[Libre-soc-dev] daily kan-ban update 11mar2022

[Luke Kenneth Casson Leighton]

On Sun, Mar 13, 2022 at 9:41 AM Tobias Platen <libre-soc at platen-software.de>
wrote:

> I have a 3.3v uart cable and will try to get it running using that
> uart.
>

yes, you'll need that.  or, a diode-circuit (the diode goes the
"wrong" way round).

UART works by pulling to GND, it's quite odd.  therefore
a diode in series (placed in the *opposite* direction from
the one you'd *think* was obvious) will drag the pin down.

when the voltage of the UART is higher, the diode *protects*
the IC by preventing the higher voltage flowing into the IC.

but yes you have to use 3.3v for this (and then make sure
the constraints file actually say 3.3v for those pins).

you also need to configure the UART to ***RECEIVE***
3.3v power ***FROM*** the ECP5 FPGA, like this:

https://libre-soc.org/HDL_workflow/ft232.png

basically what is happening there is that you need the
ECP5 FPGA to provide the 3.3v power as a "Reference"
(VREF).

if you attempt to provide 3.3v power to something that
is already providing 3.3v power, you will end up with a
dangerous current-fight, and either the UART or the FPGA
will lose.

l.