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TC1796
System Units (Vol. 1 of 2)
LMB External Bus Unit
User’s Manual
13-21
V2.0, 2007-07
EBU, V2.0
Note: In TC1796, only Arbiter arbitration mode is supported in emulation mode.
Furthermore, the CSEMU signal is connected to the CSCOMB chip select output
by default after reset.
13.4.3
External Boot Mode
The External Boot Mode of the EBU allows the EBU to boot (i.e. run all start-up code)
from external memory. Immediately after reset a system may have no knowledge as to
the type of memory connected to the external bus. When External Boot Mode is
selected, the EBU will automatically read a 32-bit Boot Configuration Value from an
external memory (connected to CS0, chip select region 0). The Boot Configuration Value
in the external boot memory makes it possible to initialize the EBU with appropriate
configuration values for the external boot memory. These configuration values will, in
turn, be used for the subsequent read accesses from the external boot memory (i.e.
instruction fetches).
In External Boot Mode, the EBU can be configured for either Arbiter or Participant
Arbitration modes. When configured as Participant, the EBU must be granted the bus by
an external master before the Boot Configuration Value access can be made. When
configured as Arbiter, the EBU owns the bus immediately after reset and can, therefore,
perform the Boot Configuration Value access immediately after reset.
13.4.3.1 Boot Process
If External Boot Mode is selected, the EBU will perform one external bus read access to
the dedicated address 000004
H
of the memory device attached to chip select line CS0.
The data read by this read access is used to configure the EBU with parameters (see
). Any internal PLMB requests will be acknowledged with RETRY code until
the external bus boot read access is complete. The boot read access itself is performed
as an asynchronous access cycle with all timing parameters set to their maximum
values. This access scheme supports demultiplexed ROMs, EPROMs or Flash
memories.
shows a timing example of booting from a standard
demultiplexed asynchronous memory device.