ST10R272L - EXTERNAL BUS INTERFACE
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9.1
Single chip mode
Single chip mode is entered when pin EA is high during reset. Register BUSCON0 is cleared
(except bit ALECTL0 and bits BTYP0[1:0] = P0L.[7:6]), which resets bit BUSACT0 of
BUSCON0 register. No external bus is enabled.
In single chip mode the ST10R272L operates from internal resources. No external bus is
configured and no external peripherals and/or memory can be accessed, and no port lines
are occupied for the bus interface. The ROMless ST10R262 cannot operate in single chip
mode, and the EA pin must be forced at 0 during reset.
9.2
External bus modes
All external memory accesses are performed by the on-chip External Bus Controller which
can be programmed either to single chip mode when no external memory is required, or to
the following external memory access modes:
In the demultiplexed bus modes, addresses are output on PORT1 and data is input/output
on PORT0/P0L, respectively. In the multiplexed bus modes both addresses and data use
PORT0 for input/output.
When the external bus interface is enabled and configured (bit BUSACTx=’1’ and bitfield
BTYP), the ST10R272L uses a subset of its port lines together with some control lines to
build the external bus.
The bus configuration (BTYP) for the address windows (BUSCON4...BUSCON1) is selected
via software typically during the initialization of the system. The bus configuration (BTYP) for
the default address range (BUSCON0) is selected via PORT0 during reset, provided that pin
EA is low during reset. Afterwards, BUSCON0 may be modified via software just like the
other BUSCON registers.
The 16 MByte address space of the ST10R272L is divided into 256 segments of 64 KByte.
The 16-bit intra-segment address is output on PORT0 for multiplexed bus modes or on
PORT1 for demultiplexed bus modes. When segmentation is disabled, only one 64 KByte
segment can be used and accessed. Otherwise additional address lines may be output on
Port 4, and/or several chip select lines may be used to select different memory banks or pe-
ripherals. These functions are selected during reset via bitfields SALSEL and CSSEL of reg-
ister RP0H, respectively. For applications which require less than 16 MBytes of external
memory space, this address space can be restricted to 1 MByte, 256 KByte or to 64 KByte.
16-bit data
demultiplexed
16-/18-/20-/24-bit addresses
16-bit data
multiplexed
16-/18-/20-/24-bit addresses
8-bit data
multiplexed
16-/18-/20-/24-bit addresses
8-bit data
demultiplexed
16-/18-/20-/24-bit addresses