AT90USB646 datasheet(77/435 Pages) ATMEL

77 / 435 page

7593D–AVR–07/06

AT90USB64/128

Figure 10-3. 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 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

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.

XXX

in r17, PINx

0x00

0xFF

INSTRUCTIONS

SYNC LATCH

PINxn

r17

XXX

SYSTEM CLK

pd, max

t pd, min

out PORTx, r16

nop

in r17, PINx

0xFF

0x00

0xFF

SYSTEM CLK

r16

INSTRUCTIONS

SYNC LATCH

PINxn

r17

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