MEMORY SECURITY CIRCUIT

Abstract: Apparatus for controlling the operation of a memory device (34) is effective to prevent unauthorized utilization of the memory device (34) upon removal thereof. A memory energizing signal (84) is delayed, the delayed signal being provided to logic means which produces an enabling signal (113) for the memory device (34) if the delayed signal (84) occurs during a predetermined time period determined by a control signal (100) produced in dependence on the energizing signal (84).

Description: EMORY SECURITY CIRCUIT Technical Field This invention relates to apparatus for con¬ trolling the operation of memory devices. The invention also relates to methods for controlling the operation of memory devices. Background Art The invention has a particular application to controlling the operation of read only memory (ROM) devices. An example of a ROM which is at present com¬ mercially available is the Intel 2316E manufactured by Intel Corporation. Once a program or other data has been stored in such known memory device and the memory device is associated with a widely-used microprocessor, it is easy for an unauthorized person to remove the memory device and copy the program from the memory, device using a similar processor. Thus the known memory device has the disadvantage that the contents thereof can readily be utilized or copied by unauthorized persons. Disclosure of the Invention It is an object of ' the present invention to provide apparatus of the kind specified wherein the aforementioned disadvantage is alleviated. Therefore, according to the present invention there is provided apparatus for controlling the opera¬ tion of a memory device, including generating means adapted to generate an energizing signal for said memory device, characterized by delay means adapted to delay said energizing signal for a first predetermined time period to thereby provide a delayed signal, and control means adapted to provide . an enabling signal to enable operation of said memory device if said delayed signal occurs during a second predetermined time period the commencement of which is dependent on the generation of said energizing signal. OMPI It will be appreciated that in apparatus according to the invention the need for the delayed energizing signal to occur during a predetermined time period in order to enable operation of the memory device assists in preventing unauthorized use of the memory device, since if the delayed energizing signal does not occur during such predetermined time period, the memory enabling signal will not be produced and the memory device cannot be accessed. Thus a high degree of security against unauthorized use of the memory device is achieved. A further advantage of apparatus according to the invention is that such apparatus is simple in construction and low in cost. According to another aspect of the invention, there is provided a method for controlling the operation of a memory device, characterized by the steps of gen¬ erating a memory energizing signal, delaying said ener¬ gizing signal for a first predetermined time period to provide a delayed signal, generating a control signal for a second predetermined time period in dependence on the generation of said memory energizing signal and generating a memory enabling signal in response to the simultaneous occurrence of said delayed signal and said control signal. Brief Description of the Drawings One embodiment of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of a ROM memory unit and associated control circuit; Fig. 2 is a block diagram of the ROM memory unit? Fig. 3 is a diagram showing various control pulses used in the control circuit for controlling the operation of the ROM memory unit; CMFI Fig. 4 is a block diagram of the control logic portion of the control circuit; and Fig. 5 is a block diagram of a combination logic circuit included in the control logic portion. Description of the Preferred Embodiment Referring now to Fig. 1 there is shown a block diagram of a portion of the security control circuit for controlling the operation of a semiconductor memory unit. Included in the circuit is a delay network 20 comprising a resistor 22 and a capacitor 24 forming a well-known RC circuit which outputs a voltage signal after a predetermined time delay as a result of the charging of the capacitor in a manner that is well-known in the art. One end of the resistor 22 is connected over line 26 to a conventional power supply 27 on which appears the supply voltage signal V which, in the present embodiment, is five volts. After a time delay during which the capacitor 24 is charged, the delayed five volt signal V (now labeled the security check signal SEC) is transmitted over line 28 to one of a number of chip select inputs labeled SEC of a ROM memory unit 30 which also receives over bus 32 the address of the data to be accessed in addition to the chip selec¬ tion signals CS, and CS 2 transmitted over lines 29 and 31 respectively. Included in the memory unit 30 as shown in Fig. 2 is a 16384 bit cell matrix 34 to which is coupled a decoder 36 which decodes the address bits A Q - _ received from the address input buffer 38. The address bits Ag-A, 0 transmitted over lines 40 are in- putted into a decoder 42 which decodes the data read out of the matrix 34. The decoded data bits D Q -D 7 are outputted to the output buffer 44 for transmission over lines 