參數(shù)資料
型號： AT80C51-25R
廠商： TEMIC SEMICONDUCTORS
元件分類： 微控制器/微處理器
英文描述： 8-BIT, MROM, 25 MHz, MICROCONTROLLER, PQFP44
文件頁數(shù)： 109/170頁
文件大?。?/td> 4133K
代理商： AT80C51-25R
43
ATtiny4/5/9/10 [DATASHEET]
8127F–AVR–02/2013
Figure 10-3. Switching Between Input and Output in Break-Before-Make-Mode
10.2.4
Reading the Pin Value
Independent of the setting of Data Direction bit DDxn, the port pin can be read through the PINxn Register bit. As
shown in Figure 10-2 on page 41, the PINxn Register bit and the preceding latch constitute a synchronizer. This is
needed to avoid metastability if the physical pin changes value near the edge of the internal clock, but it also intro-
duces a delay. Figure 10-4 shows a timing diagram of the synchronization when reading an externally applied pin
value. The maximum and minimum propagation delays are denoted t
pd,max and tpd,min respectively.
Figure 10-4. Synchronization when Reading an Externally Applied Pin value
Consider the clock period starting shortly after the first falling edge of the system clock. The latch is closed when
the clock is low, and goes transparent when the clock is high, as indicated by the shaded region of the “SYNC
LATCH” signal. The signal value is latched when the system clock goes low. It is clocked into the PINxn Register at
the succeeding positive clock edge. As indicated by the two arrows tpd,max and tpd,min, a single signal transition
on the pin will be delayed between and 1 system clock period depending upon the time of assertion.
When reading back a software assigned pin value, a nop instruction must be inserted as indicated in Figure 10-5
on page 44. The out instruction sets the “SYNC LATCH” signal at the positive edge of the clock. In this case, the
delay tpd through the synchronizer is one system clock period.
out DDRx, r16
nop
0x02
0x01
SYSTEM CLK
INSTRUCTIONS
DDRx
intermediate tri-state cycle
out DDRx, r17
0x55
PORTx
0x01
intermediate tri-state cycle
Px0
Px1
tri-state
0x01
r17
0x02
r16
XXX
in r17, PINx
0x00
0xFF
INSTRUCTIONS
SYNC LATCH
PINxn
r17
XXX
SYSTEM CLK
t
pd, max
t pd, min
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