Redundant storage device having address determined by parity of lower address bits.

0001The invention relates to checking memories, and in particular to the checking of addresses used to access memories.

0002Errors in the address used to access a memory may result in incorrect data being retrieved from the memory. Checking logic may not detect errors in the memory itself or in the addressing lines of the memory. U.S. Patent No 4,271,521 detects errors in addressing as well as in the transfer and storage of data. Both odd and even parity are alternately assigned to data words as a function of the memory address where a particular data word is stored. A parity check is performed on both the data and address. While errors are detected, no means of correcting the errors are provided.

0003In U.S. Patent No. 4,404,647, two arrays contain identical data. When data is read, it is read from one of the arrays. If a data parity error is detected on readout, the other array is accessed. In this patent, no address line checking is provided. Data retrieved from a wrong address will appear to be correct.

0004It is an object of the invention to provide checking of addresses used to access a memory. It is a further object of the invention to provide for successfully retrying the memory in the event an error is detected. It is still a further object of the invention to provide parity on both data and address in conjunction with providing redundant copies of the data. It is yet a further object of the invention to provide a memory tolerant of having one particular address line or data access line impaired. A memory is arranged for storing a first copy and a second copy of words. The different copies of words are stored with a different value of parity as a function of their addresses. A sequencer presents addresses to the memory corresponding to desired words having a first type of parity. A parity checker checks the parity of the word and provides an error signal if the parity is incorrect. On detection of the error signal, an address modifier modifies the address as a function of the error signal such that the second copy of a word having a second type of parity is addressed. The parity checker then checks the word using the second type of parity for comparison.

0005The parity checker monitors the error signal to determine which type of parity a given word will have. There is no requirement that the parity checker receive the address for the data in order to determine that a word at the wrong address was retrieved. Thus, much circuitry which would be required to provide the address to the parity checker has been eliminated, resulting in a simplified design.

0006A single selected address bit is used to differentiate the copies of words. One feature of the sequencer is that it generates the second type of parity from the address of the desired word. That parity is used as the differentiating address bit to select the location where a word is stored with the first type of parity during the first access attempt. The differentiating address bit is exclusive ORed with the error signal, such that a second access using the exclusive OR result as the differentiating address bit, following a first access failure, produces the copy of the word having the second type of parity.

0007A benefit of the present invention is that memory array and parity errors are correctable. Data and address line errors are detectable. In fact, some data and address line errors are correctable. If either the data line providing the parity stored with the data, or the address line carrying the differentiating address bit have failed, one of the access attempts will still be successful. <ul id="ul0001" list-style="none"><li>Fig. 1 is a block circuit diagram of a memory arrangement in accordance with the present invention.</li><li>Fig. 2 is a table of addresses and data stored in one particular embodiment of the circuit of Fig. 1.</li><li>Fig. 3 is a logic table of the operation of the memory arrangement in accordance with Fig.s 1 and 2 demonstrating the various modes of operation of the circuit.</li></ul>

0008A memory checking system is indicated in block diagram form generally at 10 in Fig. 1. A memory array 12, such as a non-volatile random access memory (NVRAM) or read only memory (ROM) stores data in the form of words in at least two locations. As shown, the pairs of words are stored in locations differing by a high order address bit, thus logically and visually dividing the array 12 in two halves. i words are stored in the array 12. Each word is stored with a parity bit P0 through Pi, twice, with a different type parity each time. The bit used to differentiate the location of the pairs of words could easily be any bit in the address. The high order bit was chosen for simplicity.

0009One version of the words is stored with odd parity, while the other version of each word is stored with even parity. The type of parity a word is stored with at each location is a function of the address of that location. The half of the array from which a word is fetched is determined by a sequencer 13 which contains an address queue 14. The address contains the low order bits, defining where, within both halves of the array, a word is located.

0010Odd parity is generated on the low order bits of address of a word to be accessed by an odd parity generate block 15. Bit Pa0 in block 15 is the odd parity of the address bits generated by the sequencer 13. The odd parity of the address differentiates the half of array 12 where the word is located and also signifies where the word is stored with odd type parity. Thus, words stored with odd parity are spread to both halves of the array, with the other copy of the words stored with even parity spread to both halves of the array, with an address differing by only one bit. One use for array 12 involves the storage of vital product data (VPD) for I/O devices. VPD is stored in array 12 at the time of manufacture. The storage pattern is determined according to the scheme previously described. The data stored may contain items such as Part No., Serial No., date of manufacture, etc. NVRAM is written by external devices in a common manner.

0011When a word in array 12 is to be accessed, as caused by a clock pulse on a line 16, the sequencer 13 presents an address on address lines 18 to array 12. The odd parity of the address is provided unchanged through an exclusive OR block 20, to the array 12. The word and parity is then provided in a standard manner from the array 12 to a parity checker 22, via a data bus 23. Parity checker 22 generates parity by performing an exclusive OR of the word and its parity. An even number of logical "1's" in the word and parity will produce a "1" by the parity checker 22, while an odd number of "1's" produce a "0".

0012Since, during the first access attempt, words having odd parity are accessed, the exclusive OR by parity checker 22 provides no error signal on its output line 24 when the data and parity is correct, and when the data is retrieved from the correct location. Both the conditions that the data be correct, and that the data be retrieved from the correct location are covered by the parity bit. Since the location of the word to be fetched is determined consistent with the expectation of the word having odd parity, there is no requirement to compare the address used to fetch the word with the parity bit. The parity of the address was already checked by using it to identify the half of the array containing the word stored with even type parity.

