During SRAM write cycle, usually the !OE is disabled, while !WE and !CS are enabled. But for HM6264 SRAM chip (http://esd.cs.ucr.edu/webres/hm6264b.pdf), it allows the !OE to be enabled during write cycle.

it seems it either causes BUS contention or writes its own data? (i.e., write what it reads from itself).

Any thoughts?

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  • \$\begingroup\$ I thought, and I stress thought, that 6264 internally gates off the bus drivers while /WE is enabled, regardless of /OE. But I'm not dead certain. \$\endgroup\$ – TonyM May 16 '17 at 17:03

\$WE/\$ turns off the output drivers during the write cycle so no it does not write it's own data.

HOWEVER, when driving the chip in this mode, you need to drop \$WE/\$ before you apply the write data to the data pins. Data should not be driven onto the bus from the writer till after \$T_{whz}\$ later. Further, when \$WE/\$ is brought high you must remove your driving data before \$T_{oz}\$ time expires but not before \$T_{DH}\$ expires.

Unless there is a pressing reason not to use \$OE/\$ I strongly suggest you use it.

  • \$\begingroup\$ what is the benefit of using this approach (vs the normal approach: !OE high, !WE low, !CS low)? \$\endgroup\$ – Ale May 16 '17 at 17:22
  • \$\begingroup\$ @Alex if it is driven by an appropriate synchronous state-machine (eg FPGA) then you can theoretically save a couple of clock cycles at each end of the cycle if the gate delays are engineered appropriately. \$\endgroup\$ – Trevor_G May 16 '17 at 17:26
  • \$\begingroup\$ This approach saves one control line from the controlling device as OE is just connected to GND, but it limits number of devices on the data bus to one. \$\endgroup\$ – Anonymous May 16 '17 at 17:32
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    \$\begingroup\$ "you must remove your driving data before Toz time expires" but not before Tdh expires. While this saves control signals, it requires a control sequencer which can accurately model these analog delays that may have variation over lot, unit, and temperature \$\endgroup\$ – Chris Stratton May 16 '17 at 17:36
  • \$\begingroup\$ @ChrisStratton good spot. Added.. \$\endgroup\$ – Trevor_G May 16 '17 at 17:44

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