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DAC108S085: Question about SPI timing

Hao Wang14
Expert 4415 points
Part Number: DAC108S085

Hi Team,

The datasheet says there is limits for tSS and tSH, my questions are:

1. Does the Units(limits) correspond to Typical or Limits? If to Typical, most of the data seems to be reasonable, but how to explain the ts becase 4.5us<6us?

2. For tss, should it be (1/fsclk - 3) or 1/(fsclk - 3)? What's the unit of fsclk in the formula, Hz or MHz? 

3. What does "Average Outgoing Quality Level" mean for this table?

Thanks and Best Regards!

Hao

  • 0
    TI__Mastermind 46290 points

    Hi Hao,

    I will respond in-line.

    Hao Wang14 said:
    1. Does the Units(limits) correspond to Typical or Limits? If to Typical, most of the data seems to be reasonable, but how to explain the ts becase 4.5us<6us?

    The units apply on both. ts is settling time which "typically" can be faster than 6us, but the maximum - or slowest - limit could be 6us.

    Hao Wang14 said:
    2. For tss, should it be (1/fsclk - 3) or 1/(fsclk - 3)? What's the unit of fsclk in the formula, Hz or MHz?

    The first case without the parenthesis.

    Hao Wang14 said:
    3. What does "Average Outgoing Quality Level" mean for this table?

    This is an older part from National Semi so it's hard for me to say for certain, but given my experience I would be inclined to believe that they were trying to express that this is not a production level test - instead something guaranteed or specified by design or other qualification tests.

  • 0 in reply to Kevin Duke
    TI__Expert 4415 points
    Hi Kevin,

    Does the max/min in the bracket of unit(Limits) apply for Typical or Limits column?
    For example, if the clock frequency fsclk is 20MHz, tss is 47ns calculated by the formula (1/fsclk-3), so is 47ns the maximum or minimum time?
    If the answer is maximum, when the tss set by customer exceeds 47ns, what's the consequence?
    If the answer is minimum, it's consistent with the test result from customer. Because their tss is much longer than 47ns, and it works good.
    Could you please help confirm?

    Thanks and Best Regards!
    Hao
  • 0 in reply to Hao Wang14
    TI__Mastermind 46290 points

    Hao,

    That is a good point I did not notice with your initial question. I believe the intention of the table is for the "max" or "min" comment within the parenthesis to indicate the conditions of the limit. However, it is not usual for set-up or hold time specifications to have a maximum specification - it is also odd for anything like this to have any relationship to the SCLK frequency.

    Even more confusing is the top-line Fsclk specification. The typical value is 40MHz while the limit (max) value is listed as 30MHz. Those would clearly contradict one another.

    I'm not immediately sure of what resources would be available to drive any better clarity on this, certainly an odd way to provide this table. I will see what I can find.

  • 0 in reply to Kevin Duke
    TI__Expert 4415 points
    Hi Kevin,

    Do you have any update about this issue? Because customer is confused about it and worried that there might exist some potential risks about the timing sequence.

    Thanks and Best Regards!
    Hao
  • 0 in reply to Hao Wang14
    TI__Mastermind 46290 points
    Hao,

    Sorry about the delays - Friday was a holiday in the US and I was out of office for most of Monday as well.

    I wasn't really able to find anyone remaining internally that would have had any experience with this device, however in the end I went back to inspecting the datasheet a bit closer and see what is going on now.

    The top-line conditions for the AC and Timing Characteristics table indicate that the specifications are with supply voltages from 2.7V to 5.5V, a maximum capacitive load of 200pF, from codes 12 to 1011, and at room temperature. Anything in bold applies over the wider temperature range.

    So the SCLK frequency specification is indicating that at room temperature you can go up to 40MHz, but if operating from -40C to 125C there is some corner where the maximum specified speed is actually 30MHz.

    For Tss this is a 3ns minimum and (1/FSCLK ) - 3 maximum. Over temperature Tss changes to 10ns minimum but still (1/FSCLK ) - 3 maximum. Tsh is very similar, this is 0ns minimum and (1/FSCLK ) - 3 maximum and over temperature 3ns minimum and (1/FSCLK ) - 3 maximum.

    Though it is unusual to see a maximum setup or hold time, we have to assume that there was a valid reason for it being included in this datasheet. The customer may have units that are working right now, but there could be some corner case where they could suffer in the future without compliance.
  • 0 in reply to Kevin Duke
    TI__Expert 4415 points

    Hi Kevin,

    Thanks!

    Hao