Field of the Invention
This invention relates to circuitry for ascertaining the state of
memory cells and, more particularly, to a transistor circuit which draws very little
Background of the Invention
The need for a fast-acting circuit for reading data from and writing
data into the memory cells, in particular, those that buffer packet data is described
briefly in US patent 5,309,395 issued May 3, 1994 entitled "Synchronous Static
Random Access Memory". The '395 patent solved the speed problem by providing circuitry
for reading from and writing into a memory unit in a single memory cycle. The circuitry
included a precharge circuit, a pair of cross-coupled sense amplifiers, a latch
generator, a read latch and a write circuit. As shown in Fig. 5 of the '395 patent,
during a first half-cycle, (t0 - t1), of a clock defining
the memory cycle, a precharge circuit charged a pair of 0-bit and 1-bit lines
threading through the memory array at the same time the sense amplifiers were clamped
to produce a balanced output. At the end of the first half-cycle the precharging
of the bit lines terminated. During the second half-cycle of the clock, a word
was selected in the memory and a path was completed to allow the sense amplifiers
to draw current from the bit lines. After a self-timed delay, (t1 -
t2) provided by analog circuit elements, the clamping of the sense amplifiers
was removed allowing the amplifiers to follow the voltage changes on the bit lines
as determined by the stored binary information state of the selected word. During
this interval, cross-coupling between the amplifiers provides positive feedback
which rapidly amplifies the small voltage difference on the bit lines into complimentary
logic states on the outputs of the sense amplifiers. During the final interval,
(t3 - t4), the sense amplifiers generate a signal to latch
the data read out and to allow new data to be written into the memory cells. It
was the provision of a common latch signal to the read latch circuit and to the
write circuit that enabled the circuit of the '395 patent to perform both a read
and a write operation on a memory cell within a single half-cycle of the memory
clock. A stated advantage of the '395 patent circuitry was that its regeneratively
cross-coupled amplifiers only slightly discharged the bit lines during a read operation
and thereafter drew only negligible current from the bit lines once the contents
of the memory cells had been read. Leaving the bit lines close to the supply potential
facilitated the rapid precharging of the bit lines after the sensing operation
While the circuitry of the '395 patent functioned satisfactorily
in many applications, and had low power drain once the memory cell contents was
read, its cross-coupled amplifiers did draw a significant amount of current from
the bit lines during the precharging and sensing portions of the memory cycle in
order to attain fast operation. It would be advantageous to have a sense amplifier
that drew less current from the bit lines and which, accordingly, would offer lower
power consumption than that of the '395 patent. It would also be advantageous
to reduce the complexity of the overall circuitry by eliminating the need for a
separate latch circuit.
Summary of the Invention
In accordance with the principles of the present invention, in one
illustrative embodiment thereof, the speed problem is solved by employing a clock
that defines four phases of circuit operation instead of relying on self-clocking
circuit elements, while the power conservation problem is solved by a sense amplifier
arrangement which minimizes the period of time during which the read current is
taken from the bit lines by the sense amplifier.
In particular, the time is minimized during which cross-coupled sense
amplifiers are permitted to draw current from the bit lines when the memory cells
are being read during the memory cycle. In addition, the duration of the current
path from the bit lines through the cross-coupled amplifiers to ground (which lasted
throughout the "SELECT" waveform of the '395 patent) is restricted so that it
is only present during a short portion of the SELECT interval which, in accordance
with the present invention is termed the evaluate interval ("EVAL") and not during
the precharge interval. Further economies are realized by circuit changes which
allow the sense amplifiers to perform the function of the separate latch generator
required in the '395 patent.
Further in accordance with our invention, the memory cycle is divided
into four distinct phases ("PRECHARGE", "SENSE", "EVAL", and "HOLD"), instead of
the two phases ("clock" and "select") followed by indeterminate length self-timed
intervals as provided in the '395 patent.
