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 11: Generic/Boot Bus Page
373
G
ENERIC
B
US
E
RRORS
There are five error conditions that will raise the io_bus_int interrupt. When the interrupt signal is raised the
associated address, data and parity are stored and the interrupt cause recorded. The causes are:
1
A data parity error is detected on an access to a generic bus region.
2
An access is performed to an address in the generic bus range that does not match any of the configured
regions.
3
An access is performed to an address that matches multiple regions.
4
A region is configured in acknowledgement mode and the external device does not signal ready within the time
limit set in the timeout register.
5
A cacheable coherent access is made to the generic bus and in the response phase of the access an agent
asserts both R_SHD and R_EXC to indicate it has a tag error and cannot resolve the ownership.
Error cases (
), and (
) are detected early so that no external access is made.
After an error has been signalled accesses to the generic bus may continue. However, if another error occurs
before the previous interrupt was serviced by the CPU, it will be ignored. The interrupt will be cleared only when
the
io_interrupt_status
register is read (a cacheable read of the register will not clear the interrupt). To ensure
consistent data, software should therefore read the address, data and parity registers before this status
register.
In the acknowledgement mode, if the access times out because the device does not signal ready the current
transaction will be aborted (in the case of burst mode, the rest of the burst will not be completed). If the access
was a read then data marked with a bus error will be returned. The next request will then be serviced. When
the timeout occurs, the latency from the timeout being detected on the external bus to the bus error being
returned is UNPREDICTABLE, and could be as high as 64K cycles of the IO_CLK100.
D
RIVE
S
TRENGTH
C
ONTROL
The output drive strength and internal slew rate may be set for all the general 3.3V outputs. This covers the
generic bus, the serial ports, the SMBus ports, the Ethernet/packet FIFO interfaces and the GPIO pins. The
drive strength effectively sets the number of transistors that will be used to switch the output. The drivers can
be varied in 2mA steps from a nominal source/sink current of 2mA up to 8mA for low strength drivers and from
6mA to 12mA for high strength drivers. The slew rate can be varied from fast (recommended default) to slow
in four steps. The drivers are grouped by function for setting the drive strength, and into larger groups for
setting the slew rate. The parameters are described in more detail in the Hardware Data Sheet.
The drive strength registers for all of the general 3.3V outputs are included in the generic bus configuration
register region of the address space.
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