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Ibis model for C6457 DSP - DDR2 interface

Kevin Lewandowski
Prodigy 20 points
Other Parts Discussed in Thread: TMS320C6457

The ibis model for the C6457 DSP (tms320c6457c.ibs) defines some models based on termination, 30/45/50/55 ohm, yet there is no mention of this in the DDR2 users guide (SPRUGK5C).

The only selection seems to be in the DDR2 Memory Controller Control Register (DMCCTL):

bits 5-4 DDRPVTCNTL bits.

0b00 No termination.

0b01 Half termination (recommended value).

0b1x Full termination.

(as well as slew rate through the external pin, DDRSLRATE)

From IBIS model (tms320c6457c.ibs) :

|               -BSHTLTCSCDVGPBFZ
|                 the following portions of the model have been updated
|                  Driver:
|                   a) Impedance                 - 45/50/55 ohms impedance
|                   b) Slew Rate Operation       - Fast and slow modes of operation
|                   c) Voltage Levels            - 1.5/1.8V
|                   d) Voltage Nom variation     - 10%/5%
|                   Reciever -
|                   a) Termination Modes  - Half/Full Termination as well as no termination modes
|                   b) Termination Impedance     - 45/50/55 ohms impedance
|                   c) Voltage Levels            - 1.5/1.8V
|                   d) Voltage Nom variation     - 10%/5%

The user guide also mentions that there is no ODT on the C6457 (from page 10 of the users guide) "On-die termination signal(s) to external DDR2 SDRAM. Note: There are no on-die termination resistors implemented on the die of the C6457 DSPs."  So why are there models in the ibis for ODT?

Also, for no termination models, why are there different impedances (30/45/50/55)?  Receiver Impedance?  Driver impedance?  How are they selected/changed in the DSP?

45OHM_5PER_NOTERM_1P8  RX MODE,1.8V +/- 5PERCENT,WITHOUT TERMINATION
|45OHM_5PER_NOTERM_1P8;

50OHM_5PER_NOTERM_1P8  RX MODE,1.8V +/- 5PERCENT,WITHOUT TERMINATION
|50OHM_5PER_NOTERM_1P8;

55OHM_5PER_NOTERM_1P8  RX MODE,1.8V +/- 5PERCENT,WITHOUT TERMINATION
|55OHM_5PER_NOTERM_1P8;

30OHM_5PER_NOTERM_1P8  RX MODE,1.8V +/- 5PERCENT,WITHOUT TERMINATION
|30OHM_5PER_NOTERM_1P8;

I am trying to do some timing verification in HyperLynx and I need to know what are valid models to use for the DSP.  Thanks.

  • 0
    TI__Mastermind 42545 points

    The 30/45/50/55 ohm in the model description refers to the driver termination which is defined by the resistor connected to the PTV18 pin.  The recommended value for this resistor is 47ohms which is the value used to close timing on the interface.  Based on this the best model to use would be the 45ohm version. 

    The note in the users guide concerning the lack of on-die termination resistors is an error.  The C6457 does support ODT as stated in section 2.3 of that document.  We will remove the inaccurate note from the next revision of the document.  

  • 0 in reply to Bill Taboada
    Prodigy 20 points

    One additional question, on a previous DDR2 design I worked on, a Vtt termination voltage using a sink-source supply was necessary for the addr, cmd, and control groups.  There is no mention of this as necessary with the C6457 DDR2 interface and the simulation results seem to support that, however, I just want to verity that Vtt is definetly not needed.  Thanks again.

  • 0 in reply to Kevin Lewandowski
    TI__Mastermind 42545 points

    Vtt termination voltage using a sink-source supply is not expected or supported with the C6457 as detailed in the TMS320C6457/TMS320TCI6484/TMS320TCI6487/88 DDR2 Implementation Guidelines.  http://focus.ti.com/lit/an/spraag6d/spraag6d.pdf  Series termination of the command, address and data lines is recommended and detailed in the above mentioned document.