0013If parity checker 22 generates a logical "1" on its output line 24, the "1" is clocked into an error latch 26 by a second clock 28 which occurs before first clock 16 clocks in a new address in sequencer 14. The error latch 26 provides the error signal on a line 30 to the sequencer 14, exclusive OR block 20 and parity checker 22. During normal, error free operation, the error signal line 30 has no effect on these three elements, it being at a logical "0". When an error is indicated by the parity checker on the first try in accessing a word, the error signal line 30 changes to "1". This triggers a retry by sequencer 14 on the next Clock 1 pulse. The address is again supplied to array 12, but this time, exclusive OR block 20 combines the parity of the address with the error signal, resulting in an address designating the half of the array having the word stored with even type parity.

0014The word stored with even type parity is then supplied to parity checker 22 which performs an exclusive OR of the word and its parity, and the error signal from line 30, which is a "1". If all is well, the parity checker provides a logic "0" on line 24 to the error latch 26, signaling that correct data and address was achieved on the second try. The error signal on line 30 is then changed back to "0" on the next pulse of Clock 2, and normal operation continues. If parity checker 22 provides a "1" on the retry, the sequencer signals a machine check, and operation stops.

0015In Fig. 2, a simple 8 word array is shown with addresses (Ap,A0,A1), data (Dp,D1,D2, parity Dp) and parity type (even or odd) indicated. There are four words stored twice, "101", "001", "110", and "111". At address location "000", the word "101" is stored with even type parity, the parity bit being "0". Word "101" is also stored at address location "100" with the odd type parity bit being "1". The word stored at address location "001" is stored with odd parity, and with even parity at the corresponding "101" address. Address locations "010" and "110" contain a word having odd and even parity respectively. Address location "011" and "111" contain words having even and odd parity respectively. The concept is easily expandable to larger arrays.

0016Several access attempts are represented in Fig. 3. The first address in the queue 14 of the sequencer 13 is "00". The parity checker of the sequencer 13 generates an odd parity, resulting in an address of "100", which is presented to the array 12. The data expected is the same as the data seen, "1101", where the "1" on the left is the parity bit included with the data. The data, including the parity bit, has an odd number of "1's", resulting in a "0" generated by the parity checker 22, and hence, a successful access.

0017The remaining access attempts in Fig. 3 deal with problems which may potentially be encountered with memories and their associated address and data lines. In attempt number 2, the high order address line from sequencer 13 is stuck at "0". When address "11" is presented by sequencer 13, it creates address "111". However, array 12 first sees address "011". The parity checker 22 is expecting data "0111", which has an odd number of "1's", but instead sees "1111" and generates a "1" on line 24, indicating an error. Even though there was no direct check on the address lines by parity checker 22, an error was detected.

0018Access attempt number 2 continues, this time with the error signal line 30 active, at logical "1". Exclusive OR block 20 changes the address created by sequencer 13 to "011" by exclusive ORing the parity address bit of "1" with the error signal of "1", resulting in "011" as the created and seen addresses. Thus data having an even parity type is expected, and seen at parity checker 22. The word and parity "1111" is combined at the parity checker 22 with the error signal on line 30 of "1" resulting in an odd number of "0's". Thus, parity checker 22 provides a "0" on line 24, indicating successful access. The address parity line was stuck at "0", yet a successful access occurred. Similar success would be encountered if the address parity line was stuck at "1" on the first try. Exclusive OR block 20 could take many forms, such as an inverter which is activated only when the error signal of "1" is present. Its main function is to switch the high order address bit, or discriminating bit to access the word with opposite parity on second attempts.

0019In access attempt number 3, address line 0 is stuck at "0". When an address of "11" is given, address "111" is created, but address "101" is presented to the array 12 resulting in data "1001" and a retry. On retry, the address line error continues, and the data seen is "0001". When that data is combined with the error signal, an even number of "1's" is seen at parity checker 22, and the sequencer is informed that an error condition exists.

0020The data parity bit line in the fourth access attempt is stuck at "1". The first try results in a retry because the parity bit should have been a "0" instead of the "1" that was seen at the parity checker 22. In the second attempt, success was achieved because even though the data parity line was stuck at "1", a "1" was expected by the parity checker 22. If the data parity bit line had been stuck at "0", success would have been achieved on the first try.

0021When an address line other than the address parity line is stuck, an error condition exists. Similarly, as indicated in access attempt number 5, a stuck data line will result in an error condition.

0022In access attempt number 6, an array bit error in data position "0" in the address "100" is encountered. The data expected is "1101" and the data seen, "1001" causes a retry. The retry addresses location "000", and the correct data "0101" resulting in success. All single array bit errors are detectable and correctable.

0023While the invention has been described with respect to an 8 word array, it can be seen that a much larger array would benefit from use of the invention. The invention finds particular use in cheaply storing device identification data in a low cost read only storage. Low cost memories inherently exhibit more errors than higher cost memories. By using the above described invention, the reliability of low cost memories is made very high. The simple circuitry used to access and check the data without processor intervention contribute to a low cost in implementing the invention.