User’s Manual
4-4
05.99
External Bus Interface
C513AO
4.3.1 Address Latch Enable (ALE)
The main function of ALE is to provide a properly timed signal to latch the low byte of an address
from Port 0 into an external latch during fetches from external memory. The address byte is valid at
the negative transition of ALE. For that purpose, ALE is activated twice every machine cycle. This
activation takes place even if the cycle involves no external fetch. The only time no ALE pulse
comes out is during an access to external data memory when RD/WR signals are active. The first
ALE of the second cycle of a MOVX instruction is missing (see Figure 4-1 b). Consequently, in any
system that does not use data memory, ALE is activated at a constant rate of 1/6 of the oscillator
frequency and can be used for external clocking or timing purposes. In systems that do not use
external memory at all and do not use ALE as clock, external ALE generation can be suppressed
by resetting the EALE bit in the SYSCON register. This can help reduce system RFI. Because ALE
can be enabled/disabled dynamically, it is also possible to enable ALE only when external memory
is accessed. This can be useful if the external memory is accessed only rarely.
The C513AO allows the ALE output signal to be switched off. If the internal program memory is used
by setting EA = 1 and ALE is switched off by setting EALE = 0, ALE will only go active during
external data memory accesses (MOVX instructions) and code memory accesses with an address
greater than 3FFFH (external code memory fetches). If EA = 0, the ALE generation is always
enabled and the bit EALE has no effect.
After a hardware reset, the ALE generation is enabled.
Special Function Register SYSCON (Address B1H) Reset Value: XX10XXX0B
Bit
Function
–
Not implemented. Reserved for future use.
EALE
Enable ALE output
EALE = 0: ALE generation is disabled; disables ALE signal generation
during internal code memory accesses (EA = 1). With EA = 1,
ALE is automatically generated at MOVX instructions and
code memory accesses with an address greater 3FFFH.
EALE = 1: ALE generation is enabled
If EA = 0, the ALE generation is always enabled and the bit EALE has no
effect on the ALE generation.
7
6
5
4
3
2
1
0
EALE
RMAP
–
B1H
SYSCON
Bit No.
MSB
LSB
–
–
–
–
XMAP