Address transition detection circuit

Abstract: An address transition detection circuit (30) having a number of cells (1) supplied with respective address signals and output connected in a wired NOR configuration to generate a pulse signal (WN) on detecting transitions of their respective address signals. The pulse signal is supplied to a source stage for generating an address transition signal (ATDO) having a first and second switching edge on receiving the pulse signal. The source stage has a monostable stage (80) for generating an end-of-transition signal (ATDY) with a predetermined delay following reception of the pulse signal; and an output stage (35, 70) connected to the cells (1) and to the monostable stage (80), which generates the first switching edge of the address transition signal (ATDO) on receiving the pulse signal (WN), and the second switching edge on receiving the end-of-transition signal. The monostable stage (80) presents a compensating structure (40, 42, 44) for maintaining the delay in the switching of the end-of-transition signal (ATDY) stable alongside variations in temperature and supply voltage.

Description: The present invention relates to an address transition detection circuit. As is known, the first operation when addressing a flash memory is to detect when the level of the address signals has changed. This is done inside the memory itself by a so-called ATD (Address Transition Detection) circuit, which generates a signal (hereinafter referred to as ATDO) presenting a pulse of appropriate duration, which is used by the other devices in the read portion of the memory. Figure 1 shows the switching sequence of an address ADD, and the corresponding generation of signal ATDO. As can be seen, the two parameters governing signal ATDO are the pulse duration τ and the delay T with which the pulse is generated after an address transition. At present, known ATD circuits are so designed as to provide for an extremely variable pulse duration τ (whereas delay T depends strictly on the speed with which the circuitry detects the address transition). Basically, known ATD circuits comprise a number of cells (one for each address) connected in a wired NOR configuration to detect the switching of any one address signal and generate a corresponding pulse, and a chain of inverters to prolong the pulse duration; and each cell comprises a latch circuit in turn comprising two push-pull inverters. The above known ATD circuit is shown in Figure 2 and will now be described by way of reference. Figure 2 shows a number of identical cells 1, only one of which is shown in detail, and each cell 1 comprises a first and second, in this case NMOS, input transistor 2, 3, the gate terminals of which (defining respective inputs of cell 1) are supplied respectively with a direct address signal A and an inverted address signal AN. Input transistors 2, 3 are designed to present a high W/L (channel width to length) ratio, and have the source terminals grounded, and the drain terminals (defining respective nodes 10, 11) connected to one terminal of respective capacitors 4, 5 (the other terminals of which are grounded) and to the input of respective inverters 6, 7; the output of inverter 6 is connected to node 11; the output of inverter 7 is connected to node 10; and nodes 10 and 11 are also connected to the inputs of a NOR gate 12, the output of which defines the output 13 of cell 1. The output 13 of each cell 1 is connected to the gate terminal of a respective NMOS detecting transistor 15, the source terminal of which is grounded, and the drain terminal of which is connected to a common node 16 to which detecting transistors 15 are all connected. Common node 16 is also connected to the drain terminal of a PMOS load transistor 17, the source terminal of which is connected to the supply line 20 (at V cc ), and the gate terminal of which is grounded (and therefore always on). Common node 16 is also connected to a number of cascade connected inverters 18, the last of which defines the circuit output 19 feeding signal ATDO. The state of each cell 1 is determined by address signals A and AN, which control charging and discharging of capacitors 4, 5. More specifically, in standby mode, only one of the address signals, e.g. AN, is high, while the other (in this case, A) is low, so that capacitor 5 is discharged and kept grounded by respective input transistor 3, and capacitor 4 is charged by virtue of transistor 2 being off. The output of NOR gate 12, which receives at the inputs a low and a high signal, is therefore low, and respective detecting transistor 15 is off. In the absence of address transitions, transistors 15 are all off, and transistor 17 keeps node 16 high, so that output signal ATDO presents a predetermined, e.g. low, logic state. Upon transition of an address signal, signals A and AN of the relative cell 1 switch. In the example shown, this means signal AN switches to low and A to high, so that transistor 2 is turned on and rapidly discharges capacitor 4; and transistor 3 is turned off so that capacitor 5 is charged via