Figure 10-3. Synchronization when Reading an Externally Applied Pin value
SYSTEM CLK
XXX
XXX
in r17, PINx
INSTRUCTIONS
SYNC LATCH
PINxn
0x00
tpd, max
0xFF
r17
tpd, min
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 ½ and 1½ 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 10-4. 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 1 system clock period.
Figure 10-4. Synchronization when Reading a Software Assigned Pin Value
SYSTEM CLK
0xFF
r16
out PORTx, r16
nop
in r17, PINx
INSTRUCTIONS
SYNC LATCH
PINxn
0x00
tpd
0xFF
r17
The following code example shows how to set port B pins 0 and 1 high, 2 and 3 low, and define
the port pins from 4 to 7 as input with pull-ups assigned to port pins 6 and 7. The resulting pin
values are read back again, but as previously discussed, a nop instruction is included to be able
to read back the value recently assigned to some of the pins.
76
AT90USB64/128
7593A–AVR–02/06
ATMEL [ ATMEL ]
