Low-voltage digital ROM circuit and method

In order to increase the functional capacity of digital integrated circuits (ICs), IC designers necessarily implement progressively smaller geometries for metal-oxide-semiconductor field effect transistors (MOSFETs, or FETs) and other IC devices so that more circuitry per unit area may reside on an IC die. However, generally accompanying the smaller geometries and higher circuit densities are lower transistor breakdown voltages and higher power consumption, thus requiring the use of lower power supply voltages. Unfortunately, the use of a lower power supply voltage typically reduces the available amount of voltage margin, which allows an IC to operate properly in the presence of unavoidable variations in manufacturing processes, supply voltage, and operating temperature. As a result, circuit designs utilized in larger geometry ICs may become less fault tolerant when the same design is translated into an IC utilizing a smaller geometry.

An example of such a design is an IC read-only memory (ROM) circuit. A ROM is commonly employed to great advantage as a functional portion of a larger IC design, such as a control information store, or as a standalone IC. A typical IC ROM circuit arrangement from the prior art is shown in FIG. 1 . In this case, an n-word by m-bit ROM comprises individual ROM bits 110 for each data bit to be stored. Each word of the ROM is enabled for reading by one of n separate READOUT signal lines, which are normally generated by an address decoder (not shown). At most one READOUT signal line is active at any particular time. The outputs of each ROM bit 110 associated with the same data bit of each word are connected to form an OUT signal line. Each OUT signal line is then an input for a signal buffer 120 associated with that bit of the data word to be read. Thus, the outputs of all signal buffers 120 collectively represent DATA, the m-bit data word currently being read.

FIG. 2 shows ROM bit 110 and signal buffer 120 in greater detail. ROM bit 110 is implemented by a single n-channel FET (NFET) N R , the gate of which is driven by one of the READOUT signals, READOUT x . The source of NFET N R is connected to either a ground reference via a ground connection 230 , or to a power supply voltage reference V DD via power supply connection 240 , depending on which of the two logic states, LOW or HIGH, is to be represented by ROM bit 110 . When READOUT x is HIGH, N R turns ON, allowing the voltage at the source of N R to pass to OUT signal line OUT y , less the turn-on voltage across N R . The voltage on OUT y is then passed to the input of signal buffer 120 , which in this case is a standard complementary MOS (CMOS) pair consisting of a p-channel FET (PFET) P B and an NFET N B . The output of signal buffer 120 drives the data signal line DATA y , which indicates the logic state of ROM bit 110 when READOUT x is HIGH. The use of single NFET N R in FIG. 2 ensures that the space required by ROM bit 110 is minimal, thereby allowing several thousand ROM bits 110 to occupy a relatively small space on the IC die. Additionally, ROM bit 110 dissipates virtually no power when READOUT x is in the LOW state, thus maintaining low overall power consumption for the ROM.

Unfortunately, as smaller device geometries are used for the ROM to increase IC functional capacity, the power supply voltage V DD is normally lowered, typically causing voltage margin problems, as depicted in FIG. 3 . When READOUT x becomes active, the voltage level rises from V OFF substantially to V DD . Assuming power supply connection 240 (of FIG. 2) is utilized in ROM bit 110 , the voltage level on OUT y becomes V DD less a significant voltage drop across N R , or V ON , because the voltage on READOUT x at the gate of N R is essentially the same as that at the source, causing N R to drive a weak logic HIGH level. As seen in FIG. 3, V ON may not be much higher than V TH , the threshold voltage at which signal buffer 120 perceives the voltage on OUT y as a logic HIGH instead of a logic LOW. The difference between V ON and V TH is V M , the resulting voltage margin, which becomes smaller as the power supply voltage V DD is lowered with successive IC design generations implementing smaller device geometries. Thus, variations in the IC manufacturing process, along with variations in the operating temperature and power supply voltage of the IC, may cause V ON to drop below V TH , thereby causing improper operation of the ROM.

From the foregoing, a new IC ROM design that minimizes the die area required for each ROM bit and dissipates almost no power when not being read, while allowing for greater voltage margins in the presence of relatively low power supply voltages, is desirable.

Embodiments of the invention, to be discussed in detail below, provide a ROM architecture that comprises a means for selectively driving one of two complementary logic state signal lines substantially to a voltage reference upon one of a plurality of readout signals becoming active so that a particular ROM bit may be read. The logic state of the ROM bit being read is indicated by which of the complementary logic state signal lines is being driven. A pair of inverting means is then cross-coupled with the two logic state signal lines so that the logic level of each logic state signal line is inverted and driven onto the opposing logic state signal line, resulting in both lines being driven to their proper state, even though only one logic state signal line is being driven directly as a result of the active readout signal. Therefore, only one of the logic state signal lines needs to be monitored to determine the proper state of the particular ROM bit being read.

