Re: [Qemu-devel] [PATCH 2/2] hw/armv7m_nvic: Add global variable for SysTick external reference clock

[Peter Maydell]

On 12 October 2012 07:53, Andre Beckus <address@hidden> wrote:
> As a case study, the STM32 does have a reference clock.  It is simply
> the system clock divided by 8 (maybe not ARM's intention for it to be
> tied so closely to the system clock).  The documentation says the TENMS
> field is hardwired to 9000, which corresponds to a 1 ms period when the
> external reference clock is selected and the system clock is running at
> 72 Mhz.  So, the TENMS field will not be accurate if the system clock is
> running at a different frequency (the SKEW bit is hardwired to 1).

OK, so the board needs to be able to separately set all of:
 * TENMS calibration field
 * system clock
 * reference clock

> Looking at the big picture, it seems that QEMU could benefit from a new
> "clock line" type for handling clock signals.  They could be exposed by
> devices in a similar manner to GPIO lines (there would be both input and
> output clock lines).  I could see them being useful (at least in the
> microcontroller world) for passing clock signals back and forth between
> peripherals, interfacing timer peripherals to machines, setting
> oscillator frequencies, and serving as the plumbing for clock trees.  I
> know I had to do a lot of hacking with the STM32 implementation to
> propagate the clock controller's signals to the other peripherals.
>
> When I searched on the topic, I saw that you discussed/requested a
> common clock framework back in July (in regards to the an exynos4210
> patch).  Do you know if any progress was made?

I haven't seen anything since then... I agree that a 'clock line'
connection might be useful.

-- PMM