why manual routing used in case of memory designs like DRAM and SRAM? why not auto ?

[kpraneethin007]

why manual routing is used in case of memory designs like DRAM and SRAM ?? why not automatic routing.

I have designed an ordinary 16 bit ALU where we performed Automatic Routing using Encounter. But while designing DRAM or SRAM we were told to do manual routing. Please tell me whether automatic routing for DRAM or SRAM is not possible using current tools??

 

[artmalik]

ALU is digital block which is purely combinational with flip flops. While both DRAM and SRAM have analog blocks like sense amplifier,pre-charge circuits,some timing feedback loop so they need manual routing and post layout simulations. Auto-routers work only on purely digital blocks. Once the SRAM design is done, the final SRAM from the interface point of view "looks" like a digital block. An SRAM in chip can be auto-routed but not SRAM itself.