--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/sim-card-detect	Thu Nov 26 04:45:20 2020 +0000
@@ -0,0 +1,21 @@
+On the Mother's D-Sample board the SIM card status register (FFFE:0002)
+reads:
+
+0x000A (CD bit 0) when the SIM socket is empty
+0x000B (CD bit 1) when a card is inserted and the switch is shorted
+
+These register readings match CAL207 register document.
+
+On FCDEV3B Calypso SIM_CD input is tied high following Leonardo and Openmoko,
+and the register reads 0x000B - thus we reach the conclusion that CAL000
+document is correct in that logic high on SIM_CD is supposed to indicate SIM
+inserted, whereas logic low is supposed to indicate an empty SIM socket.
+
+However, the netlist extracted from the PADS PCB file for E-Sample (Calypso+
+chipset) shows the opposite arrangement: it shows a pull-up resistor to V-IO on
+the SIM-CD net, and the switch in the SIM socket shorts this net to ground.
+Perhaps the sense of this signal is inverted on Calypso+?  We have no way of
+knowing because we have no documentation for Calypso+, but we do know that its
+ABB companion Syren reversed the sense of its test reset signal (relative to
+Iota) for no apparent reason, hence such signal polarity reversals are not out
+of the question.