the pull-up transistor of inverter 6 (sized to present a low W/L ratio and therefore to be flowed by little current). Capacitor 5 is therefore charged slowly, so that, when capacitor 4 is discharged and until capacitor 5 is charged, NOR gate 12 presents two low input signals and switches to supply output 13 with a high signal, which turns on the respective detecting transistor 15, lowers common node 16, and switches ATDO. Upon capacitor 5 reaching the threshold voltage of NOR gate 12, this switches to switch its output 13 back to low; respective detecting transistor 15 is turned off; common node 16 switches back to high; and signal ATDO switches back to low to terminate the pulse. In the Figure 2 circuit, the chain of inverters 18 provides for prolonging the duration of the ATDO signal pulse, which duration is highly sensitive to variations in temperature and in supply voltage V cc . In fact, in the known circuit featuring simple chains of inverters, slight variations in threshold voltage may result in marked variations in the current supplied by the inverters, especially when the supply voltage is low (from 2 to 3.8 V). This variability is a disadvantage in memories in which correct operation of the read circuits requires a precise predetermined pulse duration which is substantially stable in relation to both temperature and supply. EP-A-0 336 319 discloses an address transition detection circuit comprising a monostable stage including selectively activated delay means in order to generate a transition detection signal having a standard length pulse or an increased length pulse according to an external control signal. The transition detection signal depends on temperature and power supply. EP-A-0 558 079 discloses an address transition detection circuit comprising a flip-flop circuit and a delay circuit, wherein a pulse signal is fed to a set terminal of the flip-flop circuit and an output signal of the flip-flop circuit is fed, through the delay circuit, to a reset terminal of the flip-flop circuit. Thereby, a constant width signal, which is independent of the waveform of an address signal and which responds only to the change of address, can be obtained as an address transition signal for a SRAM. By initialising an internal circuit of the SRAM through, malfunctions caused by the dependence of the initialisation time upon the waveform of the address signal may be prevented. It is an object of the present invention to provide an ATD circuit designed to overcome the drawbacks of known circuits, and which in particular generates an address transition signal of predetermined, stable duration. According to the present invention, there is provided an address transition detection circuit as claimed in Claim 1. A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows the timing of the address transition signal; Figure 2 shows the electric diagram of a known address transition detection circuit; Figure 3 shows an electric diagram of the address transition detection circuit according to an embodiment of the invention; Figure 4 shows a time plot of a number of signals in the Figure 3 circuit. Number 30 in Figure 3 indicates an address transition detection circuit, which is inserted in a nonvolatile, e.g. flash, memory 100, and comprises a number of cells 1 identical to the known cells in Figure 2. In known manner, cells 1 (one for each address signal) present an output 13 connected to a respective detecting transistor 15, and detecting transistors 15 are connected in a wired NOR configuration to common node 16 in turn connected to supply line 20 via load transistor 17. In Figure 3, common node 16 (presenting a signal W) is connected to the input of an inverter 31, the output of which defines a node 32 presenting a signal WN; node 32 is also connected to the input of an inverter 33, to the reset input of an SR flip-flop 34, and to one input of a two-input NOR gate 35; the output of inverter 33 (presenting a signal COM) is connected to the set input of flip-flop 34; the direct output Q of flip-flop 34 (presenting a signal ATDN) is connected to a monostable stage 80; and the inverted output QN of flip-flop 34 (presenting a signal ATD) is connected to the second input of NOR gate 35. Monostable stage 80 comprises a PMOS input transistor 36 with its gate terminal connected to output Q of flip-flop 34, its source terminal connected to supply line 20, and its drain terminal connected to one terminal of a resistor 37, the other terminal of which is connected to the anode of a diode 38. Diode 38 is in the form of an NMOS transistor with shorted gate and drain terminals and a grounded cathode; the anode of diode 38 is connected to the drain terminal of an NMOS transistor 39, and to the gate terminal of a native NMOS transistor 40 (i.e. presenting an unmodified, i.e. low, threshold). Transistor 39 has the gate terminal connected to output Q of flip-flop 34, and the source terminal grounded; and native transistor 40 has a gate terminal defining a node 41, the source terminal grounded via a resistor 44, and a grounded bulk. Resistor 44 