1、附录 附录 1 外文文献 C8051F020 （ PORT INPUT/OUTPUT） The C8051F020/1/2/3 are fully integrated mixed-signal System on a Chip MCUs with 64 digital I/O pins (C8051F020/2) or 32 digital I/O pins (C8051F021/3), organized as 8-bit Ports. The lower ports: P0, P1, P2, and P3, are both bit- and byte-addressable throu
2、gh their corresponding Port Data registers. The upper ports: P4, P5, P6, and P7 are byte-addressable. All Port pins are 5 V-tolerant, and all support configurable Open-Drain or Push-Pull output modes and weak pull-ups. The C8051F020/1/2/3 devices have a wide array of digital resources which are avai
3、lable through the four lower I/O Ports: P0, P1, P2, and P3. Each of the pins on P0, P1, P2, and P3, can be defined as a General-Purpose I/O (GPIO) pin or can be controlled by a digital peripheral or function (like UART0 or /INT1 for example), as shown in Figure 17.2. The system designer controls whi
4、ch digital functions are assigned pins, limited only by the number of pins available. This resource assignment flexibility is achieved through the use of a Priority Crossbar Decoder. Note that the state of a Port I/O pin can always be read from its associated Data register regardless of whether that
5、 pin has been assigned to a digital peripheral or behaves as GPIO. The Port pins on Port1 can be used as Analog Inputs to ADC1. The Priority Crossbar Decoder, or “Crossbar”, allocates and assigns Port pins on Port 0 through Port 3 to the digital peripherals (UARTs, SMBus, PCA, Timers, etc.) on the d
6、evice using a priority order. The Port pins are allocated in order starting with P0.0 and continue through P3.7 if necessary. The digital peripherals are assigned Port pins in a priority order which is listed in Figure 17.3, with UART0 having the highest priority and CNVSTR having the lowest priorit
7、y. The Crossbar assigns Port pins to a peripheral if the corresponding enable bits of the peripheral are set to a logic 1 in the Crossbar configuration registers XBR0, XBR1, and XBR2, shown in Figure 17.7, Figure 17.8, and Figure 17.9. For example, if the UART0EN bit (XBR0.2) is set to a logic 1, th
8、e TX0 and RX0 pins will be mapped to P0.0 and P0.1 respectively. Because UART0 has the highest priority, its pins will always be mapped to P0.0 and P0.1 when UART0EN is set to a logic 1. If a digital peripherals enable bits are not set to a logic 1, then its ports are not accessible at the Port pins
9、 of the device. Also note that the Crossbar assigns pins to all associated functions when a serial communication peripheral is selected (i.e. SMBus, SPI, UART). It would be impossible, for example, to assign TX0 to a Port pin without assigning RX0 as well. Each combination of enabled peripherals res
10、ults in a unique device pinout. All Port pins on Ports 0 through 3 that are not allocated by the Crossbar can be accessed as General-Purpose I/O (GPIO) pins by reading and writing the associated Port Data registers ， a set of SFRs which are both byte- and bit-addressable. The output states of Port p
11、ins that are allocated by the Crossbar are controlled by the digital peripheral that is mapped to those pins. Writes to the Port Data registers (or associated Port bits) will have no effect on the states of these pins. A Read of a Port Data register (or Port bit) will always return the logic state p
12、resent at the pin itself, regardless of whether the Crossbar has allocated the pin for peripheral use or not. An exception to this occurs during the execution of a read-modify-write instruction (ANL, ORL, XRL, CPL, INC, DEC, DJNZ, JBC, CLR, SET, and the bitwise MOV operation). During the read cycle
13、of the read-modify-write instruction, it is the contents of the Port Data register, not the state of the Port pins themselves, which is read. Because the Crossbar registers affect the pinout of the peripherals of the device, they are typically configured in the initialization code of the system before the peripherals themselves are configured. Once configured, the Crossbar registers are typically left alone. Once the Crossbar registers have been properly configured, the Crossbar is
