ST18-AU1 Datasheet, PDF (48/87 Pages) STMicroelectronics – SIX-CHANNEL DOLBY AC3/MPEG2 AUDIO DECODER

ST18-AU1
12 BUS SWITCH UNIT
The bus switch unit (BSU) is a bi-directional switcher. It switches the 3 internal buses (I, X and
Y) to the external (E) bus.
12.1 BSU control registers
The BSU is programmable via six control registers mapped in the Y-memory space. These
define the type of memory used, internal to external boundary address crossing, exchange
type (external direct or through the BSU) and software wait-states count.
There are 2 registers per memory space, making it possible to define 2 sets of boundaries and
wait state numbers.
Figure 12.1 Default and user mapping examples
64K
EXTERNAL
63K
INTE RNAL1
62K
INTERNAL 0
0
DEFA ULT MAPPING (RESET)
64K
EXTE RNAL
VALUE 1
INTERNAL1
VALUE 1
VALUE 0
INTERNAL0
VALUE 0
0
USER MAPPING
(CAN CHANGE BY 1K STEP)
The BSU control registers include a reference address on bits 4 to 9, where the internal/
external memory boundary value is stored (see Figure 12.1), and software wait-states count
on bits 0 to 3, allowing up to 16 wait-states.
External addressing is recognized by comparing these address bits for each valid address
from IA, XA and YA, to the reference address contained into the corresponding control
register.
If the address is greater or equal to the reference value, an external access proceeds.
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