Other embodiments of the invention take the form of a method of storing digital read-only data as a plurality of addressable bits. First, one of a pair of complementary logic state signal lines is selectively driven substantially to a voltage reference when one of a plurality of readout signals is active. The particular complementary logic state signal line that is being driven indicates the logic state of the ROM bit being addressed by the active readout signal. Each of the complementary logic state signal lines is then inverted and driven onto the opposing logic state signal line so that both lines are driven to their proper state, no matter which is being driven directly as a result of the active readout signal, allowing for other circuitry to read the logic state of the ROM bit from either line.

Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

FIG. 1 is a block diagram of a ROM architecture according to the prior art.

FIG. 2 is a schematic diagram showing greater detail of the ROM bit and signal buffer circuits of the ROM architecture of FIG. 1 .

FIG. 3 is a waveform diagram displaying the voltage margin associated with the prior art ROM architecture.

FIG. 4 is a block diagram of a ROM architecture according to an embodiment of the invention.

FIG. 5 is a schematic diagram showing greater detail of the ROM bit and inversion block circuits of the ROM architecture of FIG. 3, using a single NFET for the ROM bit.

FIG. 6 is a schematic diagram showing greater detail of the ROM bit and inversion block circuits of the ROM architecture of FIG. 3, using a single PFET for the ROM bit.

FIG. 7 is a flow chart of a method of storing digital ROM data according to an embodiment of the invention.

The overall ROM architecture for an embodiment of the invention is displayed in FIG. 4 . Instead of a single output for each ROM bit 110 (from FIG. 1 ), each new ROM bit 410 has two complementary logic state signal lines, BIT and nBIT. Similar to the prior art, each column of ROM bits 410 corresponds to a particular bit of a data word to be read from the ROM. Each ROM bit 410 within a particular column is individually addressable by a separate readout signal READOUT. As a result, BIT and nBIT of each ROM bit 410 of a particular column are tied together and used as input for an inversion block 420 , the output of which is the data information DATA to be read from the ROM.

FIG. 5 shows specific embodiments for ROM bit 410 and inversion block 420 in greater detail. In this case, ROM bit 410 consists of a single NFET N R . N R represents one logic state when nBIT connection 530 is made, and represents the opposite logic state when BIT connection 540 is made. Either connection 530 or 540 is made; simultaneous presence of both connections 530 and 540 is not employed. Readout signal READOUT x , connected to the gate of N R , addresses ROM bit x,y when at a logic HIGH state, thereby turning ON N R . Therefore, when READOUT x goes HIGH, either BIT y or nBIT y is driven substantially to a ground reference that is attached to the source of N R , depending on the logic state which N R represents by way of connections 530 and 540 . Hence, either BIT y or nBIT y will be driven substantially to the ground reference, while the other is not driven to any particular voltage.

BIT y and nBIT y from a particular column are used as inputs to an inversion block 420 , which comprises two cross-coupled inverters, first inverter 500 and second inverter 510 . In the embodiment of FIG. 5, inverters 500 and 510 are standard CMOS inverters using a p-channel and n-channel FET in series, with inverter 500 comprising P BIT and N BIT , while inverter 510 utilizes P nBIT and N nBIT . Other inverter circuits not discussed herein may also be utilized. Inverters 500 and 510 allow the logic state signal line that is not driven by N R to assume the proper complementary logic state displayed by the logic state signal line being driven by N R . For example, when nBIT connection 530 is made, nBIT y is driven substantially to ground, while BIT y initially remains in an undriven state. However, second inverter 510 , by virtue of nBIT y being in a LOW logic state, forces a HIGH state onto BIT y . First inverter 500 then takes that HIGH signal to drive nBIT y LOW, which is the same state being driven by N R onto that signal line. Thus, BIT y is HIGH and nBIT y is LOW, representing the logic state of ROM bit x,y. Alternately, if BIT connection 540 is present instead of nBIT connection 530 , BIT y is driven substantially to ground, while nBIT y is initially floating. First inverter 500 then takes the LOW logic state of BIT y as input, and forces a HIGH logic state onto nBIT y . Consequently, second inverter 510 takes this HIGH as input, driving a LOW back onto BIT y , which is compatible with the level being forced onto BIT y by N R . Hence, BIT y is LOW while nBIT y is HIGH, reflecting the opposite logic state for ROM bit x,y from that shown in the previous case.

