[time-nuts] GPScon running on Raspberry Pi 3b

Andrew E Mileski andrew.mileski at gmail.com
Mon Jun 26 15:52:59 EDT 2017

On Mon, Jun 26, 2017 at 2:04 PM, Andrew E Mileski <andrew.mileski at gmail.com>

> The RaspberryPi 3 is a bit different.
> All the Pi models, 1 & 2 & 3, have two UARTs:  a full-featured UART, and a
> mini-UART.
> On the PI 3, the header pins for the serial port are routed by default to
> the mini-UART, and the full-featured UART is used for Bluetooth.  On the
> older Pi models, the full-featured UART is routed to the header,
> It is possible to swap them back via software, but then you lose Bluetooth
> functionality.
> The mini-UART has issues with its baud-rate generator:  it is derived from
> the system clock, not an independent clock generator like the full-featured
> UART, so it doesn't work when dynamic system clocking for power management
> is enabled, so you either have to run at max or minimum speed only.
> FWIW, I'm running a custom Fedora 25 on all my Pi3, as the default doesn't
> support all Pi3 devices (and I don't want the U-Boot boot-loader either).
> I'm using cheap $0.99 USD CH340 based USB-to-Serial converters from E-Bay
> (I bought 10), as well as my bench equipment which are mostly FTDI (hacked
> some to use standard VendorId and DeviceId) or PL.
> Beware that USB-to-Serial converters are pretty horrible for precise
> timing applications, like PPS on the CD pin.  It can be done on a GPIO pin
> instead, but requires some hacking and re-compiling.

I should have cited my sources:  most details can be found on the RPi
Foundation's web site at:


Google:  raspberry pi mini-uart

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