User Manual
BCM1250/BCM1125/BCM1125H
10/21/02
B r o a d c o m C o r p o r a t i o n
Document
1250_1125-UM100CB-R
Section 4: System Control and Debug Unit
Page
57
T
IMERS
There are two types of timers on the part: watchdog and general. Watchdog timers can be used to detect the
liveliness of the system and will cause a full system reset if software does not prove its correct operation by
regularly resetting the timer. General timers are available for any use and can be set to generate a regular
interrupt or act as one shot timers. Both sorts of timer count in 1 us steps and have a maximum timeout of over
8 s.
The timers all have a similar programming interface. Each timer module consists of an initial count register, a
down counter which is loaded with the initial count and then decrements, and a configuration register. The
initial count register should only be written while the timer is disabled. Whenever the timer enable bit in the
control register is written with a 1 the down counter is loaded with the initial count and begins decrementing.
When the counter reaches zero an interrupt is raised.
W
ATCHDOG
T
IMERS
The watchdog timers have a 23 bit counter and are clocked by a 1 MHz clock derived from the 100 MHz
reference clock. This allows a maximum count of 8388608, giving the maximum timeout of 8.3888608 s.
A watchdog timer is normally not permitted to timeout. System software should regularly re-enable the timer
by writing a 1 to the enable bit in the configuration register. This will restore the count to the initial count and
force the counter to restart decrementing from there. If the timer is permitted to reach zero then on the next
clock tick an interrupt will be raised, the counter will reload from the initial count and decrementing will continue.
This interrupt warns the system software that it has not been re-enabling the timer. The interrupt will be cleared
by any write to the configuration register. If the timer times out for a second time (i.e. before the interrupt has
been cleared) the default behavior of the watchdog is to signal the system controller to perform a full reset of
the part (and any external devices that take reset from the RESETOUT_L, LDT_RESET_L/HT_RESET_L or
PCI P_RST_L signals). Once the watchdog has signalled a reset it will be disabled.
There are two watchdog timers on the part. They are identical other than using different registers (distinguished
by appending register names with
_0
and
_1
) and different interrupt lines. On the BCM1250 this allows the
liveliness of each processor to be separately monitored. This will be of benefit when the two CPUs are running
loosely coupled (for example when one runs a real-time task and the other does general system control). The
interrupt mapper can be used to route a timeout interrupt from either watchdog to either or both processors
(this allows each of them to monitor the other).
The watchdog can be configured to do a more selective reset when it times out for the second time. It can send
an sb_softres (as described in
Section: “System Control” on page 41
), reset just one CPU, or (on the
BCM1250) reset both CPUs but no peripherals. Care must be taken if this is done, the reset software will need
to probe for devices that are hung or in inconsistent states. The behavior should only be changed from the
default in high availability systems or in some cases where the two CPUs are running distinct operating
systems.
Care must be taken in systems that boot from an SMBus EEPROM, none of the selective resets set the SMBus
channel to boot operation if it has been returned to normal operation. An additional problem with resetting only
a CPU is that it will immediately be removed from the coherence machinery. It is possible that the CPU had
just been detected as exclusive owner of a cache block and was in the process of providing it when reset. Since
the data will never be supplied the ZBbus transaction will never be terminated and the requester will hang.
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