C541U_99 Datasheet, PDF (35/192 Pages) Siemens Semiconductor Group – 8-BIT CMOS MICROCONTROLLER

External Bus Interface
C541U
4.4 ALE, Address Latch Enable
The main function of ALE is to provide a properly timed signal to latch the low byte of an address
from P0 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.
The C541U allows to switch off the ALE output signal. If the internal OTP program memory is used
(EA=1) and ALE is switched off by EALE=0, ALE will only go active during external data memory
accesses (MOVX instructions) and code memory accesses with an address greater than 1FFFH for
the C541U (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 : XX10XXXXB
Bit No. MSB
LSB
7
6
5
4
3
2
1
0
B1H
–
– EALE RMAP –
–
–
– SYSCON
The function of the shaded bit is not described in this section.
Bit
–
EALE
Function
Not implemented. Reserved for future use.
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 1FFFH.
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.
Semiconductor Group
4-4
1997-10-01