We have used an external SD slot
connected to SD pins on APU2. Unfortunately CD and WP pins are not exposed
so it is not exactly the same situation. However, we have managed to connect
to CD pin as well. Leaving WP unconnected should not make the difference -
controller will treat card as write-protected, but that is not an issue in this
case. So to make it clear: we have all SD signals (except for WP which is left
unconnected) coming from the external SD slot on prototype board.
To make below tests even more similar, we have moved WP switch down so even
in internal slot card will be seen as write-protected.
When GOOD card is inserted into the external slot it behaves the same (in terms
of command response) as if it would be inserted into APU2 internal slot. No
matter if it is a cold- or warmboot.
When BAD card is inserted into the external slot it behaves the same (in terms
of command response) as GOOD card. When it is inserted into APU2 internal
slot it does not respond to any command.
We have not managed to boot from the external slot, however. It is possible that length and quality of joints is not sufficient enough when frequency rises from initialization 400 kHz to operational frequency.