must present a low thermal coefficient (below 2x10 -3 ), is preferably formed by an N + diffusion in the substrate of the integrated device, and has, for example, a resistance of 12 KΩ. Output Q of flip-flop 34 is also connected to the input of an inverter 45, the output of which is connected to the gate terminal of a PMOS load transistor 46, the source terminal of which is connected to supply line 20, and the drain terminal of which is connected to node 41. Node 41 is grounded via a capacitor 42 (e.g. of 600 fF) and is connected to the input of an inverter 43, the output of which is connected to one input of a three-input NAND gate 48 to which it supplies signal ATDY. NAND gate 48 comprises three NMOS transistors 50, 51, 52 in series to each other, and three PMOS transistors 53, 54, 55 in parallel to each other. More specifically, PMOS transistors 53, 54, 55 have the source terminals connected to one another and to supply line 20; the drain terminals connected to one another and defining a node 56; and the gate terminals connected respectively to the output of inverter 43, to the output of inverter 33, and to output QN of flip-flop 34, so as to respectively receive signals ATDY, COM and ATD. Transistors 50, 51, 52 are connected between ground and node 56, and have the gate terminals connected respectively to the gate terminals of transistors 53, 54, 55. Circuit 30 presents an initialization branch 59 presenting an enable input 60 supplied with an inverted enable signal ENN. Input 60 is connected to the gate terminals of a PMOS transistor 61 and an NMOS transistor 62 connected in series between supply line 20 and ground. The intermediate node 63 between (and connected to the drain terminals of) transistors 61 and 62 is connected to one terminal of a capacitor 64 (the other terminal of which is grounded) and to the input of an inverter 65. The output of inverter 65 is connected to the input of a further inverter 66, the output of which is connected to one input of a three-input NAND gate 68, a second input of which is connected to node 56, and a third input of which is connected to output QN of flip-flop 34 and therefore receives signal ATD. The output of NAND gate 68 is connected to a clock input CK of flip-flop 34, to which it supplies clock signal CK. Finally, the output of NOR gate 35 is connected to the input of an inverter 70, the output 71 of which defines the output of circuit 30 and presents address transition signal ATDO. Operation of the Figure 3 circuit will now be described with reference also to Figure 4. In the absence of address transitions, and with the circuit in the steady standby condition (signal ENN low), detecting transistors 15 are off, node 16 is high, signal WN is low and COM high. Moreover, as explained in more detail later on, signal CK is high, flip-flop 34 is set (so that signal ATDN is high and ATD low), and signal ATDO is low. Consequently, transistor 36 is off, the output of inverter 45 is low, and transistor 46 is on and connects node 41 to the supply. At the same time, transistor 39 is on and grounds the gate terminal of native transistor 40, which is therefore off, so that node 41 is high, signal ATDY is low, and capacitor 42 is charged to V cc . As soon as a cell 1 detects a transition of its own address (instant t 1 in Figure 4), respective transistor 15 is turned on, thus switching signal W to low; signal WN switches to high; signal COM switches to low; flip-flop 34 switches; signal ATD switches to high; signal ATDN switches to low; and output signal ATDO switches to high. High switching of ATD switches NAND gate 68 (whose other two inputs are high) and signal CK to low, thus disabling any further switching of flip-flop 34. Moreover, the switching of ATDN turns on transistor 36 and the branch comprising components 37-38, turns off transistors 39 and 46, and turns on native transistor 40, the gate terminal of which presents a voltage versus ground equal to the voltage drop across diode 38 - in turn equal to the threshold voltage of a standard, non-native transistor - and therefore substantially constant. Consequently, capacitor 42 is grounded via transistor 40 and resistor 44, and discharges a constant current, by virtue of transistor 40 being so sized (i.e. presenting such a W/L ratio) as to determine the current in the branch. Within the branch, resistor 44 provides for reducing the voltage drop V GS between the gate and source terminals of native transistor 40 as a function of the amount of conducting current of transistor 40. The voltage at node 41 therefore falls gradually until, after a period of time τ depending on the current flow in capacitor 42, it reaches the trigger threshold of inverter 43, the output signal ATDY of which switches to high (instant t 3 ). In the meantime, since the circuit is so designed that inverter 43 always switches after the end of the address transition, signals W, WN and COM have once more switched back to the standby state (instant t 2 in Figure 4). Since flip-flop 34 is prevented from switching, however, such switchings do not affect the rest