C509-L_97 Datasheet, PDF (56/290 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

External Bus Interface
C509-L
4.1.6 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
C509-L system that does not use data memory, ALE is activated at a constant rate of 1/3 of the
oscillator frequency and can be used for external clocking or timing purposes.
As a reserved function for future versions, the C509-L allows to switch off the ALE output signal by
a bit in SFR SYSCON. If EA=0 (this is always the case for the C509-L), the ALE generation is
always enabled and resetting of bit 5 of SFR SYSCON has no effect.
Special Function Register SYSCON (Address B1H)
Reset Value : 1010XX01B
Bit No. MSB
7
6
5
4
3
B1H CLKP PMOD 1 RMAP –
LSB
2
1
0
– XMAP1 XMAP0 _SYSCON
Bit
Bit 5
The functions of the shaded bits are not described in this section.
Function
Reserved bit for future use: Enable/disable ALE output.
Bit 5 is at “1” after reset and can be written with “0” or “1”.
Semiconductor Group
4-4
1997-10-01