
18-2
MSM66577 Family User's Manual
Chapter 18 Bus Port Functions
Table 18-1 P0, P1, P2, P3_1 and P4 Operation During Program Memory Access
(Separate Bus Type)
Memory to be
accessed
Address
P0 operation
P1, P2, P4
operation
P3_1 operation
Internal program
When EA = H,
0H to 1FFFFH
I/O port
External program
When EA = H,
20000H to FFFFFH
After set as
secondary
function output,
program data input
After set as
secondary
function output,
address output
After set as
secondary
function output,
PSEN output
When EA = L,
0H to FFFFFH
Program data input
Address output
PSEN output
Multiplexed bus type (the SELMBUS pin at a low level)
When accessing internal program memory (addresses 0H to 1FFFFH with the
EA pin at a
high level), P0, P1, P2, P3_1 and P4 operate as I/O ports.
When accessing external program memory (the
EA pin at a low level or addresses 20000H
to FFFFFH with the
EA pin at a high level), P0 operates as the address output and program
data input port, P1 and P2 operate as addresses output ports, P3_0 operates as the ALE
output port, and P3_1 operates as the
PSEN output port.
If the
EA pin is at a low level, P0, P1, P2, P3_0 and P3_1 are automatically switched
(secondary function control registers and mode registers are set) to bus port and control
signal functions (hereafter referred to as bus port functions) when reset (
RES signal input,
execution of a BRK instruction, overflow of the watchdog timer, opcode trap). If the
EA pin
is at a high level, bus port functions are not automatically switched. It is necessary to switch
to bus port functions before external program memory is accessed by setting secondary
function control registers and mode registers on software.
Of the ports that are automatically set as bus port functions when the
EA pin is at a low level,
if upper address or other output is unnecessary, then after reset, those ports can be
operated as normal I/O ports by resetting their secondary function control register.
Table 18-2 lists the operation of P0, P1, P2, P3_0 and P3_1 in the multiplexed bus type
during a program memory access.