參數(shù)資料
型號: Q67120-C508
英文描述: IC-SM-8-BIT CPU 12-MHZ
中文描述: 集成電路釤8位CPU的12兆赫
文件頁數(shù): 44/121頁
文件大?。?/td> 1000K
代理商: Q67120-C508
Semiconductor Group
6-10
On-Chip Peripheral Components
C501
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 6-10
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.
Figure 6-10
Port Timing
MCT03231
P1
P2
S4
S5
P2
P1
S6
P2
P1
S1
P2
P1
S2
P2
P1
S3
P2
P1
XTAL2
Input sampled:
e.g. MOV A, P1
Old Data
New Data
Port
P1 active for 1 State
(driver transistor)
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