7 connected as described above. The fsm is also used to control the MUX Ml . An embodiment of the MUX Ml is shown in FIG 7. It comprises seven AND gates A1-A7 and four OR gates 01-04. The main function of the MUX Ml is that the enable signal enl causes the data2 to be the current output signal on ul and en2 causes the data3 to be the current output signal on ul and so on until en7 which causes the datal to be the current output signal on ul . Thus, it should be apparent that the present invention provides an LFSR that fully satisfies the aims and advantages set forth above. Although the invention has been described in conjunction with specific embodiments thereof, alternatives, modifications and variations are apparent to those skilled in the art. For example, a conventional LFRS as shown in FIGs 1 and 2 may have a different feedback polynomial such as and additional EXOR gate having first and second input terminals connected to the output of the flip-flop 6 and the flip-flop 7. Instead of the output terminal of flip- flop 7 an output terminal of the additional EXOR gate is connected to the second input terminal of the EXOR E. A conversion of the LFSR provided with this different feedback polynomial into a low power LFSR according to the invention is accomplished by a similar conversion of the conventional prior art LFSR as described for the conventional LFSR shown in FIGs 1 and 2, in which memory means such as flip-flops not consuming power when they are disabled are used, and wherein the register does not shift any bits but still generates the same sequence as the conventional linear feedback shift register. However, in one embodiment of an LFSR provided with this different feedback polynomial another seven EXOR gates are needed for the design.

Claims: CLAIMS 1. A low power LFSR comprising an ordered set of register steps including memory means (1-7), c h a r a c t e r i z e d by enabling means (F1-F7, N1-N7) , enabling a single current memory means (1-7) at every shift operation, register steps, each comprising a low power memory means (7) consuming a minimum of power when it is disabled, feedback means (E7) , an output terminal thereof being connected to an input terminal of the memory means (7), the feedback means (E7) having at least first and second input terminals connected to an output terminal (data7) of the memory means (7) and an output terminal (data2) of a second subsequent memory means (2) , respectively, in the set, and the output terminal (datal-data7) of each memory means (1-7) being connected to selection means (Ml) , selecting at every shift operation the output terminal (datal) of a first subsequent memory means (1) following the current memory means (7) being enabled at the current shift operation. 2. A low power LFSR according to claim 1 c h a r a c t e r i z e d by enabling means (F1-F7, N1-N7), enabling a single current flip-flop (1-7) at every shift operation, register steps, each comprising a low power flip-flop (7) consuming a minimum of power- when it is disabled, afeedback EXOR gate (E7) , an output terminal thereof being connected to an input terminal of the flip- flop (7) , the EXOR gate (E7) having at least first and second input terminals connected to an output terminal (data7) of the current flip-flop (7) and an output terminal (data2) of a second subsequent flip-flop (2), respectively, in the set, and the output terminal (datal-data7 ) of each flip-flop (1-7) being connected to selection means (Ml) , selecting at every shift operation the output terminal (datal) of a first subsequent flip-flop (1) following the flip-flop (7) in the set being enabled at the current shift operation. 3. A low power LFSR according to claim 1 or 2 c h a r a c t e r i z e d by enabling means comprising an ordered set of low power memory means (F1-F7) , each comprising a data output connection (d) connected to a first input terminal of a NOR gate (N1-N7) , an output terminal thereof being connected to an enable input connection (en) of the current memory means (F1-F7), the data input terminal of a subsequent memory means, and a second input of a subsequent NOR gate (N1-N7) , and the output terminal of the current memory means being connected to the enable input (enl-en7) of the corresponding memory means (1-7) in the LFSR. 4. A low power LFSR according to any of the preceding claims, c h a r a c t e r i z e d by selection means (Ml) , wherein an enable signal (enl-en7) generated by the enable means (F1-F7, N1-N7) for a particular memory means (1-7) of the LFRS and a data output terminal of a subsequent memory means (1-7) of the LFRS are connected to an AND-gate (Al- A7), the output terminal thereof being connected an input of an OR gate (01-04), the output of which is the current output signal (ul) of the register.

Inventors: HANSSON MATTIAS, HANSSON, MATTIAS
Applicants: ERICSSON TELEFON AB L M, TELEFONAKTIEBOLAGET LM ERICSSON
Application year: 1999
Main classification: G11C19
IPC codes: G11C19