PCIe expansion

[Leonid Bloch]

Hi,

I saw in the specs that the X15 supports PCIe via an expansion. I'm wondering, will such an expansion be available with the board?

Thanks!

[Matthijs van Duin]

On 27 August 2015 at 11:09, Leonid Bloch <leo...@daynix.com> wrote:

I saw in the specs that the X15 supports PCIe via an expansion. I'm wondering, will such an expansion be available with the board?

I suspect such an expansion is not going to be entirely trivial, since you'd probably want to pass-through other signals to allow stacking, somehow mechanically accommodate the PCIe card, and provide the power supplies for the PCIe card consisting of:

• 3.3V @ 3A
• 12V @ 0.5A (some cards may request up to 2.1A for full operation)
• optional 3.3Vaux @ 375mA to allow card to request wakeup from D3_cold power state

I don't know if the BBX15 happens to have such supply rails available; if not then the expansion card will probably also need to become the main power supply (assuming it's possible to power the X15 from the expansion headers) or have a separate supply (ugly).

Multiple cards (or pass-through of PCIe to stacked expansion card) could be supported using a PCIe bridge, though power supply requirements obviously get even worse.

This is assuming you mean to use PCIe as root complex. In endpoint mode (to connect the BBX15 to a PCIe slot of a computer) things are different and possibly simpler. To escape the form-factor of a PCIe card special flatcables exist, although their length is limited obviously. It would be desirable to supply the X15 from the PCIe bus in this case, which would also reduce (but not fully eliminate) power domain crossing issues. The PCIe clock and data lines are capacitively coupled hence not realy a worry, but there's also some auxiliary signals:

• PERST# (driven by host) — power-on reset, goes high (3.3V) when power supplies and PCIe clock are stable
  
• presence detect (connection between outermost pins of card, used by host to detect insertion)
• WAKE# (optional, open drain, pulled up to 3.3Vaux or lower) — driven low by card to request power up
  
• I²C/SMBus (optional, open drain, pulled up to 3.3Vaux)
• JTAG (optional, I suspect rarely supported in real world)

An alternative to the standard PCIe interface is mini-PCIe, probably most well-known for WLAN and WWAN cards, especially in laptops. Being designed for portable devices, it drops the rather heavy PCIe power supplies in favor of:

• 1.5V @ 500 mA peak, 300mA normal (may be removed in D3_cold power state)
• 3.3Vaux @
• 2750 mA peak (averaged over 100μs)
• 1100 mA normal (averaged over 1s)
• 250 mA if 1.5V supply removed and wakeup enabled

However, an added complication of mini-PCIe is that is also includes a USB 2.0 bus (mandatory afaict). It also adds lots more aux signals, though most of them optional I think, e.g. signals for the USIM slot for a WWAN module.

Matthijs

[Gerald Coley]

The board has expansion headers.

No PCIe expansion card has been created yet for the PCIe interface fro the BB-X15..

The board does support such an interface and it does work..

Gerald

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