FreeCalypso > hg > freecalypso-schem2
view venus/doc/USB-and-mobile-domains @ 79:cf39d9352394
R407 intnoconn: use different terminal numbering on further thought
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Sun, 05 Dec 2021 03:49:14 +0000 |
parents | 32b848a081a3 |
children | beb6519a3be5 |
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1. Introduction FC Venus board will consist of two principal circuit domains: the mobile domain corresponding to a battery-powered mobile phone and a USB computer interface block which becomes the USB domain. The USB domain will be based around FT2232D USB to dual UART chip, it will be strictly bus-powered (never taking any power from the battery), and its sole purpose is to provide a host computer interface to the mobile, i.e., make the two Calypso UARTs accessible via USB. The USB domain will get powered only when a host computer (or a USB charger) is connected, and will be totally unpowered at all other times, i.e., when the board is untethered and runs on battery power like a true mobile phone. At the top level of circuit hierarchy (see src/top/board.v), our board consists of the two principal domains (mobile and USB) interconnected with just a few wires: common ground, the set of UART signals going between the two domains, a charging power supply rail (user-switched, so charging won't always happen when a USB host is connected) and a couple of boot control signals. 2. UART signals between domains The connection of UART interface signals between USB and mobile domains is arguably the most challenging and the most complicated part of our entire built-in USB-serial arrangement. The challenges and the circuit complexity which we came up with in response to these challenges stem from the need to avoid bad things happening in partial power-down scenarios. Most of the background information is provided in section 1.12.2 of our FreeCalypso Handset Specification (FC-handset-spec in freecalypso-docs), and this document provides further details that were elucidated only in the process of capturing the actual circuit design for FC Venus. 2.1. Set of signals The set of UART interface signals implemented on FC Venus will be the same as listed in FC Handset Specification section 1.12.1. We have a total of 5 signals running from Calypso outputs to FT2232D, and a total of 4 signals running from FT2232D to Calypso inputs. 2.2. Buffer ICs Two Nexperia LVC family buffer ICs will be used on FC Venus for the purpose of ferrying our set of UART interface signals between the two domains. These two ICs are U401 (74LVC125A) in the mobile domain and U705 (74LVC541A) in the USB domain. U401 is powered from the mobile domain's Vio rail, and it serves only the 4 signals running from FT2232D to Calypso inputs, neatly using all 4 slots of 74LVC125A. This LVC buffer serves as a barrier, preventing feeding of power from USB domain outputs into the Calypso+Iota chipset's Vio rail in the PPD scenario where a USB host is connected, but the charging switch is off or the battery is critically low and undergoing precharge, with the chipset switched off. No corresponding mobile domain buffers will be used for Calypso outputs: these 5 signals will be connected directly between Calypso pads and the interdomain interface, where they will go to U705. On the USB side, U705 will be powered from a USB domain 3.3V rail, i.e., the output of a 3.3V regulator powered from USB. (This USB domain supply will be the only part of FC Venus board using 3.3V - there are no regulated voltages higher than 2.8V in the mobile domain.) The 8 slots of this 74LVC541A buffer are split between 5 serving the Calypso to FT2232D signal direction and the other 3 serving FT2232D to Calypso signals. The purpose of the 5 U705 buffer slots serving the Calypso to FT2232D signal direction is to stop the flow of current out of Calypso UART and GPIO outputs in the "normal mobile usage" PPD scenario when there is no USB host connected. With this buffer present, Calypso UART outputs will "see" a high impedance and won't source any battery-drawn current (beyond leakage of about 0.1 uA typical per pin, according to Nexperia datasheet), no matter whether the USB domain is powered or not. OTOH, if this buffer were omitted and Calypso outputs were connected directly to FT2232D inputs, the latter inputs would appear as shorts to ground when USB power is absent, putting an unacceptable load on Calypso outputs. The other 3 slots of U705 serve FT2232D-to-Calypso signals TxD, DTR and TxD2. On these 3 signals there are pull-up resistors to VBAT in front of U401 inputs (see section 2.4), and the U705 buffer's Ioff feature will allow those pull-up resistors to work as intended. If these pulled-up-to-VBAT nets going to U401 inputs were sourced directly from FT2232D outputs without going through U705, the powered-off FT2232D chip's I/O pins would present a lower resistance to GND than the pull-up to VBAT, drawing excessive current from the battery and making the pull-up ineffective. There is just one FT2232D-to-Calypso UART interface signal that does not go through U705, and it is RTS. It has a pull-down resistor to GND instead of a pull-up to VBAT in front of its U401 input. With only 8 signals rather than all 9 needing to go through a USB domain LVC buffer, the 8 slots of a single 74LVC541A IC become sufficient. 