Hi,
FT232 chip should have level converter in it.
If you worry about 3.3V to 1.8V could damage the board, you can connect them with level converter.
I use this setup for 2 years, just from my beginning to board porting.
3.3v is far beyond the specification of the pins. Even if it works most of the time it cannot be described as “safe”.
Whether it works will depend on a number of things such as how much current you adaptor can dump into an over volted pin and what logic level you adaptor observes 1.8v to be (in TTL logic 1.8v is an indeterminate value; neither 0 nor 1).
If I talk about FT232R datasheet at https://www.ftdichip.com/Products/ICs/FT232R.htm :
- Integrated 3.3V level converter for USB I/O .
- Integrated level converter on UART and CBUS for interfacing to 5V - 1.8V Logic.
- True 5V / 3.3V / 2.8V / 1.8V CMOS drive output and TTL input.
My cable is TTL-232R-3V3. I am not sure about this cable. The datasheet is also not clear. Not sure if 232R is same as FT232R
OK. So I read the datasheet for TTL-232R cables and the very first line says
"Each TTL-232R cable contains a small internal electronic circuit board, utilising the FT232R, which is
encapsulated into the USB connector end of the cable. "
and since FT232R datasheet says it has integrated level converter for 1.8V interfacing, it means it should work on Hikey with no need for external level converters. I will now go ahead and connected TX-RX and RX-TX and try again. hopefully, it should work.
Moral of the story “read datasheets for all the confusions” 
Specification also says (about VCCIO pin):
+1.8V to +5.25V supply to the UART Interface and CBUS group pins
(1...3, 5, 6, 9...14, 22, 23). In USB bus powered designs connect this pin
to 3V3OUT pin to drive out at +3.3V levels, or connect to VCC to drive
out at 5V CMOS level. This pin can also be supplied with an external
+1.8V to +2.8V supply in order to drive outputs at lower levels. It should
be noted that in this case this supply should originate from the same
source as the supply to VCC. This means that in bus powered designs
a regulator which is supplied by the +5V on the USB bus should be used.
So, in short, voltage on VCCIO defines the TTL level, and I suppose it’s hardwired depending on cables/packages (TTL-232RG-VREG1V8-WE, TTL-232R-3V3…).
If you have doubt on voltages I assume you can only connect GND and board TX (PC RX) pin.
Please, also ensure you do not enable any flow control on PC side.
@yuhsien-chen Have you also connected the Vcc cable from FT 232 to Hikey in addition to TX, RX and GND?
Because, as Loic pointed out, the source for Vcc should be same , so if I understood correctly Vcc of FT232 cannot be connected to Hikey directly.
Also worth noting is that a lot of the “FTDI breakout” boards do NOT adhere to the voltage they claim to. One that I have which is supposedly a “3.3v” is actually running the I/O lines at 3.8. I would never hook that up to any 96boards SBC (and funny thing is that it was actually delivered WITH a hikey960! Scary!). For me that particular board is for ESP8266. While still concerning that it is over spec, ESP8266 are so cheap (<$1) that if I did cook one, I wouldn’t lose any sleep over it.
I don’t connect VCC cable to hikey.
Just need TX/RX and GND.
That can show uart message.
I finally got the logs on serial with hikey…with connecting just 3 cables RX, TX and GND. Looks like the assumptions were correct.
But it didn’t last long.
After 2-3 minutes I started getting random characters. Didn’t do anything, it happened itself.
Now I get random characters on minicom even if I bring my hand close to my laptop or even touch the rubber portion of the cable. Even if I touch the metallic legs of my table on which my setup lies, I get random characters on screen. If I leave the setup idle,nothing comes on minicom.
I am not an electrical/electronic engineers but I guess some static electricity issue is there… I am confused what to do now. Facing this first time in my life.
Confused whether to laugh or bang my head, I can fix those nasty kernel issues, but this is really annoying me now. 
Seriously demotivating.
I’m pretty sure it comes from the incorrect IO voltage level (or issue with GND connection).
You just have to use a correct UART converter.
You can also use opt for the cheap audio mezzanine, which among other things, includes an FTDI:
I would be really thankful if you can point me to the url of a supported serial converter. (Please suggest the cheapest and reliable)
The audio mezzanine doesn’t look to be a good investment as its not completely compatible as per this post : Use of Audio Mezzanine with Hikey 960.
Please suggest a working Alternative chip.
It’s not a so bad investment for $5, and you gain level shifters on I/Os at the same time, though the audio part will not be functional. You can also use the UART mezzanine or any 1V8 FTDI (e.g. TTL-232RG-VREG1V8).
Will logic level converters e.g. TXS0108E work?
Because its not just UART, later some day, I will need level shifting for other GPIO pins. And logic level converters are generic as they can be used with any board. Not like audio mezzanine or 960 UART adapters which are specific to hikey.
Are logic level converters helpful here with Hikey?
AFAIK, there is common ground among USB bus and hikey (with GND cable connected), so a 1V8 output from LS connector [1] on hikey should work for VCCIO pin.
Can you please tell the GPIO pin number to which you are referring as LS Connector [1]…Pin 35?
Pin 35 isn’t attached to a GPIO. It is simply a reference voltage (although on some 96Boards the power management chip needs kicking to configure the pin).
No.
TX and RX carry signals at 3.3v or higher. This is OUTSIDE OF SPEC and could DAMAGE YOUR BOARD.
I’m not aware of ANY board that is “specific to hikey”. The UART mezzanine is compatible with ALL 96boards devices.
And I’m not quite sure why you object to the audio mezzanine. The only feature of that board that is limited to dragonboard is the audio, which will not hinder the rest of the functionality of the board on non-dragonboard SBCs. And for $5 with free shipping, you can’t go wrong with it.
Indeed. This has grown into a surprisingly complex thread given the answer to the question in the subject title can be given in one word (“yes”).
So, for clarity (and especially for future readers of this thread):
When working with TTL UARTs then the TX pin will be at logic 1 when the transmitter is idle. That means you can attach a voltmeter to it. If you are working with an unknown (or otherwise suspect) USB <-> UART device then measure the voltage on the TX pin. If it is much greater than 1.8v then I would not recommend connecting it to any 96Boards device.
