I am building a memory module for an 8-bit computer, and am running into an issue. (Following this schematic, but using an IDT6116 instead of the AS6C6264 shown)

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The chip is receiving 5V, which is within the tolerances listed on the datasheet. I have connected the outputs to 8 LEDs and tied all address inputs low. The CE and OE pins have been tied low (as shown in the schematic), and the WE pin is tied high. At first everything works as expected. I can view each byte of junk data the SRAM has been initialized with. After a while in this state the chip will begin to heat up and then all of the outputs switch to low, regardless of their previous state and without any intervention on my part. The datasheet I am referring to for the IDT6116 SRAM chip can be found here. The SRAM chip does not appear to be damaged by this, as when power is cycled off and back on again it returns to working normally. I have observed this behavior with all 3 IDT6116 chips I ordered.

  • \$\begingroup\$ What speed grade are your RAM chips, and where did you get them? \$\endgroup\$ – Bruce Abbott Jan 13 '18 at 6:18
  • \$\begingroup\$ What value is the LED current-limiting resistor? \$\endgroup\$ – Bruce Abbott Jan 13 '18 at 6:36

The Alliance AS6C6264 memory part has a Voh of 2.4V, with the IoH being -1mA or 2mA. The IDT6116SA/LA has a Voh of 2.4V, with the IoH being -4mA or 8mA.

So typically the output voltage of the IDT could be higher. An LED's IV curve is exponential, so if you give it a little more voltage, you get a lot more current. So I believe what is happening here is the IDT part is sourcing much more current and burning up much more power.

Increase the current limit the LED's (or remove them, yeah its cool to see the data go by but useless) and the problem in all likelihood will go away.

  • \$\begingroup\$ Better still, use another buffer to drive the LEDs. And be sure to use a separate resistor for each LED. \$\endgroup\$ – Dave Tweed Jan 13 '18 at 11:41

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