of the circuit, so that signal CK remains low, and the output of NAND gate 48 is kept high by signal ATDY at its input. Conversely, when ATDY switches, NAND gate 48 is supplied with three high logic levels, which turn on all three transistors 50-52, and turn off transistors 53-55, so that the output of NAND gate 48 switches to low; signal CK switches to high, thus enabling flip-flop 34, which switches back to standby with a low ATD and a high ATDN signal; gates 35 and 70 also switch to terminate the pulse of signal ATDO, which switches to low; monostable stage 80 is restored to the initial condition, by virtue of capacitor 42 being discharged rapidly when native transistor 40 is turned off and transistor 46 is turned on; and signal ATDY switches to low after a brief high pulse which resets the whole of circuit 30. Initialization branch 59, which is turned on together with memory 100, provides for setting circuit 30, when this is turned on, to a definite operative condition using the overall inverted enabling signal ENN of memory 100. In the absence of branch 59, in fact, flip-flop 34, when circuit 30 is turned on, could be set to a metastable condition in which output signals ATD and ATDN fail to present a definite logic state and are stopped halfway (V cc /2). In which case, NOR 35 would be supplied with a low signal (WN) and an intermediate signal (ATD), and may switch its output to low, so that output signal ATDO may not switch to 0. What is more, monostable stage 80 would also be in a metastable operating condition in which transistor 36 is partly conductive, transistor 39 is on, transistor 40 is off, inverter 45 is at an intermediate operative level, and transistor 46 is turned on slightly but enough to bring node 41 to a voltage of over V cc /2. As a result, signal ATDY would be low, the output of NAND gate 48 would be high, NAND gate 68 would receive a high signal and signal (ATD) at an intermediate level (in which case, there would be no third input of gate 68), and signal CK would be uncertain, probably low, and incapable of definitely setting flip-flop 34. Initialization branch 59, on the other hand, prevents this from happening. That is, when memory 100 is turned on, the initially high ENN signal switches to low, so that transistor 61, which forms an inverter with transistor 62 and is initially off (node 63 low), is turned on and begins charging capacitor 64 which was previously discharged. While capacitor 64 is being charged, the output of inverter 66 remains low to ensure signal CK is high, even if signal ATD is in a metastable condition as described above; and the presence of a"1" at input CK of flip-flop 34, together with a "0" at input R and a "1" at input S, ensures the flip-flop is set to the desired standby condition with output Q (signal ATDN) high and output QN (signal ATD) low. When capacitor 64 is charged to the trigger threshold of inverter 65, the output of inverter 66 switches to high, but the output of NAND gate 68 (and signal CK) does not switch, since ATD is now definitely low; and circuit 30 is now in the standby condition, ready to detect address transitions. The advantages of the circuit described are as follows. In particular, it provides for achieving ATDO pulses of a duration insensitive to variations in temperature (particularly between -40°C and 125°C) by virtue of diffused resistor 44. In fact, for a given V GS (gate-source voltage drop) of native transistor 40, a fall in temperature increases current flow through native transistor 40 and hence through resistor 44; and resistor 44 (which presents a resistance with a low thermal coefficient) presents a voltage drop proportional to the current flowing through it, and therefore reduces the voltage drop V GS across native transistor 40 by limiting current flow through it. In other words, resistor 44 performs a negative feedback function to limit the increasing discharge current of capacitor 42 (and vice versa) alongside variations in temperature. The discharge current of capacitor 42 is also substantially independent of variations in supply voltage V cc , by being imposed by the voltage drop across diode 38 (equal to the threshold voltage of the transistor forming it) and by the voltage drop V GS of native transistor 40. Simulation tests by the Applicant have shown the circuit according to the invention is capable of maintaining very small temperature-induced variations in the duration τ of the ATDO signal pulse (less than 10% of the ambient temperature value at -40°C, and less than 20% of the ambient temperature value at +125°C), and is practically insensitive to variations in supply voltage between 2 and 3.8 V. Moreover, circuit 30 does not require precise external voltage references, and consumes no current in the absence of address transitions. Clearly, changes may be made to the circuit as described and illustrated herein without, however, departing from the scope of the present invention. For example, resistor 44 may be formed differently (i.e. need not necessarily be diffused), providing a low thermal coefficient is assured.

