Abstract:
Circuitry for serial read memory access utilizing a random starting address is disclosed. Fast read access is provided without upsetting the original data pattern stored in the memory core if the sequential read is terminated in midstream. After the last memory address is reached, the access automatically rolls over to the first address. The circuit provides both random and sequential access functions and allows the memory to be used as a shift register of variable length.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to integrated circuits and, in particular, to circuitry which provides fast read access of serial memories utilizing a random starting address. 
     2. Description of the Prior Art 
     U.K. Patent Application GB No. 2 183 374 A is directed to a sequential access memory which provides serial read access, the ability for simultaneous read and write of data, and eliminates the need for complex addressing and refresh circuitry by using the memory as one large shift register. 
     The memory device disclosed in the above-identified U.K. application includes individual FET memory cells arranged in a matrix with a common set of column bit lines driven by bit line transistors to prepare the cells for read or write. The rows of the matrix are selected by a pointer register, a shift register in which a defined bit condition circulates to select the rows sequentially. The data input is offered to all cells in the first column and the cell outputs and inputs are merged so that the whole device acts as a serial shift register. When a cell is selected for read-out, its neighbor is selected for write-in, the selection using the appropriate gate primed from the shift register. As a result of the merging of inputs and outputs, a bit read from one cell goes to the cell in the next adjacent column and one up from the cell in the signal column. Thus, the data progresses through the memory matrix to the output. 
     However, the above-described sequential access memory suffers from a number of major disadvantages. First, it does not provide random access capability. Second, the data pattern in the array is altered if the sequential read is terminated in midstream. Third, the memory cannot be used as a shift register of variable lengths. These disadvantages limit the use of this memory device to specific applications. 
     U.S. Pat. No. 4,422,160 issued to Watanabe on Dec. 20, 1983, discloses a memory device which features serial access in a page-mode type of operation for RAMs and semi-random access capability. Its data pattern is not altered if sequential read is terminated in mid-stream. Fewer pins are required since row and column address information is introduced through the same set of address terminals in synchronism with row address strobe and column address strobe signals, respectively. 
     As stated above, in the Watanabe memory device, row address information and column address information are incorporated through the same set of address terminals in response to a row address strobe signal and a column address strobe signal, respectively. Furthermore, a shift register, the output of which is adapted to select a column of a memory cell matrix, is provided in addition to a column decoder. The shift operation of the shift register is effected each time the column strobe signal is made active under the active state of the row address strobe signal. The significant feature of the Watanabe memory device resides in the fact that the column address decoder and the column address inverter are made their active states when the column address strobe signal is first made active under the active state of the row strobe signal; these states of the column address inverter and column decoder are maintained irrespective of subsequent change of the column address strobe signal by the time the row strobe signal goes inactive. The shift register and an input/output circuit are repeatedly made active in synchronism with changes in the level of the column strobe signals between its active and inactive states under the active state of the row address strobe signal. Thus, in the operation of the Watanabe device, the shift operation in the shift register and activation of the input/output circuit can be repeatedly performed at a high speed and with low power consumption without repeating the active and inactive state of the column address inverter buffer and the column decoder. 
     However, the Watanabe memory device also suffers from several disadvantages. First, it does not provide a fully random access capability because a new row address must be provided after all the columns along a particular row have been read out. Second, the Watanabe device cannot be used as a shift register of variable lengths. Therefore, like the device described in the above-identified UK application, the Watanabe memory device is also limited to use in specific applications. 
     SUMMARY OF THE INVENTION 
     The present invention provides circuitry for serial read memory access with a random starting address. Fast read access is provided without upsetting the original data pattern stored in the memory core if the sequential read is terminated in midstream. After the last memory address is reached, the access automatically rolls over to the first address. Both random and sequential access are provided and the memory can be used as a shift register of variable lengths, making it more generally applicable. 
     An embodiment of circuitry for sequential read access of a serial memory array in accordance with the present invention comprises an address latch which stores an address used to access the memory array to read data from a corresponding data register in the array. The address latch includes a counter which increments the stored address upon receipt of an address increment signal. The data bits from the accessed memory register are serially shifted to an output pad by a data shift register which receives the parallel output of the array. A data stream counter keeps track of the number of bits output on the data out pin, issues the increment address signal to the address latch at a fixed count during the read sequence and zeroes itself after a word of data is output on the data out pin. In this way, the address stored in the address latch is sequentially incremented such that each of the data registers in the array is read out serially. 
     Other features and advantages of the present invention will be understood and appreciated by reviewing the description of the invention provided below which should be considered in conjunction with the accompanying drawings. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram illustrating a serial read access circuit in accordance with the present invention. 
     FIG. 2 is a timing diagram associated with the serial read access circuit shown in FIG. 1. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1 shows the basic elements of an electrically erasable programmable read only memory (EEPROM) device. The device includes an EEPROM array 2 which is divided into 256 storage registers of 16 bits each. N registers in the array can be protected against data modification by programming into a special on-chip memory protect register 3 the address of the first register in the array 2 to be protected. Thereafter, all attempts to alter data in the storage register whose data address is equal to or greater than the address stored in the memory protect register 3 will be ignored. This data protection technique is more fully described in copending and commonly assigned U.S. Pat. Application Serial No. 149,373, titled PROGRAMMABLE MEMORY DATA PROTECTION SCHEME, filed of even data herewith by Boddu et al, which application is hereby incorporated by reference. 
     As shown generally in FIG. 1, and described in greater detail below, data is written into a selected storage register within EEPROM array 2 by serially clocking a WRITE instruction into instruction register 4 and then clocking 16 bits of data to be written into the address specified in the WRITE instruction into data shift register 5. After the 16 bits of data have been clocked into data shift register 5, this data is transferred in parallel to the specified storage register in array 2 via data-in drivers 6A in a single, self-timed WRITE cycle. 
