An Embedded DRAM for CMOS ASICs

The growing gap between on-chip gates and off-chip I/O bandwidth argues for ever larger amounts of on-chip memory. Emerging portable consumer technology, such as digital cameras, will also require more memory than can easily be supported on logic-oriented ASIC processes. Most ASIC memory systems are P-load SRAM, but this circuit technology is neither dense nor power efficient. This paper describes development of a DRAM, compatible with a standard CMOS ASIC process, that provides memory density at least 4x improved over P-load SRAM in the same layout rules. It runs at speeds comparable to logic in the same process and uses circuitry that is reasonably simple and portable. The design employs Vdd-precharge bit lines, half-capacitance, full-voltage dummy cells, and a simple complementary sense amp. DRAM is organized as a number of small pages, allowing simple circuit design and low-power operation at modest expense in area overhead. The paper also described a power-conserving low-voltage-swing bus design that interfaces multiple pages to full-voltage-swing circuitry. Circuit and layout details are provided, along with experimental results for a 100MHz 786K-bit embedded DRAM in a 0.5μ process.