Some differences between 51 MCU and AVR MCU

1. Most of the I/O ports of 51 MCU are quasi-bidirectional ports. During reset, all ports output high level. The input and output operations of the ports are also directly performed through the address of the I/O port. The I/O ports of AVR are standard bidirectional ports. During reset, all ports are in the input state without pull-up resistors (high impedance state, the pin level is completely determined by the external circuit), which is very useful in situations where the reset state is emphasized. Each port of AVR corresponds to three addresses, namely DDRX, PORTX and PINX.

DDRX is the port direction register. When a certain position of DDRX is set to 1, the pin of the corresponding port is used as an output; when a certain position of DDRX is cleared to 0, the pin of the corresponding port is used as an input. PORTX is the port data register. When the pin is used as an output, the data of PORTX is output by the corresponding pin; when the pin is used as an input, the data of PORTX determines whether the pull-up function of the pin of the corresponding port is turned on. PINX is the input pin address of the corresponding port. If you want to read the logic level value of the corresponding pin, you must read PINX instead of PORTX.

2. There are two basic usages of the timer/counter of the 51 single-chip microcomputer, namely, the timer working mode with the 12-frequency signal of the crystal frequency as the input, or the counter working mode with the input signal on the external pins INT0 and INT1. When the AVR timer/counter is used as a timer, its input signal can be selected as a frequency-divided signal of the crystal frequency, and the frequency-dividing ratio is 1, 8, 64, 256, and 1024. When used as a counter, it can be triggered by either the rising edge or the falling edge.

3. The 51 MCU has 6 interrupt sources (5 interrupt entry addresses), which are divided into two priorities. The IE register controls the interrupt enable and the IP controls the interrupt priority. The AVR has different numbers of interrupt sources according to different MCUs.

4. The baud rate generation of the 51 single-chip microcomputer Kawaguchi communication requires the use of a timer, and the supported baud rate is relatively low. The AVR single-chip microcomputer can have a higher baud rate, up to 115200bps. And it has a dedicated baud rate generator.