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DS125MB203: Programming a internal intup termination

Nikita Alisov
Prodigy 140 points
Part Number: DS125MB203

Tool/software:

Hi,

We have a device based on the DS125MB203, and now we would like to turn off the input internal termination for some research. We use RXDET registers: 

We set 0x15[3:2] = 0b00, then we read 0x15 and can see that it was set. Before that we set 0x08 as 0x0C, which means we set Override MODE/RXDET bits.

But if we read register 0x18m we can see there 0xE0 or 0x80 (don't understand why sometimes different value for the same settings) but the 0x18[7] bit is always "1".

I also cannot see any changes from HW side, looks like the device works regardless of setting RXDET bits.

It's bit of our initialization below:

Read Device ID 0x51: 0x46

Write 0x06: 0x18
Read 0x06: 0x18

Write 0x08: 0x0C
Read 0x08: 0x0c

NC-S_INA0:
Write RXDET 0x0E: 0x00
Read RXDET 0x0E: 0x00
Write EQ 0x0F: 0x00
Read EQ 0x0F: 0x00

D_OUT0-S_INB0:
Write RXDET 0x15: 0x00
Read RXDET 0x15: 0x00
Write EQ 0x16: 0x00
Read EQ 0x16: 0x00
Write VOD 0x17: 0xaf
Read VOD 0x17: 0xaf
Write DEM 0x18: 0x00
Read DEM 0x18: 0xe0

... similarly for the rest of the channels ...

Write 0x5e: 0x07
Read 0x5e: 0x07
Write 0x5f: 0xfc
Read 0x5f: 0xfc

Let me ask:

1. Should RXDET Status register show the relevant state of the termination if the termination was manually turned off?

2. Is there a particular sequence of initialization for the re-driver? Could you please share some examples of the re-driver initialization?

3. What is different when DEM register is either 0xE0 or 0x80? What does bits 6:3 of DEM register show?

4. What is the right sequence of setting RXDET bit (e.g. 0x18[7]) and Override bits (0x08[3:2])?

Thank you!

  • +1
    TI__Mastermind 27820 points

    Hi Nikita,

    I believe input termination is remaining 50-ohm to VDD because of your INPUT_EN selection in register 0x5F.

    • When you first write to register 0x18, 0x5F=0x00, which selects "input always enabled with 50 ohms."
    • When you write 0x5F=0xFC, "input always enabled with 50 ohms" is still selected.

    To configure input termination with RX Detect setting, please write 0x5F[3:2]=10.

    To answer your questions:

    1. Yes
    2. There is no particular sequence necessary, your initialization sequence looks sufficient.
    3. 0x18[6:5] are reserved status bits, it is normal for these to change value sometimes.
    4. If override is written first, RXDET=Hi-Z will immediately take effect as this is the default register value. If the override is written last, then whichever configuration is programmed to RXDET bits will take effect at this time.

    Best,

    Lucas

  • 0 in reply to Lucas Wolter
    Prodigy 140 points

    Hi Lucas, thank you for the support, I'll ask an FW team to change the initialization!

    I've noticed 0x5F[3:2]=10 disables the fanout. We use the fanout functionality for one channel D_IN1 -> S_OUTA1, S_OUTB1 as below:

    So, is it possible to keep FANOUT and turn off the RX termination?

  • +1 in reply to Nikita Alisov
    TI__Mastermind 27820 points

    Hi Nikita,

    Unfortunately it's not possible to enable fanout and also configure the input as Hi-Z. This is because input termination is programmed through the Rx-Detect mechanism which is used for PCIe applications. Fanout will not work with PCIe Rx-Detect because an end-to-end link is expected, therefore DS125MB203 does not allow the RX-Detect mechanism to be used while fanout is enabled.

    Alternatively you can power down individual channels with register 0x01 and 0x02[0], however I'm unsure if this explicitly changes the input termination. I believe the power down function simply disables the channel and all related functional blocks to reduce power consumption.

    Best,

    Lucas

  • 0 in reply to Lucas Wolter
    Prodigy 140 points

    Hi Lucas, 

    Ok. I see. Thank you for the support!

    BR
    Nikita