Memory Schemes

This invention relates to improvements in memory schemes particularly to Single in Line Memory Modules (SIMMS) which is an industry standard module.

A SIMM may be used in a large number of applications and is typically produced having a number of Dynamic Read Access Memory (DRAM) modules mounted on a Printed Circuit Board (PCB).

In the past a SIMM (10) has been produced which includes both DRAM memories (12) and a parity memory (14) as shown in FIG. 1. The solution has been used in the past in applications which require a parity bit which could not be provided by the DRAM cells. The parity bit is an extra hit over and above the other bits provided by the DRAM cells and are often needed in applications such as large memories in work stations.

Use of this type of SIMM allows detection of single errors by an external memory controller. This scheme is becoming unpopular and the trend is towards DRAM only SIMMS, due in part at least to the improvements which have been made in the quality of DRAMS.

Accordingly parity memories which have been made are now going to waste. In addition, it is not uncommon for a batch of DRAMS to contain a percentage of devices which are not fully operational and include one or more deflects. The nature and extent of the defects are typically identified, when the device undergoes testing. This results in a large number of devices having known defects being disgarded.

It is proposed by this invention that the functionality provided by a DRAM in a SIMM can be provided by other types of memories or in fact DRAMS which are damaged or working at below optimum operating conditions, for example a below optimum operating speed.

According to one aspect of the present invention, there is provided a memory module having a required predetermined functionality comprising a plurality of defective, substandard and/or un-conventional memory devices arranged to produce said predetermined functionality.

This has the advantages that a SIMM can be produced with any combination of available memories which when combined form the required SIMM functionality.

Preferably the memory module includes control logic which is able to control the memory devices to have the predetermined functionality.

Preferably the required predetermined functionality is based on a memory module having a predetermined number of standard DRAM devices.

Advantageously the plurality of memory devices include parity memories; defective DRAM memories and DRAM memories having below optimum operating parameters.

Preferably the memory module further comprising a printed circuit board (PCB) on to which the memory devices are mounted and in one embodiment of the invention the memory devices are mounted on both sides of the PCB.

According to a second aspect of the invention there is provided a memory cell having a standard operating functionality comprising a plurality of defective, substandard and/or un-conventional memory devices arrange such that the standard operating functionality is obtained.

Reference will now be made, by way of example, to the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a prior art SIMM;

FIG. 2 is a schematic diagram of the top surface of a SIMM in accordance with an embodiment of the invention;

FIG. 3 is a schematic diagram of a conventional DRAM for use in a SIMM;

FIG. 4 is a schematic diagram of the present invention of a memory cell having the same functionality as the memory of FIG. 3;

FIG. 5 is a schematic diagram of a SIMM according to an embodiment of the invention; and

FIG. 6 is a schematic diagram of a second SIMM according to another embodiment of the invention.

Referring to FIG. 2 a SIMM is illustrated which includes a plurality of memory devices (18) mounted on a Printed Circuit Board (PCB) (16). The top surface (20) of the PCB includes eight memory devices of the type or types described below. There may be an equivalent number of memory devices on the underside of the PCB (not shown) in certain applications which will be described in greater detail below.

Referring to FIG. 3 a DRAM (30) is illustrated with only some of its inputs/outputs (I/O's) showing. In particular it can be seen that the DRAM produces four DQ outputs (32). The DRAM is organised in words of four bits each, each bit is controlled by a dedicated Column Address Strobe (CAS).

Built in this chip is the capability to utilise partial working units, more specifically chips that do not exhibit failures on any combination of two I/O's at least; these two I/O's at laser repair level, can be always mapped on chip pins DQ0 and DQ1 each I/O is controller by a dedicated CAS line; these chips are known as Dual-CAS and their capacity is half of the original chip.

Using the configuration in FIG. 4 it is possible to implement a functionality equivalent to the chip of FIG. 3. Two parity or Dual-CAS chips (40, 42) are used. Each chip (40, 42) is controlled by Address Strobes CAS0, CAS1 generated by a conventional CAS signal and each produces two outputs DQ0, DQ1 and DQ2, DQ3 respectively.

The PCB is formed in a specific manner such that it connects all the CAS lines together and connects the two only functional DQ's to the proper SIMM DQ in order to emulate a conventional SIMM.

Moreover a conventional SIMM with parity check is usually required to have an access time of <70 nS, therefore QUAD-CAS chips that do not operate within the required speed limitation cannot normally be used in an SIMM.

It is possible to utilise Quad-CAS chips on the same PCB as well, the four CAS lines will be connected together, but no specific care will be used for the DQ lines, that are compatible with standard DRAM's.

The total capacity that can be implemented, taking into account the mechanical limitation of the SIMM, is 1M words of thirty two bits each; using Dual-CAS chips sixteen units (50) will be mounted on both sides of the PCB as is shown in FIG. 5. Each memory unit will produce two outputs and be supplied with the necessary control signals as are shown in the figure. The Dual-CAS chips may be replaced by other non-standard memory cells having equivalent functionality for example a defective standard DRAM having only two of the possible four outputs operational. Using Quad-CAS only eight chips (60) will be used, populating only one side of the SIMM as show in FIG. 6. Again these chips can be replaced with others having equivalent functionality e.g. a defective eight bit DRAM which only has four valid outputs.

In the case of chips which do not operate within the required speed limitation it is possible to use two such chips and access only two of the outputs of each chip. This enables an improvement in the access time of the two chips and replaces the functionality of a conventional DRAM.

It will be appreciated that for different combinations of memory cell different wiring and control systems may be required for dealing with different problems and memory requirements.