mc68hc912bc32 Freescale Semiconductor, Inc, mc68hc912bc32 Datasheet - Page 85

• Manufacturer Part Number: mc68hc912bc32
• Description: M68hc12 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Chapter 6
Bus Control and Input/Output (I/O)
6.1 Introduction
Internally, the MCU has full 16-bit data paths, but depending upon the operating mode and control
registers, the external bus may be eight or 16 bits. There are cases where 8-bit and 16-bit accesses can
appear on adjacent cycles using the LSTRB signal to indicate 8-bit or 16-bit data.
6.2 Detecting Access Type from External Signals
The external signals LSTRB, R/W, and A0 can be used to determine the type of bus access that is taking
place. Accesses to the internal RAM module are the only accesses that produce LSTRB = A0 = 1,
because the internal RAM is specifically designed to allow misaligned 16-bit accesses in a single cycle.
In these cases, the data for the address that was accessed is on the low half of the data bus and the data
for address +1 is on the high half of the data bus (data order is swapped).
6.3 Registers
Under certain conditions, not all registers are visible in the memory map. In special peripheral mode, the
first 16 registers associated with bus expansion are removed from the memory map.
In expanded modes, some or all of port A, port B, and port E are used for expansion buses and control
signals. To allow emulation of the single-chip functions of these ports, some of these registers must be
rebuilt in an external port replacement unit. In any expanded mode,
port A and port B are used for address and data lines so registers for these ports, as well as the data
direction registers for these ports, are removed from the on-chip memory map and become external
accesses.
In any expanded mode, port E pins may be needed for bus control (for example, ECLK and R/W). To
regain the single-chip functions of port E, the emulate port E (EME) control bit in the MODE register may
Freescale Semiconductor
LSTRB
1
0
1
0
0
1
0
1
A0
0
1
0
1
0
1
0
1
R/W
1
1
0
0
1
1
0
0
Table 6-1. Detecting Access Type
M68HC12B Family Data Sheet, Rev. 9.1
8-bit read of an even address
8-bit read of an odd address
8-bit write to an even address
8-bit write to an odd address
16-bit read of an even address
16-bit read of an odd address (low/high data swapped)
16-bit write to an even address
16-bit write to an odd address (low/high data swapped)
Type of Access
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