FreeCalypso > hg > freecalypso-docs
changeset 52:cfe8623b915d
FC-handset-spec: UART signal pull-up/down documented
| author | Mychaela Falconia <falcon@freecalypso.org> |
|---|---|
| date | Fri, 11 Jun 2021 18:29:06 +0000 |
| parents | 7933c3a1b5e0 |
| children | 016f8cf2418c |
| files | FC-handset-spec |
| diffstat | 1 files changed, 27 insertions(+), 4 deletions(-) [+] |
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--- a/FC-handset-spec Fri Jun 11 08:04:44 2021 +0000 +++ b/FC-handset-spec Fri Jun 11 18:29:06 2021 +0000 @@ -809,12 +809,12 @@ USB UART DTE signal Calypso signal ------------------------------ -TxD RX_IRDA -RxD TX_IRDA +TxD2 RX_IRDA +RxD2 TX_IRDA However, as shown in the next section, these signals cannot be simply connected -between Calypso and FT2232x, and a more complicated scheme needs to be -implemented instead. +between Calypso and FT2232x, instead a more complicated scheme needs to be +implemented. 1.12.2. USB and mobile power domains @@ -916,3 +916,26 @@ However, this current will be limited to a maximum of about 40 uA ((4.2-3.3 V) / 22 kOhm), and my feeling is that such small current won't hurt Nexperia LVC buffers. + +1.12.2.2.1. Signal pull-up/down directions + +Of the 4 UART signals going from FT2232x to Calypso (see section 1.12.1), TxD +and TxD2 MUST be sensed high by Calypso when no USB host is connected, hence +they will need the two chained LVC buffers with pull-ups to VBAT in between, as +described above. RTS and DTR are less critical in that either high or low level +would be acceptable, as long as it is stable and not floating. However, it is +slightly preferable to have DTR pulled up like TxD (to be sensed as logically +negated), and to have RTS (flow control signal going to CTS_MODEM) pulled down, +to be sensed as logically asserted. + +The approach chosen by the Mother is to have TxD, TxD2 and DTR pulled up to VBAT +as described above, and to have RTS pulled down to GND. The value of the pull- +down resistor to GND will be 47 kOhm, and the USB-side-powered LVC buffer can be +skipped for this signal - that buffer is only needed in order to work correctly +with the pull-up to VBAT. + +With only 3 FT2232x-to-Calypso signals needing to go through a USB-side-powered +LVC buffer, and with 3 slots remaining available in the 74LVC541A buffer from +section 1.12.2.1, that one 74LVC541A IC can serve both signal directions (5 +signals from Calypso to FT2232x and 3 signals going the other way), reducing the +component count.