Circuit arrangement for verifying data stored in a random access memory

Abstract: The circuit arrangement comprises, for each bit location in a column of the RAM, an input shift register, a multiplexer and a comparator. The input data bit is stored in the shift register, and the multiplexer is arranged during a write cycle, to write the data bit into the bit position. During a verification cycle, the multiplexer is arranged to write the inverse data bit into the same position, and the comparator compares the output bit position of the RAM with the inverse data bit The result is stored in the shift register, which can be down-loaded for analysis.

Description: The present invention relates to a circuit arrangement for verifying data stored in a random access memory (RAM). In standard RAM designs which require verification that the data has been stored correctly, provision is made for reading out the data once the RAM has been written and a comparison is made with the source data. However there are some applications where the data in the RAM may be secret (a military application) or the RAM may be a write only design. In such applications it is important to test whether the RAM is fully functional or whether the data written to the memory has been corrupted during the write process. In such applications an easy comparison check is impossible. To overcome this problem, a write verify circuit has been invented. An object of the present invention is to provide a circuit arrangement which examines each bit of data written into a RAM during a write cycle and which is capable of outputting signals indicative of the examination for further analysis. According to the present invention, there is provided a circuit arrangement for verifying data stored in a random access memory comprising, for each bit location of a column of the random access memory: an input shift register stage arranged to store an input data bit; multiplexer means arranged, during a write phase of a write cycle, to write the data bit into the bit position in the column of the random access memory and, during a verification phase of said write cycle, to write an inverse data bit into the data bit position in the column of the random access memory using a weak inverter circuit which will not overwrite a fully operational random access memory bit location; and a comparator arranged to compare, after completion of the verification phase, the output of the bit position in the random access memory with the inverse input data bit; the result of the comparison being stored in the input shift register stage. An embodiment of the present invention will now be described with reference to the accompanying drawings wherein; Figure 1 shows an example application of the circuit arrangement according to the invention; and, Figure 2 shows the circuit arrangement of the invention. Referring to Figure 1, a RAM array 1 is shown, columns of which are addressed by a column address selector 2 by way of address lines 2a. Each bit of a column is addressed by a write-write verify circuit 3 by way of line 3a, which interfaces the RAM array 1 with an n-bit shift register 4 which receives a d-bit input word and a 10 bit address word. A whole column of the RAM array is written to at once from the shift register 4 into which the data is loaded a word at a time. The shift register 4, write-write verify circuit 3 and the column address selector 2 are controlled by an address controller 5, by way of control lines 5a, 5b and 5c respectively. Each bit position of the shift register 4 is connected to the adjacent bit position by lines 4c. The final bit position providing an output line 4c to a write verify output pin. Referring to Figure 2 the circuit arrangement of the present invention will now be described. It should be understood that the circuit shown in Figure 2 represents the circuit required for one bit position in a column of the RAM array, and in reality a plurality of such circuits would be provided equal in number to the number of bit positions in a column. The input data bit is stored in a master slave flip-flop 6 which forms part of the shift register 4 (Fig. 1), and is controlled by way of line 5a. The input data bit is presented to the flip-flop 6 by an input multiplexer 7 which also handles an input from the previous bit position and an input from a sense amplifier, to be described later. Writing to the RAM bit position 8 occurs in two cycles, a write cycle, followed by a verification cycle. The flip-flop 6 has a 2:1 multiplexer 9 connected to its output line 4a. The multiplexer comprises inverters 10, 10 1 , and transmission gates 11, 11 1 which perform an Exclusive-OR Function. Inverter 10 1 and transmission gate 11 1 acts as a very weak inverter circuit, i.e. having high output impedance. The multiplexer 9, therefore switches the data or the inverse of the data (data bar) onto the line 3a. During the write phase data is written to the RAM bit by switching the address line 2a of transistor 13, and multiplexer 9. Transistor 13, forms part of the RAM. Each RAM cell also includes a weak inverter 14 and a multiplexer transistor 15. Once this is complete the multiplexer 9 switches over to the data bar output and an attempt to write the data bar signal by way of the very weak inverter is made. The weak inverter 10 1 is optimised against the feedback element of the RAM bit so that it will NOT overwrite a fully operational RAM cell. A period of time is allowed for the inverter 10 1 to change the state of a faulty bit and then the output of the RAM is compared with the data bar signal using a sense amplifier 12 as a comparator. This result is loaded back into the shift register bit. The shift register 4 now contains a zero for any location that has failed to write correctly. Thus if any location stuck at one or zero, the faults can be found by writing all ones and then all zeros and also any disconnected RAM bit can be detected at all times. Since the RAM can only be tested during a write cycle, the data held within is secure. As the result from the RAM write can be output serially, over line 4c, it can be very easy for the user to locate the faulty RAM cell which is useful for yield analysis and also for fault tolerant type applications. The circuit arrangement as described is intended for use with single sided fine transistor RAM cells using single address and data lines. However, it will readily be appreciated by those skilled in the art that the circuit could be modified to accommodate six transistor RAM cells.

Claims: A circuit arrangement for verifying data stored in a random access memory comprising, for each bit location of a column of the random access memory: an input shift register stage arranged to store an input data bit; multiplexer means arranged, during a write phase of a write cycle, to write the data bit into the bit position in the column of the random access memory and, during a verification phase of said write cycle, to write an inverse data bit into the data bit position in the column of the random access memory using a weak inverter circuit which will not overwrite a fully operational random access memory bit location; and a comparator arranged to compare, after completion of the verification phase, the output of the bit position in the random access memory with the inverse input data bit; the result of the comparison being stored in the input shift register stage. A circuit arrangement as claimed in Claim 1, wherein the multiplexer means comprises first and second transmission gates, wherein the first gate is arranged to pass the data bit to the bit location of the random access memory during a write cycle, and the second gate in conjunction with an inverter connected to an input of the gate is arranged to pass the inverse data bit to the bit location of the random access memory during a verification cycle. A circuit arrangement as claimed in Claim 2, wherein the inverter and second gate form said weak inverter circuit. A circuit arrangement as claimed in Claim 3, wherein an input multiplexer is connected to an input of the input shift register, and is arranged to receive the input data bit, the result from the comparator and an input from a previous bit location. A circuit arrangement as claimed in Claim 3 or 4, wherein the comparator is a sense amplifier.

Inventors: CLIFF RICHARD GUY, HASTIE NEIL STUART, CLIFF, RICHARD GUY, HASTIE, NEIL STUART
Applicants: PLESSEY SEMICONDUCTORS LTD, PLESSEY SEMICONDUCTORS LIMITED
Application year: 1990
Main classification: G06F11
IPC codes: G11C29, G06F11