BRIEF DESCRIPTION OF THE DRAWING
The foregoing and other objects and features of the present invention
may become more apparent by referring now to the drawing in which:
- Fig. 1 is a schematic diagram of the sense amplifier circuitry of the invention;
- Fig. 2 shows the waveforms controlling the operation of the circuit of Fig.
- Fig. 3 shows the circuitry of the prior art '395 patent, redrawn for purposes
of comparison with the circuitry of Fig. 1.
The circuitry of Fig. 1 will be described with respect to the waveforms
shown in Fig. 2. In Fig. 2 the convention is employed which shows the waveforms
high when in their active state. In Fig. 1 the convention is employed which uses
an overhead bar with the waveform designation to indicate that the waveforms is,
in fact, active low. Referring to the top of Fig. 1, bit lines BIT and BIT lead
to a particular memory cell (not shown) of the same type of memory array (not shown)
discussed in the '395 patent.
The circuitry of Fig. 1 operates under control of the four-phase
CLOCK waveform shown in Fig. 2(a) which controls the start and end of each of the
waveforms of Figs. 2(b) through 2(g). The circuitry of Fig. 1 is called into operation
to access the memory array by the SELECT waveform, shown in Fig. 2(f), which remains
active throughout the following three phases labelled PRECHARGE, SENSE, and EVAL.
The SELECT waveform activates transistors T1 and T2 which remain conducting through
the PRECHARGE, SENSE and EVAL phases. All of the transistors in Fig. 1 are P-channel
enhancement transistors, except transistors T5, T6 and T7 which ar N-channel enhancement
The PRECHARGE PHASE
During the PRECHARGE phase identified in Fig. 2, waveforms PRE, Fig.
2(b) and BAL, Fig. 2(c), are high (i.e., active low in Fig. 1), while waveforms
EN, Fig. 2(e), and HOLD, Fig. 2(g), are low (i.e., inactive high in Fig. 1). Waveform
PRE in Fig. 2(b) activates transistors T12 and T13 which initialize the bit lines
to the potential of the supply Vdd . Transistors T3 and T5 form a non-inverting
amplifier for responding to the information storage state of the bit line BIT while
transistors T4 and T6 form an inverter amplifier for responding to the information
storage state of the bit line BIT BAR. Transistor pair T3, T5 will hereinafter
sometimes be referred to as one of the pair of sense amplifiers while transistor
pair T4, T6 will be referred to as the other of the pair of sense amplifiers. During
this PRECHARGE phase, the source terminals of the transistors T3 and T4 of both
of the sense amplifiers are connected together by balancing circuit transistor
T11 thereby ensuring negligible voltage difference between these terminals at
the end of the PRECHARGE phase.
Also during the PRECHARGE phase, the BAL waveform shown in Fig. 2(c)
activates balance circuit transistor T8 to clamp together the gate terminals of
transistors T3, T4, T5 and T6. (Transistor T8 will remain conducting during the
subsequent SENSE phase, as well.) At the same time that the BAL waveform is active,
the EN waveform shown in Fig. 2(e) is inactive, maintaining the gate of transistor
T7 at a potential to keep this transistor in a non-conducting condition. Keeping
transistor T7 non-conducting isolates the sense amplifiers from ground so that
they cannot provide a current path to ground for the bit lines. So long as transistor
T7 remains non-conducting, the source terminals of the transistors in the sense
amplifiers track the potential on the bit lines but the transistors dissipate no
power. During PRECHARGE, transistors T14 and T15, which are connected to the bit
lines, are both off. These transistors will come into operation when the potential
of one of the bit lines begins to fall after the termination of the PRECHARGE
The SENSE Phase
During the SENSE phase, the BAL waveform remains active. The WS waveform,
Fig. 2(d), becomes active, selecting a word line linking a column of memory cells
in the array (not shown, but similar to the word line described in the '395 patent).
