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ATMEGA1280 Scheda tecnica(PDF) 83 Page - ATMEL Corporation |
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ATMEGA1280 Scheda tecnica(HTML) 83 Page - ATMEL Corporation |
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83 / 407 page  83 ATmega640/1280/1281/2560/2561 2549A–AVR–03/05 Toggling the Pin Writing a logic one to PINxn toggles the value of PORTxn, independent on the value of DDRxn. Note that the SBI instruction can be used to toggle one single bit in a port. Switching Between Input and Output When switching between tri-state ({DDxn, PORTxn} = 0b00) and output high ({DDxn, PORTxn} = 0b11), an intermediate state with either pull-up enabled {DDxn, PORTxn} = 0b01) or output low ({DDxn, PORTxn} = 0b10) must occur. Normally, the pull-up 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 MCUCR 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 34 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 34, 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 35 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 35. Synchronization when Reading an Externally Applied Pin value Table 34. Port Pin Configurations DDxn PORTxn PUD (in MCUCR) 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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