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ATMEGA8515-16AU Datasheet(PDF) 60 Page - ATMEL Corporation |
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ATMEGA8515-16AU Datasheet(HTML) 60 Page - ATMEL Corporation |
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60 / 254 page  60 ATmega8515(L) 2512G–AVR–03/05 enabled state is fully acceptable, as a high-impedant environment will not notice the dif- ference between a strong high driver and a pull-up. If this is not the case, the PUD bit in the SFIOR Register can be set to disable all pull-ups in all ports. Switching between input with pull-up and output low generates the same problem. The user must use either the tri-state ({DDxn, PORTxn} = 0b00) or the output high state ({DDxn, PORTxn} = 0b11) as an intermediate step. Table 24 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 30, 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 introduces a delay. Figure 31 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 31. 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 suc- ceeding positive clock edge. As indicated by the two arrows t pd,max and tpd,min, a single Table 24. Port Pin Configurations DDxn PORTxn PUD (in SFIOR) I/O Pull-up Comment 0 0 X Input No Tri-state (Hi-Z) 0 1 0 Input Yes Pxn will source current if ext. pulled low. 0 1 1 Input No Tri-state (Hi-Z) 1 0 X Output No Output Low (Sink) 1 1 X Output No Output High (Source) XXX in r17, PINx 0x00 0xFF INSTRUCTIONS SYNC LATCH PINxn r17 XXX SYSTEM CLK t pd, max t pd, min |
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