2.3. UART signals from Calypso to FT2232D In the case of these 5 signals running from Calypso outputs to U705 inputs, one very noteworthy detail is that there are _no_ pull-up or pull-down resistors on these nets. When the Calypso+Iota chipset is switched on, all 5 Calypso outputs will be actively driving and there are no problem issues. However, in the PPD scenario where a USB host is connected, but the Calypso+Iota chipset is switched off (the charging switch is off or the battery is precharging), the absence of pull resistors suggests that U705 inputs will be floating, which certainly looks bad at first glance. However, a more detailed analysis shows that floating CMOS inputs aren't bad "in principle", but instead they are bad when they can float around the input switching threshold, causing potential oscillations and high current draw from the buffer's Vcc supply. And in the present case, the Mother's analysis suggests that the signals in question won't float around the switching threshold - instead we expect them to be sensed as a consistent logic low in the PPD scenario in which they appear to float. In this PPD scenario, the Iota chip's VRIO regulator is off, thus the Vio rail will be at 0 V unless something feeds wayward power into it. Calypso chip outputs connected to the signal nets under consideration have clamping diodes to this Vio rail. Therefore, if a stray voltage somehow accumulates on those nets that is high enough to approach the LVC buffer's input switching threshold, it will also be high enough to drain to Vio through Calypso I/O pad clamping diodes, and ultimately to GND. What is our reason for not adding pull-up or pull-down resistors to these nets? Besides the obvious reason of not wanting to throw even more PCB real estate at this already way overcomplicated USB-serial interface arrangement, each of the two options (pull-up or pull-down) would introduce its own problems. The pull-up resistor option has already proven to be problematic on the DUART28+Caramel2 combo, particularly in the Luna configuration in which both PPD scenarios become real. When the Calypso+Iota chipset is on but there is no USB power present, current from Calypso outputs flows backwards through these pull-up resistors and feeds into the USB domain's power rail to which these resistors are connected. The USB domain output buffer then powers up erratically, feeding garbage to Calypso inputs and preventing Calypso from entering sleep modes. And in the opposite PPD scenario (USB power present, chipset switched off), enough power can flow through these resistors into Calypso I/O pad clamping diodes to feed into Vio and cause erratic LED behaviour! And on FC Venus we would have the additional problem of not having a USB-powered 2.8V regulator, as we don't need it for anything else. Pull-down resistors to GND would be much less problematic. However, if we use a resistor value that is low enough to be better than floating, such as the "canonical" 100 kOhm, we would be introducing a *continuous* drain on the battery in normal operation. Calypso outputs corresponding to host RxD, RxD2, DCD and RI will normally be high, thus each of those would be a 28 uA drain. All 4 combine into a 112 uA continuous drain for no good justification, always present whether or not there is a USB host connected, unlike the other similar drain discussed in section 2.4. Therefore, the Mother's decision is to do away with pull resistors in either direction, and hope that my reasoning is correct in that we won't get prolonged floating voltages around the input switching threshold. Because these seemingly-floating nets are expected to be sensed as logic low, the same logic low will be propagated to FT2232D inputs, which means a break condition on RxD and asserted state on all control signals. For this reason the Mother's earlier software recommendation still holds: when operating in this scenario, only operate on the second UART channel (the data leads only one that also has boot controls), and don't open the AT command UART channel except when the mobile is switched on and regular operational firmware runs normally. 