On Sat, Sep 20, 2008 at 2:56 AM, Trent Piepho <tpiepho at freescale.com> wrote: > On Sat, 20 Sep 2008, Hector Lahoz wrote: >> >> Ok, we won't use interrupts. Now, I'd like to have some kind of signal >> that warns about CPU going above max temperature. For example, through >> sensord. But the problem is that some seconds after the alarm is >> triggered the ALARM message of sensors does not appear any more. >> Shouldn't the alarm bit remain set until it is read? And if it's only >> read by show_* functions (i.e. it87_update_device() ), why does it go >> away? > > It might depend on how the device is programmed. For example, with the LM63 > in it's default alarm mode (which the driver neither checks nor sets), > alarms > work like this: > > When an alarm condition occurs the bit in the alarm register is set. It > remains set until the register is read. It whatever condition caused the > alarm goes away, it still says set. > > When the alarm register is read, it is cleared (and the alert# IRQ pin is > disabled). It doesn't matter if the condition still exists or not, the > register is always cleared. > > When the chip does its next temp conversion, which it does automatically 16 > times per second by default, it will again set the alarm bit in the register > if the condition still exists. However, the alert# pin will remain > disabled. It needs to be turned back on manually, which the driver doesn't > do. > > If something besides the lm63 driver (like i2cdump or ACPI) reads the alarm > register before the driver does then the driver might not see the alarm. > > The LM63 can be put into alarm comparator mode. In this mode the alarm bit > remains set after the register is read. If the condition no longer exists > after a temp conversion, the bit is cleared, even if the register hasn't > been > read yet. In this mode it's not possible to not see an alarm if it exists > when the driver checks, but it's also not possible to see an alarm for a > condition that no longer exists. > The it87 behaves in a similar way. I think hysteresis doesn' make any sense if not using interrupts, so I tried setting low temp to 127 deg. Celsius (comparator mode) and now it works fine. I don't understand yet why the bit is cleared. Either the chip doesn't follow the datasheet or there is another process/subsystem reading the chip register. But it works now, that is what matters. Thanks
