Electronic – Why is there a maximum time for length of write pulse to write on an EEPROM


I am still just learning about electronics on my own, so please bear with me.

The EEPROMs that I have come across (for example this one where the t_wp max is 1000 ns.) all have a time limit for the write pulse (I think this is called the Write Pulse Width). I am just curious, but

1) What is the reason that EEPROMS have this upper limit?

2) Are there any parallel EEPORMs with no upper time limit?

Please note that I am not asking about the limit on the number of times one can write to an EEPROM.

Best Answer

There are two reasons I can think of for having a limit to the write pulse length:

  1. If the part uses dynamic latches to hold the address, those latches may only be able to hold their value for a certain length of time. Since the address is latched on the falling edge of /CE & /WE, but the write doesn't start until the rising edge, giving the chip a write command that's long relative to the time required to complete a write cycle could result in the dynamic latches forgetting the address before the write cycle is complete.

  2. If the device rejects any write cycles that are excessively long, that may help guard against erroneous write operations in cases where a system operation gets disrupted (e.g. by loss of power). If that were the intended purpose, however, I would expect a specification that would indicate that write pulses within a certain range are guaranteed to be accepted, write pulses that are outside a larger range would be guaranteed to be ignored, and those between the two ranges might arbitrarily be accepted or ignored.

In either case, 1000ns seems like a curiously short maximum. The address needs to be held for an entire write cycle, so any dynamic latches would need to be able to deal with that. If the cycle limit is intended to guard against stray write events, engineering it to be usable with systems that run at slow clock speeds should have been trivial and would have improved usability.