Device for Detecting the Contents of Cells Within a Memory, Especially an EPROM Memory, Method Implemented With This Device, and Memory Provided With This Device

1. Field of the Invention

The present invention relates to a device for detecting the contents of the cells contained in a memory, especially an EPROM memory. It also concerns a method implemented with this device, as well as a memory provided with this device.

2. Discussion of Background Art

Present progress in memories of very high capacities, especially of EPROM type, has been accompanied by an increase in the working frequencies of the systems in which these memories are implemented, and consequently by the search for ever shorter times for access to the contents of the cells constituting these memories. Now, in order to minimize the access time of a memory, especially a memory of EPROM type, it is essential to preset the detection amplifier normally allowed for in order to permit the reading of the contents of each cell in a state in which the differential inputs are balanced.

Hitherto, in the earlier designs of this detection amplifier, this presetting was performed using a pulse to short-circuit the differential inputs directly. However, this has the disadvantage of generating spurious current spikes both in the reference column and in the bit row, this having the consequence of disturbing the state of the current comparator and consequently of leading to an increase in the access time.

The aim of the invention is to remedy these disadvantages by providing a device for detecting the contents of the cells of a memory, especially an EPROM memory, these cells being organized as a set of bit lines, this device comprising means for comparing a reference current associated with a reference column and a read current invoked in a cell of a bit lines and whose contents are to be read, said comparison means comprising a resistive reference element and a resistive read element through which the reference and read currents respectively travel, and which are connected, on the one hand, to a supply voltage source and, on the other hand, to a non-inverting input and to an inverting input respectively of a differential amplifier delivering as output a detection cue.

According to the invention, the device furthermore comprises means for selectively connecting the output of the differential amplifier to its inverting input for a predetermined preloading period.

Thus, it is the differential stage itself which is used to balance the differential inputs, by passing from a differential mode to a follower mode for a predetermined period. This also has the effect of compensating for the asymmetries which general exist in the input transistors of the differential stage.

Furthermore, since the entire preloading current travels towards the bit line, the reference column is not disturbed by the preloading operation, this having the effect of improving the stability of the reference column during the detection operation.

According to an advantageous embodiment, the selective connecting means comprise switching means controlled by a cyclic preloading clock signal.

According to another aspect of the invention, the method of detecting the contents of cells of a memory, especially of EPROM type, implemented in the device according to the invention, comprising, for each read sequence for a cell, a step of preloading this device, followed by a step of reading the contents of said cell comprising a comparison of the read current flowing in said cell with a reference current, this comparison being made by a differential amplifier, is such that, during the preloading step, the differential amplifier is switched into follower mode during a preloading clock cycle.

According to yet another aspect of the invention, the erasable permanent memory, especially of EPROM type, incorporating devices according to the invention, is such that it comprises control means for generating, in the course of a preloading clock cycle, a signal for controlling the means for selectively connecting the output to the inverting input of the differential amplifier of each of the detection devices.

Other features of the invention will emerge further from the description below.

In the attached drawings given by way of non-limiting examples:

FIG. 1 is a diagrammatic view of a detection device according to the invention;

FIG. 2 represents a practical embodiment of selective connecting means implemented in the invention;

FIG. 3 is an overall view of the organization of a memory incorporating the device according to the invention.

The detection device according to the invention will now be described in further detail with reference to FIGS. 1 to 3, together with the method implemented in this device.

In the embodiment illustrated in FIG. 1, the detection device according to the invention comprises a differential amplifier 2 mounted in order to compare a reference current I.sub.REF invoked in a reference column and represented symbolically by a reference current source, with a read current I.sub.L invoked by a memory cell 10 whose contents are desired to be read accurately and which is modeled in FIG. 1 in the form of a capacitance and a current source. Each reference and read current source is connected respectively to a DC supply voltage source V.sub.DD via resistive reference and read elements R.sub.REF and R.sub.L respectively, according to techniques which are well known within the area of the manufacture of semiconductors by MOS technology. Moreover, the resistive reference and read elements R.sub.REF and R.sub.L are connected to the non-inverting input V+ and inverting input V- respectively of the differential amplifier 2 according to an arrangement which is well known to the expert in the area of electronics. According to the invention, a switch 4 controlled by a clock signal CK selectively connects the output of the differential amplifier 2 to its inverting input V-. The detection signal S.sub.A output by the differential amplifier 2 is applied as input to a high-impedance amplifier 3 which generates a logic cue D representative of the contents of the memory cell read. The detection device 1 according to the invention thus has a three-state logic output which makes possible any interfacing of the memory within a digital system.