     A READ instruction loads the address of the memory register to be read from the instruction register 4 into an 8-bit address register 7. The data from the accessed storage register is then transferred in parallel to data shift register 5 via the sense amplifiers 6 and then clocked out serially to the Data-Out pin DO. 
     FIG. 2 shows serial read access circuitry 10 for sequentially reading storage registers within array 2 in accordance with the present invention. The circuit 10 comprises two primary components: an address register/counter 12 and a data stream counter 16. 
     The address register/counter 12 latches the starting address input from the instruction shift register 14 during φ1I.Io high. That is, when the start bit of the instruction which is being serially shifted into instruction shift register 14 migrates over a series of clock cycles to register location IO, the output φ1I.Io of AND gate 15 goes high, causing the parallel transfer of the starting address from the instruction shift register 14 to the address register/counter 12. As will be described in greater detail below, the address register/counter 12 has the ability to count up in increments of 1 in response to IAL pulse inputs to the address latch 12. 
     A data stream counter 16 monitors the number of clock pulses during the READ mode and generates an increment address latch (IAL) signal at a fixed count. The data stream counter 16 also generates a signal RD.φ1, during which time a new register in memory array 24 corresponding to the new incremented address in address register/counter 12 is read by the sense amplifiers 22. The new data is transferred into the masters and slaves of the data shift registers 18 while the old data bit DO is clocked out of the dummy bit 26. 
     The data shift registers 18 are loaded either serially from the data-in (DI) pin 20 (as described above in conjunction with a WRITE instruction) or can be loaded in parallel during the RD.φ1 phase from the sense amplifiers 22. As stated above, the data loaded serially into the data shift register 18 is written into the memory array 24 in parallel. The data loaded in parallel to the shift registers 18 during a read access is serially clocked out via dummy bit 26 and data output drivers 28 onto the data output (DO) pad 30. 
     The mechanics of serial read access in accordance with the present invention will be described with reference to the timing diagram shown in FIG. 3 and the FIG. 2 schematic. 
     A read operation is accomplished by inputting a READ instruction into the instruction register 14 through the data-in pin 20. The READ instruction commences with a start bit &#34;1&#34; followed by the op-code and the address of the memory storage register to be read. During the clocking sequence, while the last address bit Ao is latched into the &#34;Slave&#34; at one end of the instruction register 14 on the rising edge of the φ1 clock, the start bit is latched into the slave of the instruction register 14 at the other end making Io go high. In the illustrated embodiment, the Io signal is delayed to provide an IOPD signal which provides enough time for all bits in the instruction register 14 to stabilize. The IOPD signal going high forces the internal clock phases φ1I and φ2I clocking the instruction register 14 to stop clocking. This prevents any further data from being shifted into instruction register 14. 
     As further shown in FIG. 2, the bits of data stream counter 16 are initialized to zero when the READ signal is low by resetting the flip-flops which comprises the counter 16. With the IOPD signal going high, the READ instruction is decoded. The signal RD.φ1 is true during the time READ and 01 are true and the data stream counter 16 count is zero. During the time interval RD.φ1, the data from the register address in array 24 specified in the READ instruction resident in address register/counter 12 is latched into the data shift register 18 and the dummy zero bit is output on the data output pin 30 after a small delay through the data output driver. The dummy zero bit signals that a valid data stream is to follow. At the next clock pulse φ2, the data bits are shifted one position to the right and at the following φ1 clock phase, the new data bit (MSB of data, e.g. D15 in FIG. 3) is latched into the slave portion of the last shift register (the dummy bit). After some delay through the data output driver 28, the new data bit (e.g. D15) comes out on the data output pin 30. In this way, for every successive input clock phase, the next lower bit of data is output on the data output pin 30. 
     Because all the data bits (D15-DO in the illustrated example) corresponding to a memory address are latched into the data shift register 18 in parallel when RD.φ1 is true, the address in the address register/counter 12 can be incremented while the data from the previous address is being serially clocked out on the data out pin 30. 
     In the illustrated embodiment, a count of 7 (decimal) has been selected to generate a signal IAL (increment address latch) to increment the address register/counter 12 count by 1. Following the decoding of the READ instruction, at the sixteenth clock phase φ2, the data stream counter 16 rolls over to a count of 0 and during the sixteenth clock phase φ1 following the READ decoding, the signal RD.φ1 is true again. During the time interval when the RD.φ1 signal is high, the data corresponding to the new incremented address is transferred into the masters and slaves of all sixteen data shift registers 18. Simultaneously, the LSB (DO) of the previous data stream is clocked out of the dummy bit 26 into the data output drivers 28 and onto the data output pad 30. On the seventeenth high going phase of clock signal φ2 following the decoding of the READ instruction, the MSB (data bit D15*) of the incremented address is shifted into the master of the dummy bit and at the seventeenth high going phase of φ1, the data bit D15* is output on the data output pad 30. 
     This sequence of events repeats for each 16 bits of data. Thus, a continuous stream of data bits can be serially clocked out of the data output pad 3 without the need for providing the new addresses to the circuit 10 by inputting new READ instructions over and over again. This provides a substantial time savings. In this way, the entire memory array 24 can be read in one continuous data stream or as registers of length varying from 16 to 4096 bits. Thus, the array 24 can also be used as a shift register of variable lengths (from 16 to 256 bits). 
     It should be understood that various alternatives to the embodiment of the invention shown herein may be employed in practicing the invention. It is intended that the following claims define the invention and that circuits within the scope of these claims and their equivalents be covered thereby.