Apparatus for Programming a Dynamic EPROM

1. Field of the Invention

This invention relates generally to semiconductor memory circuits and, more particularly, to an insulated gate field effect transistor (IGFET) quasi-static decoder for programming a dynamic electrically programmable read only memory (EPROM).

2. Description of the Prior Art

Some read only memories may be programmed by causing a permanent and irreversible change in the memory mental interconnection pattern by applying electrical pulses thereto. Other ROMs have been programmed by causing a reversible change in the characteristic of a memory device within the ROM. These so-called alterable ROMs are generally deprogrammed by exposing the device to ultraviolet light. Both static and dynamic decoders have been employed to accomplish this reversible programming; however, each suffer from certain disadvantages.

In the case of a static decoder, the decoder circuitry is configured so as to draw current all the time. With the ever increasing demands for larger and larger memory arrays, the power requirements become very high. Furthermore, static decoders perform at relatively low speeds when compared to dynamic decoders.

While dynamic decoders require less power and perform at higher speeds, they are susceptible to leakage at the precharged nodes. Since it may take as much as 50 milliseconds to program a memory cell, periodic program pulsing is required to overcome the leakage problem. Furthermore, those EPROMs which are ultra-violet erasable are generally equiped with a clear lid. This permits optical infiltration which causes the nodes to discharge even faster.

It is an object of the present invention to provide a circuit for programming an EPROM cell with one low level pulse, said circuit consuming less current than a static decoder.

It is a further object of the invention to provide a circuit for programming an EPROM memory cell with low level signals, which circuit also operates at high speed and consumes less power.

Finally, it is an object of the present invention to provide a circuit for programming an EPROM cell which combines the feature of low level single pulse programming, low power consumption and high speed, which circuit can be implemented with MOS insulated gate field effect transistors.

According to a broad aspect of the invention there is provided a circuit for providing a programming signal from a first source of supply voltage to row or column conductors of an electrically programmable read-only-memory, comprising: first means associated with each of said conductors for latching each of said conductors at a voltage below that of said first source; second means associated with each of said conductors for establishing an enabling voltage with a single precharge pulse; first switching means coupled to said enabling voltage and responsive thereto for unlatching a selected one of said conductors; and third means associated with each of said conductors for coupling said first source to said selected one of said conductors.

The above and other objects, features and advantages of the invention will be more clearly understood from the Following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic diagram of the inventive EPROM memory cell program circuit; and

FIG. 2 (A-E) is a timing diagram which demonstrates the operation of the circuit shown in FIG. 1.

Referring to FIG. 1, there is shown a precharge device 2 coupled between a source of supply voltage V.sub.CC and the common drain of a plurality of word-line decode devices only two of which, 4 and 6, are shown. It is to be understood, however, that the number of word-line decode devices would correspond to the number of address bits, Ao-An, necessary to access the total number of rows in the memory. A program device 10 is coupled between node 8 and ground and has a gate electrode which receives a program enable signal (PGM) as will be discussed below.

Device 12 has a current conducting path coupled in series between nodes 8 and 14 and has a gate coupled to V.sub.CC. This device serves to isolate that portion of the circuit discussed thus far from the remaining portion of the circuit. Device 18, a word select device, has a gate coupled to node 14 and a first of its current conducting electrodes coupled to a row select signal (XS). The other current conducting electrode of device 18 is coupled in common with row select line 20 and the control gates of a plurality of memory devices only two of which, 34 and 36, are shown. It will be understood, however, that any desired number of memory bits may be coupled to one particular row line.

A latching device is also coupled to row select line 20. This latching device comprises enhancement devices 26 and 30 and depletion device 28. Device 26 has its current conducting path coupled between row select line 20 and ground, and has a gate electrode coupled to node 32. Device 30 has a current conducting path coupled between node 32 and ground, and has a gate coupled to the row select line 20. Finally, depletion device 28 has a current conducting path coupled between a source of supply V.sub.CC and node 32 and has a gate electrode coupled to its source electrode and to the gate electrode of device 26.

Word line pull-up devices, 22 and 24, are coupled in series between a second source of supply voltage V.sub.pp (typically 25 volts) and row select line 20. Device 22 has its gate coupled to its drain electrode and serves as a voltage limiter. The gate of device 24 is likewise coupled to the row select line 20.

The operation of the inventive programming circuit will now be described. During the precharge period shown in FIG. 2A, device 2 is turned on causing node 8 to achieve a voltage equivalent to one enhancement threshold below the supply voltage V.sub.CC. Since V.sub.CC is applied to the gate of device 12, node 14 is also charged to the voltage at node 8. Also during the precharge period, node 16 is at ground, the row select line 20 is at ground and node 32 is charged to V.sub.CC. Additionally, XS, PGM and all address lines are grounded during the precharge period.

After precharge, the address buffer clocks are activated (FIG. 2B) and the address bits Ao-An are applied to word line select devices 4, 6, etc. Of all of the rows in memory, the address select devices of only the addressed row will have all zero inputs (negative logic). Thus, if row 20 is the desired row, zeros will appear at the gates of select devices 4, 6, etc. Therefore, these devices will not turn on and nodes 8 and 14 will remain charged to its precharge level. However, at least one address select device for each of the other rows in the memory will have a "1" applied to its gate thus discharging nodes 8 and 14 in each of the other rows.

After a sufficient amount of time (t.sub.1) has elapsed in order to perform the decoding function, the row decode clock XS becomes high as is shown in FIG. 2C. The overlap capacitance of device 18 causes node 14 to be bootstrapped to a voltage which is above the supply voltage V.sub.CC (typically 7 or 8 volts). This turns device 18 on pulling word line 20 up to XS (approximately 5 volts). For all other rows, node 14 has been discharged and therefore, device 18 does not turn on.

During the precharge period, line 20 was at ground, node 32 was at 5 volts, device 26 having a gate coupled to node 32 was on and device 30 having a gate coupled to row line 20 was off. When the voltage on row line 20 rises to XS, device 30 turns on discharging node 32 and turning device 26 off. Thus, row line 20 is no longer held down by device 26. As long as XS is high, line 20 is held at 5 volts.

Shortly after the row decode has been accomplished, a similar operation occurs to select a desired column. As can be seen in FIG. 1, each of the devices having control gates coupled row select line 20 has a drain coupled to similar column select circuitry. This column select function is initiated by a column select signal YS shown in FIG. 2D.

When the programming signal (PGM) shown in FIG. 2E goes high, (5 volts) device 10 turns on discharging node 14. At this point, the row select line 20 pulls up to V.sub.pp (25 volts) through the action of the depletion devices 22 and 24. As a result, 25 volts is applied to the control gates of all of the memory devices coupled to that row. Similarly, 25 volts is applied to the gate of the column select device coupling the high program voltage (25 volts minus one enhancement threshold) to the drain of device 34. With 25 volts on the control gate of device 34 and the high voltage on the drain of device 34 as a result of that column being selected, charge is injected onto the floating gate of device 34 thus programming device 34. It should be clear that all, none or some of devices coupled to row line 20 may be simultaneously programmed by enabling the appropriate column select lines.

The foregoing description of a preferred embodiment of the invention is given by way of example only. Changes in form and detail may be one skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.