With reference to FIG. 2, the switch 4 can be made advantageously, implementing well-known techniques, from four transistors connected in series, a first transistor T1, of PMOS type, having its source connected to a DC voltage source Vc and its drain connected to the drain of a second transistor T2 of NMOS type whose source is connected to the source of a third transisor T3 of PMOS type. A fourth transistor T4 of NMOS type has its source connected to the earth of the device and its drain connected to the drain of the third transistor T3. It should be noted that the transistors T1 and T3 of PMOS type must have their substrate connected to their drain, so as in particular to minimize their threshold voltage.

PMOS transistor means a P-channel MOS transistor and NMOS transistor means an N-channel MOS transistor.

A clock signal CK generated by an outside clock generator (not shown) is applied to the gate of the fourth transistor T4 whilst its logic inverse CK is applied to the gate of the first transistor T1. The detection signal S.sub.A is applied simultaneously to the respective gates of the second and third transistors T2, T3, whilst the junction point of the source of the second transistor T2 and of the source of the third transistor T3 is connected to the inverting input V- of the differential amplifier 2.

When the clock signal CK is at the high logic level, the transistors T4 and T1 are on. The output S.sub.A of the differential amplifier 2 is then connected to its inverting input V- and the differential amplifier 2 is thus in a follower configuration. The output voltage S.sub.A is then substantially equal to the product of the voltage V+ times the ratio K/K+1), where K is the gain of the differential amplifier 2. Now, since K is generally much greater than 1, the signal Sa is substantially equal to the voltage V+ corresponding to the reference column. This follower configuration is maintained during the preloading cycle operated by a control logic internal to the memory. On completion of this preloading cycle, the switch 4 is then placed in the open position, resulting in a high impedance between the inverting input V- and the output of the differential amplifier 2 which then completely fulfils its role as comparator since it is no longer under feedback.

Moreover, it should be noted that with the invention there is no risk of instability and of oscillation during the preloading operation since the feedback gain is equal to 1. The differential amplifier 2 then behaves as a follower stabilized by negative loop-back.

Furthermore, with the prior-art detection devices, owing to the inevitable asymmetries existing between the input transistors of the differential amplifier, a short-circuiting of the two inputs of the differential amplifier could cause a false reading, either of a 1 or of a 0. With the device according to the invention, the use of the amplifier itself for the balancing causes automatic compensation of the aforesaid imbalance between input transistors, and the inputs of the differential amplifier are then biased to the amplifier's changeover point.

An erasable programmable permanent memory M of EPROM type incorporating detection devices according to the invention, a very simplified illustrative structure of which is represented in FIG. 3, incorporates a set 23 of memory cells organized as bit lines L1, L2, Ln with which a reference column C.sub.REF is associated. A control and interface logic 22 processes the address cues received on an address bus BA and control cues exchanged on a control bus BC. An address decoding logic provides for the pointing to the bit line relating to a read operation whilst a detection logic 20 incorporates a set of detection devices 1 according to the invention which generate read cues which are then delivered onto a databus BD. It should be noted that the combination of the addressing, control and interfacing operations can be carried out according to the customary techniques well known in the area of microelectronics.

Of course, the invention is not limited to the examples which have just been described and other contrivances and extensions may be envisaged without departing from the scope of the invention. Thus, the switch allowing the selective connecting of the output and of the inverting input of the differential amplifier may be made in many ways other than that which has just been described. Furthermore, the present invention can be applied to any present or future semiconductor technology so long as it involves detecting a logic cue within a memory by implementing a differential amplifier.