Semiconductor Memory

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Last Updated: Wednesday, 01 September 2021 22:26

Published: Thursday, 17 December 2020 07:53

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Semiconductor Memory:

Processor and Memory are 2 most important components of any digital chip. Just as transistors are used to build logic functionality on a chip (such as AND, OR gates to buld a adder, etc), the same transistors are used to serve as memory to store bits. Memory can store bit 0 (voltage=0 Volts) and bit 1 (voltage=VDD Volts)

There are 2 kinds of memory:

1. Volatile Memory:

These are the memories that lose their contents when power is turned off. In your laptop, you have a hard drive, which is non volatile memory. It keeps the contents even when power is turned off. The CPU transfers programs from hard drive to a volatile memory, and accesses it from there. That makes the programs run faster, as there is significantly lower delay accessing contents from this faster volatile memory. There are 2 kinds of Volatile memories: 

A. SRAM (static random access memory):  This is usually seen on a processor, integrated with other logic. Any circuit that has 2 back to back inverters can serve as a memory. So, we could use flops, latches, etc to serve as memory. However, flops and latches have large number of gates (usually 8 or more gates), which is very costly in terms of area. Early on, engineers started making custom version of these latches so that can be put together closeer and need fewer transistors. They came with idea of using 6 transistors to make a memory cell (very similar to latch but with less transistors). Also, they reduced the size of transistor, and optimized the layout and decode logic, to start building compact memory modules. This memory is called 6T SRAM and is used in all logic chips to make register file, caches, etc. Tese memory are fast, but costly in terms of area, so they are usually limited in size to 64MB or so. They are used in caches and other memories on microprocessors. These are not sold as stand alone memories.

B. DRAM (dynamic random access memory):  This is the memory that is usually built and sold separately. It's not integrated in the processor, but sits right next to the processor. This is slower than SRAM, as it's sitting further away, and has lot more stuff packed. However, it requires only 1 transistor and one capacitor to build 1 memory cell. This makes it much smaller than SRAM. However, in absence of back to back inverter, there is no feedback loop to hold the bit value to a 0 or 1. So, periodic refreshing of the value is needed, which slows DRAM further. Since it needs to be refreshed periodically, it's called dynamic. All the memory that you hear in news, journals, etc is this DRAM. This memory is the one used on external memory modules that you buy from BestBuy, Amazon etc (known as DDR memory cards). They can go as large as 128GB or more (Samsung already reported 512GB DRAM memory modules available). DRAM started out as SDR DRAM, and then moved to DDR style DRAM. DRAM are also known as SDRAM (synchronous DRAM), as all signals are driven synchronous to a clock. NOTE: SDRAM and SDR DRAM are referring to 2 different things.

Next we look at each of these meories in detail:

A. SDR

B. DDR

DDR1

DDR2

DDR3

DDR4

DDR5

LP DDR