Some modern DRAMs are capable of self-refresh; no external logic is required to instruct the DRAM to refresh or to provide a row address. Under some conditions, most of the data in DRAM can be recovered even if the DRAM has not been refreshed for several minutes. Memory timing Many parameters are required to fully describe the timing of DRAM operation. Here are some examples for two timing grades of asynchronous DRAM, from a data sheet published in 1998: Thus, the generally quoted number is the minimum /RAS low time. This is the time to open a row, allowing the sense amplifiers to settle. Note that the data access for a bit in the row is shorter, since that happens as soon as the sense amplifier has settled, but the DRAM requires additional time to propagate the amplified data back to recharge the cells. The time to read additional bits from an open page is much less, defined by the /CAS to /CAS cycle time. The quoted number is the clearest way to compare between the performance of different DRAM memories, as it sets the slower limit regardless of the row length or page size. Bigger arrays forcibly result in larger bit line capacitance and longer propagation delays, which cause this time to increase as the sense amplifier settling time is dependant on both the capacitance as well as the propagation latency. This is countered in modern DRAM chips by instead integrating many more complete DRAM arrays within a single chip, to accommodate more capacity without becoming too slow. When such a RAM is accessed by clocked logic, the times are generally rounded up to the nearest clock cycle. For example, when accessed by a 100 MHz state machine (i.e. a 10 ns clock), the 50 ns DRAM can perform the first read in five clock cycles, and additional reads within the same page every two clock cycles. This was generally described as timing, as bursts of four reads within a page were common. When describing synchronous memory, timing is described by clock cycle counts separated by hyphens. These numbers represent in multiples of the DRAM clock cycle time. Note that this is half of the data transfer rate when double data rate signaling is used. JEDEC standard PC3200 timing is with a 200 MHz clock, while premium-priced high performance PC3200 DDR DRAM DIMM might be operated at timing. Minimum