Getting 3.3v Flash ROM chip to work in older 5.0v Circuit

Discussion in 'The Projects Forum' started by Sukasa, Nov 17, 2009.

  1. Sukasa

    Thread Starter New Member

    Nov 17, 2009
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    0
    I'm trying to work with an older legacy device with a bad Mask ROM chip on it which runs at 5 volts DC. However, I want to be able to modify the contents of the ROM as I wish, so I'm looking at using a Flash ROM instead of a Mask ROM with a Propstick USB to program it. The problem I'm having, of course, is that the flash ROM and propstick USB run on a 3.3v supply (produced on the propstick chip via voltage regulator), which means that I can't just hook up the data and address buses/etc from the device to the flash ROM, because I'll simply destroy the chip.

    So, my question is if anyone knows a good/safe way for me to bring down the voltage and current to the Flash ROM so that the Legacy 5v system AND the 3.3v propstick USB chip can access it safely without wrecking each when the legacy system is on. I'm not particularly great with electronics (I've only had experience with same-voltage stuff), so I'm not sure where to start with this, and any help would be greatly appreciated.
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,135
    1,786
    Assuming that you have both 3.3V and 5V power supplies; there are several lines of CMOS chips that have TTL thresholds and 5V tolerant inputs. What this means is that you can conveniently build a completely robust interface between the 3.3 V sybsystem and the 5V subsystem.

    http://www.onsemi.com/pub_link/Collateral/MC74VHC1GT50-D.PDF
     
    Last edited: Nov 17, 2009
  3. Sukasa

    Thread Starter New Member

    Nov 17, 2009
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    0
    My problem is, though, I've had 0 luck finding any of these, and whichever I do, it needs to be fast enough to be able to keep up with a 10Mhz processor, too. Are there any chips or chip families I could look up specifically to help with this?
     
  4. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    Try something in this family of parts ... 74LVC126A
     
  5. davebee

    Well-Known Member

    Oct 22, 2008
    539
    46
    I've gotten a 3.3 volt microcontroller to operate 5 volt periferals by just using a plain old Schmidt trigger 74HCT14 to go from 3.3 to 5 volts, and a resistor divider with values like 3.3K/2.2K to reduce the 5 volt logic down to 3.3 volts.

    I haven't run speed tests but the HCT14 is supposed to work at 10 mHz.

    I don't know about the flash ROM, but the propstick itself has protection diodes on its input pins so you can drive its inputs safely from 5 volt logic by just passing the signal through a single series resistor of about 2.2K.
     
  6. Papabravo

    Expert

    Feb 24, 2006
    10,135
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    Surely you jest! Not fast enough for a 10 MHz. processor; did you even look at the datasheet?
     
  7. Papabravo

    Expert

    Feb 24, 2006
    10,135
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    Does that family have TTL thresholds? That is the critical feature for level translation. You absolutely don't want the thresholds to be the traditional CMOS ones at VCC/2.
     
  8. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    It is listed as

    "· Direct interface with TTL levels"
    "· 5 V tolerant inputs/outputs for interfacing with 5 V logic"

    and it has

    V(IH) = 2 V (min)
    V(IL) = 0.8V (max)

    for Vcc between 2.7 and 3.6V​
     
  9. Papabravo

    Expert

    Feb 24, 2006
    10,135
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    OK, that's the ticket. Many manufacturers add the letter "T" to the part number to indicate the use of TTL levels. Example 74HC00 has CMOS thresholds at Vcc/2 while 74HCT00 has TTL thresholds of 0.8 and 2.0 Volts. It is really easy to miss the importance of this for level shifting applications.
     
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