# HG changeset patch # User Mychaela Falconia # Date 1638949005 0 # Node ID beb6519a3be51c960799035e40f0030a7a364933 # Parent c27854430cb374a5ba75c65e46cd284027ebe78b doc/USB-and-mobile-domains: document UART rescue header diff -r c27854430cb3 -r beb6519a3be5 venus/doc/USB-and-mobile-domains --- a/venus/doc/USB-and-mobile-domains Wed Dec 08 05:32:47 2021 +0000 +++ b/venus/doc/USB-and-mobile-domains Wed Dec 08 07:36:45 2021 +0000 @@ -290,6 +290,52 @@ sourcing about 70 uA of current from its USB 3.3V supply into the pull-down resistor - well within FT2232D output current sourcing budget. +2.6. UART rescue header + +With the built-in FT2232D subsystem becoming a part of the critical path for +board bring-up and any kind of Calypso GSM functionality, we get a new concern, +now that we live in an era of severe part shortages and insanely long lead +times: what if we are unable to get some critical part for this USB subsystem, +or run out of parts in the course of troubleshooting iterations? Likewise, +what if some design mistake causes our USB subsystem to not work as intended, +thereby making the main Calypso GSM part of the board inaccessible? + +The Mother's answer to this concern consists of a 10-pin header at reference +designator J301. Out of the total of 9 UART signals running between mobile and +USB domains, 8 of these signals (all except Host_RI) plus two GND pins will be +wired to this header, in the same FreeCalypso dual UART pinout as used on +FCDEV3B and Caramel2. If the USB subsystem is fully populated on a given Venus +board and if it works as designed, the only things that can be connected to J301 +are oscilloscope probes - don't connect an external DUART28 or other adapter, +or you will cause a driver conflict between built-in USB outputs and that +external adapter. However, if the entire USB subsystem is omitted from board +population (presumably due to part shortage), or if U705 and R707 are removed +following a discovery of killer problems in this subsystem, then an external +DUART28 adapter can be connected to J301, and it will effectively take the place +of the built-in USB subsystem that has been taken out. + +2.6.1. DUART28 modifications + +If we end up having to use an external DUART28 adapter board instead of the +built-in USB subsystem, that DUART28 will need to be slightly modified, as in +surgical rework: + +* Adapter input pull-up resistors R11, R12, R14 and R16 will need to be removed + - see section 2.3 above for the reasoning. + +* A loading resistor will need to be added across C12 (perhaps soldered directly + on top of the cap), following section 2.4.1 above. + +Also because DUART28 adapter outputs are at 2.8V rather than 3.3V (DUART28 was +designed to feed signals directly to Calypso inputs, without a subsequent LVC +buffer in the Calypso domain), the current flowing from the mobile domain +battery into DUART28 P_2V8 through the 74LVC541A buffer's output p-channel +MOSFET in reverse increases from about 41 uA to about 58 uA per pin, accounting +for 2.2 kOhm series resistors on DUART28 outputs in addition to our 22 kOhm +pull-ups to VBAT. We can either live with this slightly higher current, or we +can increase R401, R402 and R403 on those Venus boards that will go without +built-in USB. + 3. Boot control signals The two boot control signals RPWON and nTESTRESET properly belong in the mobile