Claims: An address transition detection circuit (30) comprising a number of cells (1), each having a respective output (13); each said cell (1) receiving a respective address signal, and generating a first pulse signal at said respective output on detecting transitions of a respective address signal; said outputs of said cells (1) being connected to a signal generating stage (15, 17, 31, 34, 43, 80) generating an address transition signal (ATDO) having a first and a second switching edge, said first switching edge being a transition in a first direction and occurring on receiving a first arriving of said first pulse signals and said second switching edge being a transition in the opposite direction; said signal generating stage (15, 17, 31, 34, 43, 80) comprising a monostable stage (80) having an input connected to said outputs (13) of said cells (1) and determining said second switching edge of said address transition signal at a predetermined delay from said first switching edge of said address transition signal; characterized in that said monostable stage (80) comprises a capacitive element (42) having a first terminal connected to a second reference potential line and a second terminal (41) connected to a first reference potential line (20) via a first switching means (46) and to said second reference potential line via current source means (40, 44); said current source means (40, 44) comprising a diffused resistor (44) connected to said second reference potential line and a first transistor (40) having a first and a second terminals connected respectively to said capacitive element (42) and said diffused resistor (44), and a control terminal connected to a voltage source (36, 37, 38); said first switching means (46) being switched off and said voltage source (36, 37, 38, 39) being switched on at receiving said first arriving of said first pulse signals and, when switched on, said voltage source (36, 37, 38, 39) providing the control terminal of said first transistor (40) with a voltage greater than the threshold voltage thereof; said signal generating stage (15, 17, 31, 34, 43, 80) further comprising first logic gates (15, 17, 31) connected to said outputs (13) of said cells and generating a second pulse signal (WN) on receiving said first arriving of said first pulse signals; a memory, element (34) having a set input (S) connected to said first logic means (15, 17, 31), an enable input (CK) connected to a disable stage (48, 68), and an output connected to said monostable stage (80) and to an output (71) of said signal generating stage (15, 17, 31, 34, 80); and second logic gates (43) having a trigger threshold, an input connected to said second terminal (41) of said capacitive element (42) and an output connected to said disable stage (48, 68). A circuit as claimed in claim 1, characterized in that said first transistor (40) comprises a native MOS transistor; and in that said voltage source (36, 37, 38, 39) comprises a diode element (38) connected between said control terminal of said first transistor (40) and said second reference potential line. A circuit as. claimed in claim 2, characterized in that said diode element (38) is connected to said first reference potential line (20) via second switching means (36); in that each of said first (46) and second (36) switching means has a respective control terminal receiving a respective control signal; and in that said circuit comprises third logic gates (45) connected to said first logic gates (15, 17, 31) and generating said control signals of said first and second switching means (46, 36) in phase opposition to each other on receiving said second pulse signal (WN). A circuit as claimed in claim 3, characterized in that said first and second switching means (46, 36) comprise a second and a third MOS transistors. A circuit as claimed in any one of the foregoing claims, characterized in that said monostable stage (80) has an output supplying an end-of-transition signal (ATDY) presenting a pulse after said predetermined delay from said first switching edge; and in that said disable stage (48, 68) comprises fourth logic gates (48) having a first, a second and a third inputs connected respectively to said output of said monostable stage (80), to the output of said first logic gate (15, 17, 31), and to the output of said memory element (34), and an output (56) connected to said enable input (CK) of said memory element (34); said output (56) of said fourth logic gates (48) supplying a turn-on signal (CK) having a first and a second levels; said fourth logic gates generating said first level to disable said memory element (34) on receiving said first edge of said second pulse signal (WN), and generating said second level to enable said memory element on receiving said end-of-transition signal (ATDY). A circuit as claimed in claim 5, characterized by an initialisation stage (59) having an input (60) and an output; said input of said initialisation stage receiving an enable signal (ENN), and comprising delay means (61, 64) generating a delayed enable signal; and in that said disable stage (48, 68) also comprises fifth logic gates (68) having a first and a second inputs and an output; said first and second inputs of said fifth logic gates being connected to the output of said initialisation stage (59) and to the output of said fourth logic gates (48); and said output of said fifth logic gates being connected to said enabling input (CK) of said memory element (34). A circuit as claimed in any one of the foregoing claims, characterized, by an output stage (35, 70) connected between said output of said memory element (34) and said output (71) of said signal generating stage (15, 17, 31, 34, 43, 80); said output stage comprising sixth logic gates (35) connected to said output of said memory element (34) and to the output (32) of said first logic gates (15, 17, 31), generating said first switching edge of said address transition signal (ATDO) on receiving said second pulse signal (WN) and said second switching edge of said output signal upon said memory element (34) switching.

Inventors: GOLLA CARLA MARIA, ZAMMATTIO MATTEO, ZANARDI STEFANO, GOLLA, CARLA MARIA, ZAMMATTIO, MATTEO, ZANARDI, STEFANO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: H03K3
IPC codes: G11C8, H03K5