On Sonntag, 25. Februar 2018 12:32:56 CET Achim Gratz wrote:
I am using a GPS as a reference clock for NTP. It's currently attached via a USB serial adapter, but the PPS timing precision is limited by the USB interrupt handling. I want to see if things get better by attaching it to the (only) RS232 port, but AFAIK that is used (optionally? I think) for a serial console during boot. The running system doesn't seem to use the serial, at least I can find no getty process attached to it. So, do I need to do anything to disable the serial console and if so, waht exactly?
Is the PPS signal wired to one of the handshake lines? Depending on the converter, the line status registers may be signaled using interrupt transfers, are "streamed" using bulk transfers, or have to be polled. Prolific (pl2303) and CH34X support interrupt transfers for line status signaling, while Silabs CP210x and FTDI apparently do not. If the converter uses interrupt transfers, the polling rate (all USB "interrupts" are message-signalled, i.e. happen at the protocol level, not on the physical layer) varies. The latency is at least/typically one frame (1ms for Low/Full Speed). Silabs CP210x are probably the worst, as status changes have to be explicitly polled by issuing a register read. FTDI puts the status in the header of the normal data transfer. As the Bulk IN enpoint is continously polled for available data (~33us) it should be able to signal changes in the line status with at most 33us latency. Hope this helps, Stefan -- Stefan Brüns / Bergstraße 21 / 52062 Aachen home: +49 241 53809034 mobile: +49 151 50412019