Pinout diagram of MSC-51 microcontroller

　　The common MSC-51 microcontroller generally uses a dual in-line (DIP) package with a total of 40 pins. The picture shows the pinout diagram. The 40 pins can be roughly divided into four categories: power, clock, control and I/O pins.

　　1. Power supply

　　Vcc: chip power supply, generally +5V;

　　Vss: ground terminal.

　　2. Clock

　　XTAL1 and XTAL2: the inverting input and output terminals of the crystal oscillator circuit. When using the internal oscillator circuit, an external crystal oscillator is required, such as 4M, 6M, 11.0592M, 12M, etc. When using external oscillation input, XTAL1 is connected to ground, and XTAL2 is connected to external oscillation pulse input.

　　3. Control line

　　The MCS-51 microcontroller has a total of 4 control lines, 3 of which are multiplexed lines with two functions.

　　(1) ALE/PROG: Address latch enable/programming pulse.

　　ALE: ALE function during normal use, mainly used to latch the 8-bit address sent from the P0 port. Port P0 generally transmits the lower 8-bit address and data signals in a time-sharing manner, and they are all binary numbers. To distinguish whether it is the lower 8-bit data signal or the address signal, look at the ALE pin. When the ALE signal is valid, the P0 port transmits the lower 8-bit address signal; when the ALE signal is invalid, the P0 port transmits the 8-bit data signal. Generally, the content transmitted by the P0 port is locked on the falling edge of the ALE pin, that is, the low 8-bit address signal.

　　When the CPU is not executing instructions to access external RAM (MOVX), ALE is output at a fixed frequency of 1/6 of the clock oscillation frequency, so the ALE signal can also be used as the clock signal of the external chip. But when the CPU performs access to external RAM (MOVX), ALE will skip an ALE pulse.

　　PROG: When the microcontroller is programming, this pin inputs programming pulses (provided by the programmer).

　　(2) PSEN: External ROM read strobe signal.

　　When the microcontroller reads the external ROM, PSEN is effectively output twice in each machine cycle. PSEN is equivalent to the allowed strobe signal output by the external ROM chip. But it is invalid when reading on-chip ROM and reading off-chip RAM.

　　(3) RST: reset pin.

　　RST is the power-on reset input terminal of the microcontroller. As long as this pin is kept at a high level for more than two machine cycles, the microcontroller can achieve a reset operation. After reset, the program starts executing from 0000H. In general applications, RC circuits can be used to implement power-on reset of microcontrollers. In some industrial control and other demanding situations, special watchdog chips are generally used for reset and power supply monitoring. A typical RC power-on reset circuit is shown in the figure. Show:

　　(4) EA/VPP: Internal and external ROM selection/EPROM programming power supply.

　　EA: During normal operation, EA is the internal and external ROM selection terminal. The addressing range of the MCS-51 microcontroller is 64KB, of which 4K is on-chip and 60K is off-chip. When EA is high, the on-chip ROM is accessed first. When the program length exceeds 4K, it will automatically transfer to the external ROM for execution. program. When EA is low, the microcontroller only accesses the external ROM. For the old 8031 ​​microcontroller (because there is no ROM on the chip), EA must be connected to ground. Most of the current microcontrollers come with their own ROM, so EA is also connected to high level in general applications.

　　VPP: For microcontrollers with internal EPROM, this pin is used to apply programming power during on-chip EPROM programming.

　　4. I/O pins

　　The MCS-51 microcontroller has four 8-bit parallel I/O ports and a total of 32 programmable I/O pins. Each of the four I/O ports has its own function. Under normal circumstances, P0 is dedicated to transmitting the lower 8-bit address signal and 8-bit data signal in a time-sharing manner, the P2 port is dedicated to transmitting the upper 8-bit address signal, and the P3 port most of the time Used for secondary functions. Of course, all I/O ports can be used as ordinary input/output ports.