Date: Fri, 25 Nov 2016 09:49:24 -0700 From: Ian Lepore <ian@freebsd.org> To: "Rodney W. Grimes" <freebsd-rwg@pdx.rh.CN85.dnsmgr.net>, Hal Murray <hmurray@megapathdsl.net> Cc: freebsd-arm@freebsd.org, Michael Sperber <sperber@deinprogramm.de> Subject: Re: Can't get 11.0-RELEASE to boot on Banana PI M3 Message-ID: <1480092564.1889.70.camel@freebsd.org> In-Reply-To: <201611251642.uAPGgJmX022074@pdx.rh.CN85.dnsmgr.net> References: <201611251642.uAPGgJmX022074@pdx.rh.CN85.dnsmgr.net>
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On Fri, 2016-11-25 at 08:42 -0800, Rodney W. Grimes wrote: > > > > > > sperber@deinprogramm.de said: > > > > > > Ah, thanks ... but that's not standard RS232, right? (BPI > > > homepages says > > > "TTL".) If it isn't, what kind of hardware connects to that? > > The normal setup for RS232 is that the transmit and receive signals > > come out > > of a big chip (SOC, or PCI UART, or USB UART, or ...) and then go > > through a > > level converter which is typically a MAX-232 or one of many clones > > or > > variants. The "TTL" is telling you that it doesn't have that level > > converter > > chip. > > > > You can either add a level converter chip and then plug it into a > > real RS-232 > > port, or find some setup that also doesn't have the level converter > > and > > speaks TTL levels. Adafruit and probably many others sell a USB > > UART without > > the level converter for applications like this. > > https://www.adafruit.com/product/954 > > > > Sometimes, TTL means 3V CMOS levels and 5V from real TTL/CMOS will > > fry your > > expensive chip. Best to check carefully. The above part says > > 3V. It also > > has an extra power wire that you get to ignore. > Be SURE to ignore that extra power wire! If your USB/Serial adapter > also has > a power wire DO NOT CONNECT IT. Many of these embeded boards provide > a power > pin with the serial interface that can be used to power something > external, > like a level shifter, and many of the USB/Serial adapters also bring > out the > USB 5V rail on a wire. DO NOT CONNECT THE TWO! > Ummm... say what? I power my rpi boards using the 5v power from the USB serial adapter connected to the 5v pin on the rpi header. I can't imagine any reason not to. > That being said, there are many aftermarket USB/Serial cables > avaliable, > usually a 3.3V version of these well work everywhere as long as it > has > 5V tolerent inputs, which most of the newer ones do, check the specs > from the vendor. 3.3V outputs well satisfy the input requirements of > a 5V TTL/CMOS circuit and not cause it problems, the opposite is not > always true. > Usb serial adapter based on Prolific chipsets are NOT 5v tolerant. Those based on FTDI chips are. Those are the two big names in usb- serial chips, but there are others out there too; you have to check the datasheet to be sure. > Watch your lead length and wire sizes if you need to do anything > funny > to get this connected, capacitive loading of any kind on this type of > signal can cause character loss, especially at speeds above 9600 > baud. > Ummm... that sounds pretty bogus too, considering that I've run ftdi chips at 12mbps using breadboards with a rat's nest of wiring to carry the comms signals to other boards. -- Ian
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