I want to replace an IS62WV51216BLL-55 (http://www.issi.com/WW/pdf/62WV51216ALL.pdf) with an AS7C4098A-12 (https://au.mouser.com/datasheet/2/12/as7c4098a_v1.2-1288279.pdf), as the latter has much better speed (although less capacity, but that's not a problem in my case), to be interfaced with some STM32 I have lying around.

My question is, is the fact that the latter is a TTL part (but CMOS compatible according to the datasheet) a problem? I can see a table with the recommended voltages (and those are basically 5v), but the absolute minimum and maximum seems to be adequate (-0.5 to 7V).

Any other problems that might arise even if voltage is ok (according to the datasheets)?

  • 1
    \$\begingroup\$ The AS7C4098A is a CMOS RAM. It says so at the top of the second page of the datasheet. \$\endgroup\$ – Elliot Alderson Aug 12 '18 at 1:53
  • \$\begingroup\$ Right, but still it says it's TTL on the first page ("TTL- and CMOS-compatible, three-state I/O"). In any case I guess it's fine then? \$\endgroup\$ – Edu Garcia Aug 12 '18 at 2:16
  • \$\begingroup\$ The have not made TTL RAMs for decades, eg. 74F189 \$\endgroup\$ – Spehro Pefhany Aug 12 '18 at 5:56
  • \$\begingroup\$ No, the hyphen after TTL means that the "TTL-" connects to the "-compatible". The phrase means that the device is TTL-compatible, not that it is an actual TTL circuit. They certainly could have made the wording more clear. \$\endgroup\$ – Elliot Alderson Aug 12 '18 at 12:55

You're confusing the manufacturing process of the AS7C4098A (which is CMOS) with it's compatibility (which is TTL and CMOS). Compatibility here refers to I/O levels and means that the AS7C4098A will connect to and work with other parts that use either TTL or CMOS logic thresholds.

This is all about reading and understanding the DC characteristics section of the datasheets, comparing the Voh/Vol and Vih/Vil parameters of the devices you are connecting together. And I'm not sure you've understood the absolute maximum ratings section - the 7V you quote is an absolute maximum supply voltage above which you will damage the SRAM, so what do you mean when you say this is "adequate"?

Make sure your STM32 (which is presumably powered from 3.3V) is going to be OK working with a 5V powered SRAM, i.e. that its inputs are 5V tolerant.

  • \$\begingroup\$ What I understood from the maximum ratings section is that according to that, I can power this chip at 3.3V (as the IS62WV51216BLL was) and it should work, is that correct? Because that's what I want to do (and not power it at 5V as you mention at the end). \$\endgroup\$ – Edu Garcia Aug 12 '18 at 12:00
  • \$\begingroup\$ No: Absolute Maximum Ratings tells you what level of stress the device will survive without damage. For the device to function correctly you need to follow Recommended Operating Conditions. The power supply (for example) must be 4.5 - 5.5V. \$\endgroup\$ – amb Aug 12 '18 at 12:43
  • \$\begingroup\$ Well, I was hoping that "Recommended" was just that, a recommendation, and that 3.3V was still possible, but this of course answers my question (with a "no", which is sad indeed, I didn't want to modify or have more voltages on my board, but oh well...). Thanks! \$\endgroup\$ – Edu Garcia Aug 12 '18 at 12:46

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.