C509-L_97 Datasheet, PDF (94/290 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C509-L
6.1.3 Port Handling
6.1.3.1 Port Timing
When executing an instruction that changes the value of a port latch, the new value arrives at the
latch during S6P2 of the final cycle of the instruction. However, port latches are only sampled by
their output buffers during phase 1 of any clock period (during phase 2 the output buffer holds the
value it noticed during the previous phase 1). Consequently, the new value in the port latch will not
appear at the output pin until the next phase 1, which will be at S1P1 of the next machine cycle.
When an instruction reads a value from a port pin (e.g. MOV A, P1) the port pin is actually sampled
in state 5 phase 1 or phase 2 depending on port and alternate functions. Figure 6-12 illustrates this
port timing. lt must be noted that this mechanism of sampling once per machine cycle is also used
if a port pin is to detect an “edge”, e.g. when used as counter input. In this case an “edge” is detected
when the sampled value differs from the value that was sampled the cycle before. Therefore, there
must be met certain requirements on the pulse length of signals in order to avoid signal “edges” not
being detected. The minimum time period of high and low level is one machine cycle, which
guarantees that this logic level is noticed by the port at least once.
XTAL2
S4
S5
S6
S1
S2
S3
P1 P2 P1 P2 P1 P2 P1 P2 P1 P2 P1 P2
Input sampled:
e.g.: MOV A, P1
or
p1 active for 1 state
(driver transistor)
Port
Figure 6-12
Port Timing
Old Data
New Data
Semiconductor Group
6-16
1997-10-01