[time-nuts] Question for my new GPSDO

[Attila Kinali]

On Mon, 14 Oct 2019 20:46:14 +0000
Tobias Pluess <tobias.pluess at xwmail.ch> wrote:

> For sure the 1PPS output will be generated in hardware. I have made good 
> experiences with using a microcontroller timer: One can use an ordinary PWM 
> channel and this generates very precise pulses, and no interrupts or the 
> like are involved. And since the timer can be clocked directly by the 10 MHz 
> signal, we can even avoid PLL jitter.

This does not really work with the STM32F40x. The timer clocks are derived
from the APB clock, which in turn is derived from the AHB clock which
defines the core clock speed. So, unless you are willing to clock the
whole uC (safe for the USB interface) with 10MHz, you will end up
with the PLL in there.

Unfortunately, you will need to run the timer at higher speed (see
my previous mail).

An alternative would be to lock a 40MHz crystal to the OCXO
and use this to clock the STM32F40x (max input clock freq is 50MHz).
The added noise due to the 40MHz crystal and PLL should be negligible
compared to the noise/jitter from the uC itself.


				Attila Kinali

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