Accelerated carry generation

Details Accelerated carry generation Accelerated carry generation

2001-01-31

2002-04-02

Wambach, Margaret R. (Department: 2816)

Electrical pulse counters, pulse dividers, or shift registers: c

Applications

Including memory

C377S051000, C365S236000

Reexamination Certificate

active

06366634

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates in general to memory-devices, and in particular, to an interleaved memory readable in a synchronous mode or in a random access asynchronous mode with fast access times. The interleaved memory is readable in the synchronous mode by successive locations with a sequential type of access, commonly referred to as a burst mode.
BACKGROUND OF THE INVENTION
In a standard memory a read cycle is defined from a request of data effected by the input of a new address, to the final output of the bits stored in the addressed location (byte, word, etc.). Internally, the reading process evolves through several steps. These steps start from the acquisition of the new address, to its decoding, to the generation of synchronizing pulses of the sensing circuits, and to the output of the read data.
The ATD (Address Transition Detection) signal recognizes a change of the address input by the external circuitry, and therefore, the new request of access and initiates a new read cycle. After enabling the sense amplifiers by the signal SAenable, an equalization of the sensing circuitry takes place. At the end of which, as timed by the signal EQZ, the effective reading of the memory cells takes place. Finally, after a certain interval of time that may vary from device to device, by way of a signal SAlatch, the recording of the read data into the latches in cascade to the sense amplifiers takes place, from where the read word may be transferred to the output buffers.
In memory devices designed for a synchronous read mode with a sequential type (burst) of access, the reading process exploits the fact that the reading takes place by successive locations. That is, the subsequent memory location to be read, and therefore, its address, is predictable from the address of the location being currently read.
A subgroup of these sequential (burst) synchronous read mode memories is represented by the interleaved memories. A burst access interleaved memory is described in U.S. Pat. No. 5,559,990, for example. In this type of memory, the cell array is divided in two semi-arrays or banks, each having its own read circuitry. The read streams of the two banks are thereafter superimposed according to one of the most commonly followed approaches. They are outphased, i.e., out of phase, from each other. While on one of the two banks or semi-array the steps of evaluation and transfer of the data to the output are being performed, on the other bank or semi-array (the next location to be addressed) a new read cycle may start without waiting for the conclusion of the current read cycle that involves the first semi-array.
In interleaved memories, a basic scheme of which is depicted in
FIG. 1
, the array is divided into two independent banks or semi-arrays, EVEN and ODD, respectively, each having its own independent read path. Typically, there are two counters (one for each bank) containing the address of the currently pointed memory location. In case of simultaneous reading processes evolving respectively on the two semi-arrays, the least significant bit of the address (A
0
) supports the multiplexing between the EVEN and the ODD banks. If A
0
=0, the data coming from the EVEN semi-array will be made available at the output. If A
0
=1, the data coming from the ODD semi-array will be made available at the output.
As it is commonly known, the reading of the two semi-arrays is carried out according to one of two different approaches. A first approach is simultaneous readings and multiplexing of the outputs. A second approach involves time readings that are out of phase.
According to the first approach, the readings are simultaneous on the two banks. The data read are stored in respective output registers and made available to the outside world in synchronization with an external clock signal. According to the second approach, the readings on the two semi-arrays have an alternate and interleaved evolution over time.
The first approach, though offering a simpler hardware implementation, limits the minimization of the start times of synchronous read cycles. For a better comprehension, it is necessary to consider the basic steps that are performed when passing from an asynchronous read mode to a synchronous read mode.
With reference to the scheme of
FIG. 2
, and starting the reading from an address X, the latter will be loaded on the EVEN bank counter and on the ODD bank counter, less the least significant bit (A
0
) of the address. The two counters will point to the same location X of the respective bank or semi-array.
If A
0
=0: the first read data is relative to the address X of the bank EVEN and the successive read data is the data X of the bank ODD.
If A
0
=1: the first read data is relative to the address X of the bank ODD and the successively read data is relative to the X+1 address of the bank EVEN.
In the first case, it is sufficient to perform a simultaneous reading of the two banks and multiplex the outputs. In the second instance, it is necessary to increment the counter before starting the reading on the bank EVEN.
Usually, known synchronous memory devices do not make any initial increment and wait for the successive cycle for incrementing both counters, and therefore, read the location X+1 of the banks EVEN and ODD. This makes the times of the first read cycle and of the second sequential read cycle at best equal to the asynchronous read mode time of the memory.
In general, it may be stated that the efficient management of the read processes has a direct influence of the performance of the memory device. Many read-path architectures have been proposed. Known read-path architectures have generally been conceived for responding efficiently to either one or the other of the two modes of operation: asynchronous or synchronous.
If a memory device is designed to be read in an asynchronous mode, it will be generally provided with a rather simple control circuitry for the read data streams. This allows the use of adaptive structures, such as dummy wordlines and dummy sense amplifiers, while leaving the reading circuitry free to evolve as fast as possible in order to achieve the shortest asynchronous access delays.
In contrast, in memory devices designed to function in a burst access mode or in a synchronous read mode, the possibility of making available in output a certain number of words read and stored in advance, permits, after a first asynchronous access, as long as it may be, a series of extremely fast read cycles. In this case though, the control logic must intervene extensively to manage the sense amplifiers which should not be left to evolve freely but be enabled, equalized and read at precise instants established by the control system. Prior European Patent Application Serial No. EP-98830801, filed on Dec. 30, 1998, and Italian Patent Application Serial No. MI99A00248, filed on Nov. 26, 1999, describe burst-mode EPROM devices with the above characteristics. These patent applications are both incorporated herein by reference in their entirety, and are assigned to the assignee of the present invention.
The access mode in a reading phase of operation is set to a specific protocol of use of two of the external protocol signals. These two signals are the address input latches enabling signal ALE and of the read stimulation signal RD.
The counters of the two semi-arrays, or the counter of the first bank and a more simpler register which functions as an address counter of the second bank, are incremented distinctly from one another. This is different from what is commonly done in interleaved memory devices. The readings are thus out of phase on the two banks from the first (asynchronous) read cycle. In this way, the memory device of the invention is perfectly able to switch to a synchronous mode reading phase at any time, which practically cuts in half the access time to such a mode.
The two different reading processes, according to an asynchronous random access mode and according to a synchronous burst access mode remain congruent with

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