46 to a receiving unit. Also included in the ROM memory unit 30 is an input buffer and control logic unit 48 which receives the chip select signals ϋST and U3T in addition to the OMPI delayed security check signal SEC transmitted over line 28 (Fig. 1) from the delay network 20. The signals CS,, CS 2 and SEC are transmitted to a chip select program unit 50 which selects the chips to be accessed in ac- cordance with a program stored in the unit. The chip select signals outputted from the program unit 50 are transferred to a chip select decoder 52 which controls the enabling of the output buffer 44 to output the required data over lines 46 to the receiving unit. Referring now to Fig. 4, there is shown a block diagram of the control logic circuit located in the input buffer and control unit 48 (Fig. 2) of the ROM memory unit 30 which, in conjunction with the delay net¬ work 20 controls the enabling of the memory unit 30 in accordance with the generation of the security check sig¬ nal SEC. Included in the logic circuit is a reset gen¬ erator 80 comprising an edge detector circuit which generates a reset pulse 82 (Fig. 3) when the supply voltage signal V C appearing on line 26 and indicated by the numeral 84 in Fig. 3 goes high. The reset pulse 82 is transmitted over the line 84 to a binary counter 86 and, to-a pair of D-type flip-flops 88, 90 thereby clearing the counter and resetting the flip-flops. Also included in the control circuit is a clock generator 92 which clocks the counter 86 at a predetermined frequency in a manner that is well known in the art. The counter 86 also receives over line 97 the signal of the most significant bit of the output count of the counter 86, which signal is connected to the enabling input EN of the counter preventing the counter from counting multiple cycles. The binary signals appearing on the output lines 96 of the counter 86 and representing the current count is transmitted to a combination logic circuit unit 98 (Figs. 4 and 5) which is logically constructed to output the signal I indicated by the numeral 100 in Fig. 3. As illustrated, the signal I goes high as the output count of the counter OMPI 86 reaches a predetermined value and stays high until the output count of the counter 86 reaches a second value, at which time the signal 100 goes low. The time the signal 100. stays high must be selected to correspond to the occurrence of the delayed signal SEC (Fig. 3) generated by the circuit 20 (Fig. 1). As will be dis¬ cussed more fully hereinafter. Thus the operation of the ROM unit 30 (Fig. 1) is dependent on the delay period T generated by delay circuit 20. As shown in Fig. 5, the combination logic unit 98 in the present embodiment may comprise AND gates 99 arranged as shown to receive the binary bits present in the output lines 96 of the counter 86 (Fig. 4) to output to the OR gate 101 the signal 100 when the output count is between five and fifteen. As further shown in Fig. 3, the signal 100 is arbitrarily indicated as being high between .5T to 1.5T from the time the power supply signal V C went high. The signal I is transmitted over line 102 to the D-input of the flip-flop 90 (Fig. 4). The security check signal SEC appearing on line 28 is also coupled to the clock input of the flip-flop 88 whose D-input is connected -to the power supply 27 (Fig. 1) which generates the signal V cC As shown in Fig. 3, once the signal V * C goes high, the Q output signal SECON of the flip-flop 88 and indicated by the numeral 104 will go high upon the clocking of the flip-flop by the signal SEC. Because the signal V will remain high, the signal SECON will stay high even when the signal SEC goes low as indicated by the dotted lines 106 in Fig. 3. This latter con¬ dition may occur if the chip select input SEC (Fig. 1) is used in addressing the memory unit 30. The signal SECON is transmitted over line 106 to an edge detector 108 (Fig. 4) which outputs the pulse SECE, indicated by the numeral 110 in Fig. 3, over line 112 to the flip- flop 90 upon sensing the signal SECON going high. The signal SECE will clock the flip-flop 90 allowing the OMPI flip-flop to output a high signal 113 {Fig. 3) at its Q output if the signal I appearing at the D-input is high at this time. The high signal 113 appearing on the Q output line 114 of flip-flop 90 (Fig. 4) will enable the input buffer 49 of the ROM memory unit 30 to initiate a memory access operation in accordance with the chip select signals appearing at the chip select input lines 29, 31 (Fig. 1) of the ROM memory unit 30. Once the signal SECE goes high, the flip-flop 90 will be set and remain set until the power is shut off and the flip- flops 88, 90 are reset in the manner described pre¬ viously. It will be seen from this construction that once the signal V goes high indicating a power-up condition, the delayed security check signal SEC must go high during the time the signal I is high (Fig. 3) in order for the flip-flop 90 to output the high enabling signal 113 to the ROM unit 30. Without the delay, the signal SEC is unlikely to go high when the signal I is high. This results in the signal I being low when the flip-flop 90 is clocked by the signal SECE, which opera¬ tion prevents any access to the ROM unit 30 by an ad¬ dressing unit. Thus, the circuit prevents the un¬ authorized use of the ROM memory unit when removed from the processor for which it was programmed to be part.

