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Re: [Qemu-devel] [PATCH 2/2] hw/armv7m_nvic: Add global variable for Sys
From: |
Peter Maydell |
Subject: |
Re: [Qemu-devel] [PATCH 2/2] hw/armv7m_nvic: Add global variable for SysTick external reference clock |
Date: |
Fri, 12 Oct 2012 09:36:39 +0100 |
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