SEC12 - 12 EMBEDDED MEMORY TECHNOLOGY Embedded memory is...

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Embedded memory is any non-stand-alone memory. It is an integrated on-chip memory that supports the logic core to accomplish intended functions. High-performance embedded memory is a key component in VLSI because of its high-speed and wide bus- width capability, which eliminates inter-chip communication. Advantages of embedded memories are : Reduces number of chips Reduces pin count Multi-port memories Less board space requirements Faster response with memory embedded on chip Dedicated architecture Memory capacity appropriate for the application Power consumption reduced More cost effective at system level The main disadvantages of embedded chips are their larger size, their greater complexi- ty and the fact that the technology for embedded memories is not the same as that for embedded logic devices. Standard memory devices are manufactured using a specific memory process. With chips integrating embedded memories, the process used will often be more logic orient- ed than memory oriented. On a standard memory device, the cell size is the most criti- cal issue. In devices using embedded memory, memory cell size is not a critical factor. For this rea- son, chip designers will often use a conservative cell design for embedded memories. EPROM, flash, or DRAM may have more than one transistor per cell. SRAM will often use the CMOS 6-transistor cell design instead of the 4-transistor memory design. Figure 12-1 shows the key differences between embedded and stand-alone flash memo- ries. This same kind of comparison would be valid for other types of embedded memories. INTEGRATED CIRCUIT ENGINEERING CORPORATION 12-1 12 EMBEDDED MEMORY TECHNOLOGY
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DRAM Embedded DRAMs may become a solution as on-chip memory needs increase and the use of system-on-a-chip rises. Most of the current embedded DRAM developments are for graphic and multimedia applications. Embedded DRAMs are in the introduction phase. Due to the complexity of DRAM process technology, suppliers did not quickly develop embedded DRAMs. Embedded DRAM capacitors that store data require several processing steps not needed when mak- ing logic devices. The threshold voltage of DRAM transistors must be high enough to ensure that they will not create memory cell capacitor leakage. This constraint on low sub-threshold current may cause some speed penalty on the embedded logic devices. DRAM processes are a central issue in an embedded DRAM device using a process developed of logic chips. Memory Cell The memory cell of standard DRAM memory chips consists of one transistor and one capacitor. In the new DRAM generations, the capacitor is either a trench or stack capac- itor design. The memory cell of embedded DRAMs using a standard logic process will often be a 3- transistor design with separate read and write access. An additional capacitor is some- times added to ensure a minimum value of 30pF. The larger the capacitor, the less sensi- tive to noise or alpha particle induced soft error the memory cell will be. Figure 12-2
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SEC12 - 12 EMBEDDED MEMORY TECHNOLOGY Embedded memory is...

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