Claims: CLAIMS : 1. Apparatus for controlling the operation of a memory device (34), including generating means (27) adapted to generate an energizing signal (84) for said memory device (34), characterized by delay means (20) adapted to delay said energizing signal for a first pre¬ determined time period (T) to thereby provide a delayed signal (SEC), and control means (80, 92, 86, 98, 88, 108, 90) adapted to provide an enabling signal (113) to enable operation of said memory device (34) if said de- layed signal (SEC) occurs during a second predetermined time period (100) the commencement of which is dependent on the generation of said energizing signal (84). 2. Apparatus according to claim 1, charac¬ terized in that said control means includes first logic means (80, 82, 86, 98) coupled to said generating means (27) and adapted to provide a control signal (100) for said second predetermined time period in response to receiving said energizing signal (84), and second logic means (88, 108, 90) coupled to said first logic means (80, 82, 86, 98) and adapted to provide said enabling signal (113) in response to the occurrence of said de- layed signal (SEC) during said control signal (100). 3. Apparatus according to claim 2, charac¬ terized in that said control signal (100) commences a third predetermined time period after the generation of said energizing signal (84). 4. Apparatus according to claim 3, charac¬ terized in that said first logic means includes clock signal generating means (92), and counting means (86, 98). responsive to clock signals generated by said clock signal generating means (92) to initiate said control signal (100) in response to the attainment of a first OMPI 4. ( concluded) predetermined count corresponding to said third pre¬ determined time period and to terminate said control signal (100) in response to the attainment of a second predetermined count. 5. Apparatus according to claim 4, charac¬ terized in that said second logic means includes a first bistable device (90) having a first input adapted to receive said control signal (100) and a second input adapted to be operated in dependence on said delayed signal (SEC). 6. Apparatus according to claim 5, charac¬ terized in that said second logic means includes a second bistable device (88) adapted to receive said delayed signal (SEC), said second bistable device (88) having an output coupled to edge detector means (108) which is coupled to said first bistable device (90). 7. Apparatus according to claim 6, charac¬ terized in that said first bistable device includes a flip-flop circuit (98) having a data input arranged to receive said control signal (100) and a clock input coupled to said edge detector means (108). 8. Apparatus according to claim 1, charac¬ terized by reset means (80) coupled to said generating means (27) and adapted to provide a reset signal (82) for said control means (80, 92, 86, 98, 88, 108, 90) in response to the generation of said energizing signal (84). 9. Apparatus according to claim 1, charac¬ terized in that said delay means (20) includes an RC circuit. OMPI 10. Apparatus according to claim 1, charac¬ terized in that said memory device includes a read only memory (34) . 11. Apparatus according to any one of the preceding claims, including data processing means, characterized in that said memory device (34) is pro¬ vided with a plurality of input ports (28, 29, 31) said delay means (20) being coupled to one of said input ports (28), whereby upon removal of said memory device (34) from association with said data processing means, said delay means (20) is disconnected from said memory device (34). 12. A method for controlling the operation of a memory device (34), characterized by the steps of generating a memory energizing signal (84), delaying said energizing signal (84) for a first predetermined time period (T) to provide a delayed signal (SEC), gen¬ erating a control signal (100) for a second predetermined time period in dependence on the generation of said memory energizing signal (84) and generating a memory enabling signal (113) in response to the simultaneous occurrence of said delayed signal (SEC) and said control signal (100). OMPI

Inventors: DEVCHOUDHURY R
Applicants: NCR CO, NCR CORP
Application year: 1981
Main classification: G06F13
IPC codes: G06F9, G06F12, G11C29