2.4. Pull-up resistors to VBAT Experience with Caramel2 boards shows that Calypso UARTs don't like seeing a break condition presented to them for extended lengths of time, thus we need to design our FC Venus USB-serial interface in such a way that when there is no USB host present, Calypso inputs RX_MODEM and RX_IRDA will receive high levels (meaning RS-232 idle) rather than low (meaning RS-232 break). Having an extended break condition presented to UARTs prevents entry into Calypso sleep modes, which is obviously bad. The practical implication is that U401 inputs which this buffer propagates to Calypso RX_MODEM and RX_IRDA need to be have some kind of mobile domain pull-ups on them, as opposed to floating or pulled down. Less critically, we do the same for the DTR signal running from the USB domain to the mobile domain - thus DTR will be seen as negated when there is no USB host connected. However, if we were to implement pull-ups to the Calypso+Iota chipset's Vio rail on these nets, we would have a problem in the PPD scenario of USB power present, chipset switched off: U705 high outputs would feed power into Vio through these resistors, potentially causing Calypso peripherals to turn on erratically as has been observed on Caramel2 boards. Therefore, we are making a more unusual innovation: instead of pull-ups to Vio, we are implementing pull-ups to VBAT, i.e., to the unregulated raw positive battery terminal. The resistor value chosen by the Mother is 22 kOhm, and the rest of this section explores the implications of this unusual design under all expected conditions. First let us examine what happens in the case of "normal" mobile usage: the device is untethered and mobile, powered by the battery, and there is no USB host or charger connected. In this case the input voltage seen by U401 will be raw VBAT, which will always be higher than its 2.8V supply (but still safely below the 5.5 V limit), thus the LVC buffer will sense a good logic high as desired. The current drawn from the battery through these pull resistors will be the sum of U705 Ioff and U401 Ii, each of which is listed as 0.1 uA typical in Nexperia datasheets. The listed worst case specs are 10 uA for Ioff and 5 uA for Ii, thus under presumably very rare conditions, we could have a maximum current draw of 15 uA per each of the 3 pull-ups. 15 uA times 22 kOhm equals a voltage drop of 330 mV, thus U401 inputs will remain at 2.8 V or above (meaning no increased Vio supply current draw) with the battery as low as 3.1 V, which is below the operational range for the mobile. Now consider what happens when USB power is plugged in, either a host computer or a charger. FT2232D will power up in UART mode and start putting out logic high on all of its outputs (the state when no one opens either of the two serial ports in host computer software); U705 will also power up and start putting out the same logic high on the 3 signal nets going to the mobile domain. At this point U705 output pads are no longer in their Ioff state, instead they are in the actively driving high state, and Nexperia datasheet says that the voltage at these output pins should not exceed Vcc, which is 3.3 V in the present case. VBAT can be as high as 4.2 V, thus we are operating the 74LVC541A under somewhat adverse conditions. The undesirable adverse condition is that current will flow through the LVC buffer's output p-channel MOSFET (open for driving high) in the wrong direction. The maximum magnitude of this current is about 41 uA, computed as (4.2-3.3 V) / 22 kOhm. This current magnitude is expected to be far too low to damage the 74LVC541A IC (the limiting values Iok spec is 50 mA), thus the only remaining concern would be feeding of a higher voltage into the USB domain's regulated 3.3V rail. This concern is addressed in the following subsection. Aside from the issues of reverse current flow through the LVC buffer's output p-channel MOSFET, the other concern with this 41 uA current (at maximum VBAT) is that this current is drained from the battery. With a total of 3 such pull-ups, we expect a total battery drain current of 123 uA at maximum battery charge - but this current only turns on when USB power is applied, and the current into U705 output pins goes up from 0.1 uA to 41 uA per pin. However, when USB power is applied for extended periods of time, it is usually done for the purpose of charging the battery, and the charging current of 500 mA far exceeds the drain current of 123 uA. In order for the battery to not be charging when USB power is present, the system would have to be in one of two other states: either the user connected USB for computer interface purposes and not for charging (the charging switch is off), or the charge cycle finished but the user hasn't unplugged the charger yet. Both of these conditions are not expected to persist for unreasonably long time spans, hence the Mother deems it acceptable to impose this 123 