FreeCalypso > hg > freecalypso-schem2
comparison venus/doc/USB-and-mobile-domains @ 84:beb6519a3be5
doc/USB-and-mobile-domains: document UART rescue header
| author | Mychaela Falconia <falcon@freecalypso.org> |
|---|---|
| date | Wed, 08 Dec 2021 07:36:45 +0000 |
| parents | 32b848a081a3 |
| children |
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| 83:c27854430cb3 | 84:beb6519a3be5 |
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| 288 ADBUS2 output to U401 input) will be 47 kOhm. When USB power is applied but no | 288 ADBUS2 output to U401 input) will be 47 kOhm. When USB power is applied but no |
| 289 one opens the AT command UART channel, FT2232D will drive its RTS output high, | 289 one opens the AT command UART channel, FT2232D will drive its RTS output high, |
| 290 sourcing about 70 uA of current from its USB 3.3V supply into the pull-down | 290 sourcing about 70 uA of current from its USB 3.3V supply into the pull-down |
| 291 resistor - well within FT2232D output current sourcing budget. | 291 resistor - well within FT2232D output current sourcing budget. |
| 292 | 292 |
| 293 2.6. UART rescue header | |
| 294 | |
| 295 With the built-in FT2232D subsystem becoming a part of the critical path for | |
| 296 board bring-up and any kind of Calypso GSM functionality, we get a new concern, | |
| 297 now that we live in an era of severe part shortages and insanely long lead | |
| 298 times: what if we are unable to get some critical part for this USB subsystem, | |
| 299 or run out of parts in the course of troubleshooting iterations? Likewise, | |
| 300 what if some design mistake causes our USB subsystem to not work as intended, | |
| 301 thereby making the main Calypso GSM part of the board inaccessible? | |
| 302 | |
| 303 The Mother's answer to this concern consists of a 10-pin header at reference | |
| 304 designator J301. Out of the total of 9 UART signals running between mobile and | |
| 305 USB domains, 8 of these signals (all except Host_RI) plus two GND pins will be | |
| 306 wired to this header, in the same FreeCalypso dual UART pinout as used on | |
| 307 FCDEV3B and Caramel2. If the USB subsystem is fully populated on a given Venus | |
| 308 board and if it works as designed, the only things that can be connected to J301 | |
| 309 are oscilloscope probes - don't connect an external DUART28 or other adapter, | |
| 310 or you will cause a driver conflict between built-in USB outputs and that | |
| 311 external adapter. However, if the entire USB subsystem is omitted from board | |
| 312 population (presumably due to part shortage), or if U705 and R707 are removed | |
| 313 following a discovery of killer problems in this subsystem, then an external | |
| 314 DUART28 adapter can be connected to J301, and it will effectively take the place | |
| 315 of the built-in USB subsystem that has been taken out. | |
| 316 | |
| 317 2.6.1. DUART28 modifications | |
| 318 | |
| 319 If we end up having to use an external DUART28 adapter board instead of the | |
| 320 built-in USB subsystem, that DUART28 will need to be slightly modified, as in | |
| 321 surgical rework: | |
| 322 | |
| 323 * Adapter input pull-up resistors R11, R12, R14 and R16 will need to be removed | |
| 324 - see section 2.3 above for the reasoning. | |
| 325 | |
| 326 * A loading resistor will need to be added across C12 (perhaps soldered directly | |
| 327 on top of the cap), following section 2.4.1 above. | |
| 328 | |
| 329 Also because DUART28 adapter outputs are at 2.8V rather than 3.3V (DUART28 was | |
| 330 designed to feed signals directly to Calypso inputs, without a subsequent LVC | |
| 331 buffer in the Calypso domain), the current flowing from the mobile domain | |
| 332 battery into DUART28 P_2V8 through the 74LVC541A buffer's output p-channel | |
| 333 MOSFET in reverse increases from about 41 uA to about 58 uA per pin, accounting | |
| 334 for 2.2 kOhm series resistors on DUART28 outputs in addition to our 22 kOhm | |
| 335 pull-ups to VBAT. We can either live with this slightly higher current, or we | |
| 336 can increase R401, R402 and R403 on those Venus boards that will go without | |
| 337 built-in USB. | |
| 338 | |
| 293 3. Boot control signals | 339 3. Boot control signals |
| 294 | 340 |
| 295 The two boot control signals RPWON and nTESTRESET properly belong in the mobile | 341 The two boot control signals RPWON and nTESTRESET properly belong in the mobile |
| 296 domain. They are pulled up to VBAT inside the mobile domain (RPWON is pulled up | 342 domain. They are pulled up to VBAT inside the mobile domain (RPWON is pulled up |
| 297 to VBAT inside the Iota chip, nTESTRESET is pulled up to UPR with R208 per | 343 to VBAT inside the Iota chip, nTESTRESET is pulled up to UPR with R208 per |