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Original question:

DP83867IS: With regards to the Application Report SNLA243–October 2015

DP83867E: SNLA243 - DP83867E: Min_SR, Min_HR, IO_skew

Lauren Gresko
Prodigy 20 points
Part Number: DP83867E

Hi, 

Where did the values of  Min_SR, Min_HR, IO_skew in table 3 of snla243 come from? They are not in the DP83867E datasheet. In the datasheet, T_setupR and T_holdR is 1ns. If we use Min_SR = 1ns and Min_HR =1ns in equations 1-4, the set up and hold margins are not positive:

(1) t_sr = t_id - ID_var - T_skewT - IO_skew - PCB_skew
(2) Setup Margin = t_sr - Min_sr

(3) t_hr min = tchmin - t_id - ID_var - T_skewT - IO_skew - PCB_skew
(4) Hold Margin = t_hr - Min_hr

t_id = 2.0ns (mode 0 strap)
ID_var = 0.2ns (from snla243)
IO_skew = 0.35ns (from snla243)
Min_sr = (datasheet says 1ns)
Min_hr = (datasheet says 1ns)
PCB_skew = 0.01ns (lowered from snla243)

T_skewT = 0.5ns (from rgmii v2 spec in snla243)

tchmin = 0.45*8ns = 3.6 ns

(1) t_sr = 2 - 0.2 - 0.5 - 0.35 - 0.01 = 0.94 ns
(2) Setup Margin = t_sr - Min_sr = 0.94 - 1.0ns = -0.06ns

(3) t_hr min = 3.6ns - 2 - 0.2 - 0.5 - 0.35 - 0.01 = 0.54
(4) Hold Margin = t_hr - Min_hr = 0.54 - 1.0 = -0.46

I've tried adjusting the delay by increments of 0.25 ns, but it's not possible to get the set up and hold margins positive using the above numbers from snla243. The IO_skew of .35 ns and ID_var of 0.2 ns are too large. 

Why is the I/O buffer skew (IO_skew = 350ps) so large? Also, why is the internal ID_var so large (200ps)? Are these typos? It doesn't seem possible to get positive margin while using the internal delay. Any information/guidance would be much appreciated.

Thanks!

  • 0
    TI__Mastermind 23675 points
    Hi Lauren,

    Setup/Hold time in RGMII section of the DS are not the same as min_SR and min_HR in table 3 of the app note.
    This is why you are seeing non-positive results. Those values in table 3 should not be changed when doing the calculations.
  • 0 in reply to Ross Pimentel
    Prodigy 20 points
    Why are those values not in the datasheet? I followed the example, and yes, I know that the calculation works when using the values in table 3. My question is more - why is table 3 not in the datasheet if those set up and hold times are the values for the DP83867E?

    "To calculate the setup margin, we subtract the minimum setup time required by the receiver from the worst-case setup time. If the MinSR is less than the RGMII specified 1.2 ns, more margin can be gained by using the receiver’s MinSR."

    "To calculate the hold margin, we subtract the minimum hold time required by the receiver from the worstcase hold time. If the MinHR is less than the RGMII specified 1 ns, more margin can be gained by using the receiver’s MinHR."
  • 0 in reply to Lauren Gresko
    TI__Mastermind 23675 points
    What we spec is the RGMII specification and the requirements for a transceiver.
    If you meet the setup/hold times in the DS, you will be able to transmit error free data.
    The app note is breaking it down for better understanding. We are not going to add table 3 in the app note to the DS.