BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
364
Section 11: Generic/Boot Bus
Document
1250_1125-UM100CB-R
B
US
W
IDTH
The data width for a region is set in the
io_ext_cfg
register. If a region is in multiplexed address/data mode it
can be set to have a data bus 8 or 16 or 32 bits wide. Regions using non-multiplexed mode must always be
configured for 8 bit widths. The interface will pack or unpack data as required. The generic bus is always
ordered in a big endian manner, so the lowest memory location is on IO_AD[31:24]. If the region is configured
as 16 bit wide the odd addresses will be on IO_AD[23:16], for 32 bit regions IO_AD[15:8] and IO_AD[7:0] are
added to carry the bytes with a[1:0] equal to 2 and 3.
Writes to the generic bus can be 1-8 bytes direct from the CPU, 1-32 bytes from an uncached-accelerated
merge from the CPU or Data Mover, or 32 bytes when a cache line (which must be cacheable non-coherent)
is moved. The interface will order these and align them so that they are written on the generic bus in ascending
address order (this is useful when writing to FIFOs on generic bus devices).
Reads from the generic bus will similarly be broken down into requests that match the width of the region.
Again the requests for a single read will be made in ascending address order.
If a multiplexed address/data bus is used then the upper four bits IO_AD[31:28] are used during the address
phase as byte lane enables. These active low signals are used when doing byte accesses to 16 or 32 bit
regions, and for half-word accesses to 32 bit regions. The mapping from the byte enables to the byte lanes is
shown in
.
G
ENERIC
B
US
T
IMING
The timing of the generic bus is configurable for each of the chip select regions. When multiple devices are
connected, care must be taken to ensure that the timing for one device does not confuse other devices. The
chip select signals and idle time between cycles will normally be sufficient to ensure this.
Internally all the timing is done with reference to the 100 MHz clock (this clock is also used by the SMBus and
serial interfaces). This clock will be output on the IO_CLK100 pin if enabled by the reset configuration resistor
on IO_AD[23]. In most cases the clock is not needed externally and the interface is run as an asynchronous
one using the computed timings given in the Hardware Data Sheet.
There are two modes for timing cycles. In fixed cycle mode the timing of the access is entirely controlled by
the generic bus interface based on the parameters in the configuration registers. In acknowledgement based
mode a ready/busy signal from the device controls the length of the access. The mode and active sense of the
ready/busy signal is programmed in the
io_ext_cfg
register.
shows the parameters that can be set to control the cycle time. The next four sections
have timing diagrams that show how these parameters control the cycle. Each timing diagram includes the
IO_AD timing for both multiplexed and non-multiplexed operation. On reset the parameters for the IO_CS_L[0]
region are suitable for a slow EPROM or flash memory (the parameters for the other regions do not get useful
defaults). The parameters are programmed into the
io_ext_time_cfg0
and
io_ext_time_cfg_1
registers and
are expressed in cycles of the 100 MHz reference clock (i.e. multiples of 10ns).
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