
43
1477K–AVR–08/10
ATtiny26(L)
Table 21 summarizes the control signals for the pin value.
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 32, the PINxn Register bit and the preceding latch con-
stitute a synchronizer. This is needed to avoid metastability if the physical pin changes value
near the edge of the internal clock, but it also introduces a delay.
Figure 33 shows a timing dia-
gram of the synchronization when reading an externally applied pin value. The maximum and
minimum propagation delays are denoted tpd,max and tpd,min respectively.
Figure 33. 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 indi-
cated 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 indi-
cated in Figure 34. 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.
Table 21. Port Pin Configurations
DDxn
PORTxn
PUD
(in MCUCR)
I/O
Pull-up
Comment
0
X
Input
No
Tri-state (Hi-Z)
0
1
0
Input
Yes
Pxn will source current if ext. pulled
low
0
1
Input
No
Tri-state (Hi-Z)
1
0
X
Output
No
Output Low (Sink)
1
X
Output
No
Output High (Source)
XXX
in r17, PINx
0x00
0xFF
INSTRUCTIONS
SYNC LATCH
PINxn
r17
XXX
SYSTEM CLK
tpd, max
tpd, min