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Bug #5001

W_DISABLE should be pulled high to 3.3V

Added by marwalte about 1 month ago. Updated about 1 month ago.

Status:
New
Priority:
Low
Assignee:
Target version:
-
Start date:
Due date:
% Done:

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Description

Hi,

The radio enable/disable pin (Pin 8) is currently pulled high to 1.8V. PCIe M.2 spec requires 3.3v (version 3.0, revision 1.2).

History

#1 Updated by laforge about 1 month ago

  • Assignee set to mschramm

I think the decision to pull it to 1.8V was taken in reference to some specific modem where the manufacturer stated it to be don that way? Maybe mschramm can comment.

I guess in the worst case we should have a jumper (either physical 1x3 jumper) or some jumper SMT resistors that can be used to switch.

#2 Updated by mschramm about 1 month ago

  • Priority changed from Normal to Low
  • Start date deleted (02/01/2021)

Thanks for the feedback!

When designing this breakout board about four years ago, we reviewed the PC-SIG specs as well as several modem hardware integration manuals; amongst them were Fibocom L860, Huawei ME906s, ME936s, Sierra EM7565, and recently Quectel RM500Q. Despite the PC-SIG remark about pulling it up to V host (usually 3V3), they all allow the system integrator the usage of either power rail of 1V8 or 3V3 (btw, same is true for BODY_SAR, GPS_DISABLE and RESET (partly for WAKE_ON_WWAN)). Furthermore, it's noted in the integration guides that even leaving this pin open is also sane, as they internally pull it up to the 1V8 domain (and add a diode internally, cathode towards WWAN_DISABLE pin). As the signal is low-active, a voltage above 0,7-1,26V (depending on manufacturer) is considered HIGH, which always will be achieved by a 1V8 rail and 10k PU to it.

So far, no problem with usage of that power rail has been reported. We might revise this detail for an upcoming production run.

#3 Updated by marwalte about 1 month ago

Hi!

Thanks for the rapid response.

we reviewed the PC-SIG specs as well as several modem hardware integration manuals

Excellent due diligence!

Despite the PC-SIG remark about pulling it up to V host (usually 3V3), they all allow the system integrator the usage of either power rail of 1V8 or 3V3

I couldn't find this remark in the PCI-SIG spec, do you have a reference? PCI-SIG section 3.1.12.3 calls out W_DISABLE1 must be 3.3v explicitly, with a 100-200k pull-up resistor.

same is true for BODY_SAR, GPS_DISABLE and RESET (partly for WAKE_ON_WWAN)

Looking at Section 3.1, Table 3-11, BODY_SAR (DPR), GPS_DISABLE, RESET and WAKE_ON_WWAN are 1.8V. Additionally, W_DISABLE2 is listed as 1.8V (vs. W_DISABLE1, 3.3V)

As the signal is low-active, a voltage above 0,7-1,26V (depending on manufacturer) is considered HIGH, which always will be achieved by a 1V8 rail and 10k PU to it.

Anecdotally, I have observed that 1.8V for Pin 8 seems to be fine (for example, with an FN980m and an lt4120). Based on the the feedback here, it seems like this behaviour is the same for a lot of other devices. I think in practice this is fine, even if it might not be PCI-SIG standard.

#4 Updated by mschramm about 1 month ago

marwalte wrote:

Despite the PC-SIG remark about pulling it up to V host (usually 3V3), they all allow the system integrator the usage of either power rail of 1V8 or 3V3

I couldn't find this remark in the PCI-SIG spec, do you have a reference? PCI-SIG section 3.1.12.3 calls out W_DISABLE1 must be 3.3v explicitly, with a 100-200k pull-up resistor.

My "they" referred to the modem's hardware integration manuals mentioned, not the PCI-SIG spec.

As the signal is low-active, a voltage above 0,7-1,26V (depending on manufacturer) is considered HIGH, which always will be achieved by a 1V8 rail and 10k PU to it.

Anecdotally, I have observed that 1.8V for Pin 8 seems to be fine (for example, with an FN980m and an lt4120). Based on the the feedback here, it seems like this behaviour is the same for a lot of other devices. I think in practice this is fine, even if it might not be PCI-SIG standard.

That's right, same here: despite their deviation from PCI-SIG spec, this rather appears to be the 'normal' case. - Redesignation and RFU pins and not fully following PCI-SIG standards are unfortunately widely spread amongst the manufacturers, e.g. there are even miniPCIe modem cards which source a voltage on the sink pin 1V5, while other name this pin VBAT_3V3_IN ... While the latter can be a mess, I don't see a problem with the 1V8 rail usage mentioned here above.

#5 Updated by marwalte about 1 month ago

My "they" referred to the modem's hardware integration manuals mentioned, not the PCI-SIG spec.

Thanks for the clarification, this makes sense.

I don't see a problem with the 1V8 rail usage mentioned here above.

Yup - in practice, this seems to be the case.

Thanks for following up!

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