An FTDI 3.3 Volt USB to UART0 connection will not blowup or hurt anything. You an hook it up and leave it hooked up, and not have to worry about damaging the Pocket Beagle, A 10K pullup to +3.3 V is also OK, but not actually required for the FTDI cable to work.
Something like the TTL-232R-3V3-WE for wire pigtails. The damage warning is for things that will try to drive pins with currents high enough to damage semiconductor circuits and/or ESD protection circuits. Or, in the case of the boot pins, something that will over-ride a 100K resistor. --- Graham == On Saturday, December 16, 2017 at 12:19:28 AM UTC-6, Stuart Longland wrote: > > Hi all, > > I have one silly question to start off with… I have a PocketBeagle > coming, hopefully next week, and a Lattice IceStick FPGA development > board with which I'll be teaming it up with. > > The plan is to produce a logic analysis and stimulus tool. I'll be > using GPIOs on both the PocketBeagle and the FPGA to control and monitor > some arbitrary logic circuit constructed on a bread board. > > The project is detailed here: https://hackaday.io/project/28513 > > One question I have though regards the following warning: > > NOTE: Do not connect 5V logic level signals to these pins or the board > will be damaged. > > > > NOTE: DO NOT APPLY VOLTAGE TO ANY I/O PIN WHEN POWER IS NOT SUPPLIED TO > THE BOARD. IT WILL DAMAGE THE PROCESSOR AND VOID THE WARRANTY. > > > > NO PINS ARE TO BE DRIVEN UNTIL AFTER THE SYS_RESET LINE GOES HIGH. > -- > > https://github.com/beagleboard/pocketbeagle/wiki/System-Reference-Manual#71_Expansion_Header_Connectors > > > Now point one is fine. It's a 3v3 device, and I plan to put a voltage > clamp circuit (two resistors, a 3V3 zener and a Schottky diode) in front > of the GPIOs anyway just to prevent accidents. > > Point two worries me a bit from two points: > > 1. UART0 connection to a TTL-UART interface > 2. Pull resistors > > In the former case, it'd be nice to have access to the actual console of > the PocketBeagle. Whether I hard-wire a USB-TTL serial adaptor or > TTL-RS232 driver chip, or whether I just expose the headers for > something to be wired up later I'm not certain, but the fact that TTL > UART always idles high, means that any driver or adapter IC I wire up to > these pins, is *going* to be presenting a voltage on the UART0 RX pin. > > In the latter case, it's common for particular signals to have > pull-resistors attached. Notably I²C, chip select lines, interrupt > lines and reset lines. Again, the moment power is applied, a voltage > will appear on those pins. > > A workaround might be to hook a MOSFET switch up to the aforementioned > SYS_RESET line (which I'm guessing is *actually* called NRST on pin > P2.26; or even a software-driven GPIO), and have that switch turn on > power to the FPGA and other peripherals. > > The FPGA boots up on its own right now, but I plan to remove the EEPROM > on it so that I can just load the SRAM on it directly. I notice though > when the EEPROM is erased that there's a slight glow of the user LEDs on > the FPGA board, which suggests to me that other pins may present a > voltage too. > > 12 pins of the FPGA will be hooked directly up to PRU0 pins exposed on > PocketBeagle, the thinking being this will give me a nice high-speed > 8-bit wide comms link, with PRU0 providing the interface between the CPU > and FPGA. > > How have others handled this requirement of not supplying voltages on > the GPIO pins during power up? > > Regards, > -- > Stuart Longland (aka Redhatter, VK4MSL) > > I haven't lost my mind... > ...it's backed up on a tape somewhere. > -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to beagleboard+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/beagleboard/e48dd7b9-d342-41ce-910e-809a7105c358%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