uA battery drain during times of active computer interfacing. In the case when the charging switch is on and the charging cycle has finished, it is also important to consider that Iota ABB sleep is prevented for as long as VCHG remains applied to the chipset, which holds whenever USB power is present and the charging switch is on. In this state (charging cycle finished, but VCHG not removed yet) Calypso can enter deep sleep (the main VCXO is stopped), but Iota ABB sleep is prevented. According to TI's APN2_110.pdf document, the difference in battery power draw between deep and "superdeep" sleep modes is about 600 uA - thus leaving the charger plugged in after charge cycle completion causes even more battery drain through this other mechanism than through our VBAT pull-up resistors in the USB-serial interface. Finally, whenever any of the 3 USB domain outputs in question (TxD, DTR or TxD2) drives low, the current drawn from VBAT on that net increases from 41 uA to about 190 uA at maximum VBAT, computed as 4.2 V / 22 kOhm. However, this condition only occurs when the operator does active host computer interfacing with software (the main AT command UART needs to be opened in order for DTR to go low, and each of the two TxD lines goes low only when the host computer actively transmits bytes), thus this increased battery current draw corresponds to explicit user activity, rather than something that happens on its own. In contrast, typical user activity on an untethered phone involves the LCD and its backlight turning on, consuming tens of mA instead of hundreds of uA. It also needs to be noted that all of these currents are drawn directly from the battery without going through Iota LDO regulators, thus none of these uA numbers come out of the 1 mA sleep mode Vio current budget. 2.4.1. USB domain 3.3V load resistor One concern raised in the analysis of VBAT pull-ups is that when current flows from the battery into the USB domain's P_3V3 rail through U705 outputs in reverse, the effect may be to raise that rail above 3.3 V. To prevent this effect, the Mother's idea is to add a load resistor to the USB domain, a 3.3 kOhm load permanently placed between P_3V3 and GND, producing a permanent 1 mA draw from USB, going through our 3.3V regulator. For comparison, FT2232D is said to draw about 30 mA of supply current, although the datasheet does not specify how it breaks down between Vcc and Vccio supply currents, hence most of this 30 mA is probably drawn directly from USB 5V, without going through our 3.3V regulator. The addition of 1 mA to the overall USB current draw can be considered insignificant (especially for 500 mA charging, but even for 30 mA host computer interfacing), but with a guaranteed load of at least 1 mA on P_3V3, any wayward current coming from the battery (123 uA maximum) will always be absorbed into this load without raising P_3V3 above its intended 3.3 V. 2.5. Pull-down to GND on Host_RTS The effect of Host_RTS being sensed as low instead of high when there is no USB host connected is that any unsolicited output on the AT command channel will be flushed in this no-host-connected state, instead of being buffered in the mobile. Given that an end user mobile phone (as opposed to a dedicated cellular modem) is expected to be untethered most of the time, having buffered output accumulate while waiting for a host computer to be connected "some day" does not sound like a good idea, hence having it flushed by unblocked flow control state soundly appears to be the better option. And with only 3 out of the 4 FT2232D to Calypso signals needing pull-ups to VBAT, only 3 need U705 buffer slots, making a single 74LVC541A buffer IC sufficient for both directions in the USB domain. The value of the pull-down resistor on the Host_RTS net (running from FT2232D ADBUS2 output to U401 input) will be 47 kOhm. When USB power is applied but no one opens the AT command UART channel, FT2232D will drive its RTS output high, sourcing about 70 uA of current from its USB 3.3V supply into the pull-down resistor - well within FT2232D output current sourcing budget. 3. Boot control signals The two boot control signals RPWON and nTESTRESET properly belong in the mobile domain. They are pulled up to VBAT inside the mobile domain (RPWON is pulled up to VBAT inside the Iota chip, nTESTRESET is pulled up to UPR with R208 per Leonardo schematics), many other designs don't provide any way at all for these signals to be pulled down, but on FC Venus we have a dual OD buffer (74LVC2G07) in the USB domain that can pull either signal down on host command. This dual OD buffer is little different from a pair of dry contact pushbutton switches shorting to GND, thus the two boot control signals don't bring on any of the difficulties like those associated with the UART interface.