PCIe expansion


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 D3cold
   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 D3cold 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.


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…