The PRE waveform of Fig. 2(b) becomes inactive, turning off transistors T12 and
T13 and ending the PRECHARGE interval. The termination of the PRECHARGE interval
allows the bit lines to assume the potential dictated by the information stored
in the memory cell to which they are connected. The potential of one of the bit
lines will begin to fall. During this time one of the cross-coupled sense amplifiers
has its source tracking the falling bit line but, unlike the '395 patent, there
is no path to ground from the bit lines through the sense amplifiers because transistor
T7 is still in a non-conducting state. The gates of the transistors of the sense
amplifiers are still connected together by transistor T8 under the control of the
BAL waveform. When the potential of one of the bit lines begins to fall it, for
example the bit line BIT, transistor T14 connected to that bit line causes transistor
T15 whose drain is connected to the opposite bit line to turn on, clamping that
bit line high and preventing both bit lines from falling.
The EVAL Phase
During this phase the WS waveform remains active. The BAL waveform
becomes inactive while the EN waveform becomes active. The inactive state of the
BAL waveform renders transistor T8 non-conducting thereby ending the balancing
of the gates of the transistors of the sense amplifiers. At the same time the active
phase of the EN waveform causes transistor T7 to complete a current path to ground
from the bit lines through the sense amplifiers. The sense amplifiers quickly assume
the state corresponding to the bit line that has fallen in potential somewhat.
During this phase the bit line continues to fall as driven by the memory cell.
This is the only phase during which the sense amplifiers draw current from any
of the bit lines. The current stops when a stable state is reached.
The LATCH Phase
During this fourth phase, the HOLD waveform becomes active, turning
on transistors T9 and T10 and connecting the sense amplifier terminals B and BN
to the potential source Vdd . This latches the data in the sense amplifiers
with no power dissipation. During this phase the SELECT waveform becomes inactive
thereby turning off transistors T1 and T2 and disconnecting the memory bit lines
from the sense amplifiers. The WS waveform goes to its inactive phase as well to
save power by stopping the memory bit line from being pulled any lower. This fourth
phase ends when the PRECHARGE signal becomes active to start the next cycle (or
to idle the memory). The PRECHARGE signal can remain high until the next read
cycle begins when SELECT rises and HOLD falls.
The WRITE phase, during which information may be entered into the
memory cell by circuitry (not shown herein, but described in the '395 patent) which
impresses state-defining differential voltages on the bit lines, may occur during
or following the LATCH phase since the sense amplifiers are then disconnected from
the bit lines.
The Operation of the '395 Patent Compared
In the '395 patent, the SELECT signal causes transistor 58 to conduct,
thereby completing a path to ground for the bit lines through the sense amplifiers
and allowing them to draw current from the bit lines throughout the remainder
of the memory access cycle. The intervals t1 - t2 and t2
- t3 are self-timed, i.e., determined by analog circuit parameters.
The duration of the latter interval is determined by the voltage difference between
the bit lines. This period could be quite short, unless there was little voltage
difference between the bit lines and there would be little voltage difference
if the self-timed interval, t1 - t2, were short. Accordingly,
the shortening of one of the intervals causes a lengthening of the other. In contrast,
the circuit of our invention allows a full half-cycle for the SENSE phase to take
place, typically resulting in a significant voltage differences. Since our circuit
allows a significant voltage difference to accrue, the evaluation of the state
of the bit lines by the sense amplifiers during the EVAL phase occurs quite rapidly.
However, since the EVAL phase is not self-timed, the entire half-cycle of the EVAL
phase can be devoted to allowing the sense amplifiers to settle. As noted above,
the sense amplifiers of the '395 patent are allowed to draw current throughout
the t1 - t4 interval while in our circuit the sense amplifiers
are permitted to draw current from the bit lines only during the EVAL phase and
not during the preceding SENSE phase nor during the succeeding LATCH phase.
What has been described is deemed to be illustrative of the principles
of my invention.
Numerous modifications may be made thereto, such altering the start of the waveforms
as indicated by the dotted lines in Fig. 2.
Other modifications may be made by those skilled in the art without, however, departing
from the spirit and scope of my invention.