Chip Selects
Am186™CC/CH/CU Microcontrollers User’s Manual
5-9
5.5.5
Configuring Address and Data Buses
5.5.5.1
UCS and LCS
When UCS or LCS are asserted, the DA bit in the UMCS or LMCS register selects whether
the AD15–AD0 bus is driven during the address phase of a bus cycle.
The DA bit is still valid when UCS or LCS supports DRAM (either UDEN or LDEN is 1).
That is, even though the UCS signal is held High and the LCS signal becomes RAS0 in
DRAM mode, the address phase on AD15–AD0 is still disabled during DRAM accesses to
UCS/LCS space if DA is set to 1. In addition, the DISMEM (for memory addresses) and
DISIO (for I/O addresses) bits in the SYSCON register can act as global address disables
to prevent address bits from appearing on the AD15–AD0 bus. Setting the DISMEM bit
overrides clearing the DA bits. The block size programmed should match the size of the
DRAM being used, otherwise the full capacity of the DRAM is not utilized.
The UCSX8 signal is sampled during every external reset. If UCSX8 is 0, the LSIZ/USIZ
bit is set in the LMCS/UMCS register, which defines memory as an 8-bit space. This allows
the microcontroller to boot from an 8-bit wide device. It is possible to later clear this bit, thus
redefining the space to be 16 bits wide. Only a hard system reset can cause this bit to be
set; therefore, it is only possible to go from an 8-bit to a 16-bit space through software and
not the reverse. If the system does a watchdog timer reset, this bit reverts to the value
sampled on UCSX8 during the last external reset. The UCSX8 signal has a weak pullup
that defaults the part into 16-bit operation. If DRAM is enabled for UCS or LCS, the bus
size is forced to 16 bits. For more information about controlling the bus width, see Table 3-8
on page 3-31.
5.5.5.2
Non-UCS and Non-LCS
The OMSIZ bit determines the width of the data bus (i.e., x8 or x16) for memory accesses
between the LCS and UCS memory regions (i.e., accesses above the LCS region and
below the UCS region).
An MCS space cannot overlap LCS or UCS memory, so it always lies in the space affected
by the OMSIZ bit. A PCS space is only affected by the OMSIZ bit if the PCS space is in
memory and does not overlap an LCS or UCS region. If a PCS space overlaps an LCS or
UCS region, the PCS space is accessed as x16 memory.
If the PCB space resides in an x8 memory region, each word-wide PCB register access
generates two external bus cycles, but all 16 register bits are accessed internally on the
first cycle. For more information, see “Peripheral Registers” on page 2-4.
5.5.5.3
PCS I/O Space
The IOSIZ bit in the MPCS register determines the width of the data bus (x8 or x16) for all
I/O accesses.
If the PCB space is mapped to I/O and the I/O bus width is x8, each word-wide PCB register
access generates two external bus cycles, but all 16 register bits are accessed internally
on the first cycle. For more information, see “Peripheral Registers” on page 2-4.
Содержание Am186 CC
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Страница 18: ...Table of Contents xviii Am186 CC CH CU Microcontrollers User s Manual...
Страница 24: ...Introduction xxiv Am186 CC CH CU Microcontrollers User s Manual...
Страница 39: ...Architectural Overview Am186 CC CH CU Microcontrollers User s Manual 1 15 Figure 1 6 32 Channel Linecard CH CC...
Страница 40: ...Architectural Overview 1 16 Am186 CC CH CU Microcontrollers User s Manual...
Страница 86: ...System Overview 3 36 Am186 CC CH CU Microcontrollers User s Manual...
Страница 92: ...Emulator Support 4 6 Am186 CC CH CU Microcontrollers User s Manual...
Страница 112: ...DRAM Controller 6 8 Am186 CC CH CU Microcontrollers User s Manual...
Страница 134: ...Interrupts 7 22 Am186 CC CH CU Microcontrollers User s Manual...
Страница 186: ...Programmable I O Signals 9 8 Am186 CC CH CU Microcontrollers User s Manual...
Страница 200: ...Watchdog Timer 11 6 Am186 CC CH CU Microcontrollers User s Manual...
Страница 232: ...Asynchronous Serial Ports UARTs 13 24 Am186 CC CH CU Microcontrollers User s Manual...
Страница 242: ...Synchronous Serial Port SSI 14 10 Am186 CC CH CU Microcontrollers User s Manual...
Страница 264: ...High Level Data Link Control HDLC 15 22 Am186 CC CH CU Microcontrollers User s Manual...
Страница 332: ...Universal Serial Bus USB 18 34 Am186 CC CH CU Microcontrollers User s Manual...
Страница 348: ...Register Summary A 16 Am186 CC CH CU Microcontrollers User s Manual...
Страница 376: ...Index Index 18 Am186 CC CH CU Microcontrollers User s Manual...