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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?

enter image description here

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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
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\$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.

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  • \$\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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