In this embodiment, an additional read buffer is implemented in the form of a third inverter 520 . In this particular case, the input for third inverter 520 is nBIT y . However, BIT y can just as easily be used as input for third inverter 520 , depending on the needs of the circuit utilizing the data stored in the ROM. Other types of buffers, such as a noninverting buffer, may also be used in lieu of third inverter 520 . Furthermore, third inverter 520 , or any other type of buffering, may not be necessary, depending on the particular ROM application. In this instance, either BIT y or nBIT y can be used as the DATA y output.

A benefit of this embodiment and others is that NFET N R will be turned ON completely, thereby preventing voltage margin problems. When READOUT x is HIGH, the voltage on the drain of N R will not be higher than the READOUT x signal voltage on the gate of N R , whether nBIT y connection 530 or BIT y connection 540 is present. Thus, regardless of which connection is made, either BIT y or nBIT y will be substantially driven to ground, allowing either of first inverter 500 or second inverter 510 to be driven properly with sufficient voltage margin to protect the operation of the circuit against variations in manufacturing processes, operating power supply voltage, and operating temperature. Additionally, the embodiment of FIG. 5 uses a single NFET as in the prior art circuit of FIG. 2, providing a space-efficient solution. The embodiment also does not dissipate significant current when READOUT x is inactive, since N R is shut OFF in that case, in similar fashion to the operation of the prior art circuit.

Another embodiment of the invention involves a ROM bit based on a PFET P R , as shown in FIG. 6 . The source of P R is connected to V DD , and READOUT x is active when placed in the LOW logic state. When HIGH, READOUT x effectively shuts OFF P R . When READOUT x goes LOW, either BIT y or nBIT y is forced substantially to V DD , depending on whether nBIT connection 630 or BIT connection 640 is present. As a result, either BIT y or nBIT y will sustain a voltage close to V DD because the voltage on READOUT x , being close to the ground reference when active, will be lower than either the drain or source voltage of P R , causing P R to be completely ON. Thus, no voltage margin problem will exist when either BIT y or nBIT y drive first inverter 500 or second inverter 510 , respectively. In an analogous fashion to the embodiment of FIG. 5, either BIT y or nBIT y will be driven HIGH, while the opposing signal line will initially remain undriven. When BIT y is HIGH as a result of BIT connection 640 being made, first inverter 500 drives nBIT y LOW, which, in turn, causes second inverter 510 to force BIT y HIGH, in agreement with the level being driven by P R . Conversely, when nBIT y is HIGH due to N BIT connection 630 being present, second inverter 510 forces BIT y LOW, which then causes first inverter 500 to drive nBIT y HIGH, along with P R . With both BIT y and nBIT y being driven to their proper logic states based on nBIT connection 630 and BIT connection 640 , either BIT y or nBIT y can be used as the output signal for ROM bit x,y. Also, third inverter 520 , or a noninverting buffer, may be used to buffer either BIT y or nBIT y from other circuitry accessing the ROM data. Alternately, third inverter 520 may be eliminated if the buffering it provides is not required in a particular application.

In most IC process technologies, p-channel FETs require more surface area to implement than n-channel FETs. Thus, for many applications, embodiments utilizing n-channel FETs for ROM bit 110 will be more desirable.

Another embodiment of the present invention takes the form of a method of storing digital. read-only data. First, one of two complementary logic state signal lines is selectively driven substantially to a voltage reference upon one of a plurality of readout signals being active (step 700 ). Thus, the state of the ROM bit addressed by the active readout signal is exemplified by which of the two complementary logic state signal lines is being driven. Next, the logic state of each of the complementary logic state signal lines is inverted to drive the other complementary signal line to the opposite logic state (steps 710 and 720 ). As a result, both complementary logic state signal lines are driven to the proper logic state, regardless of which is actually being selectively driven as a result of the active readout signal. Either complementary logic state signal line could then be buffered, if necessary, for use by other circuitry (step 730 ). However, depending on the particular application, such buffering may not be necessary or desirable.

From the foregoing, the embodiments of the invention have been shown to provide a ROM circuit and method that are particularly useful in low power supply voltage environments. Other specific circuits and methods embodying the invention are also possible. Therefore, the present invention is not to be limited to the specific forms so described and illustrated; the invention is limited only by the claims.