SP2328 serial port expansion special chip and its interface circuit with microcontroller

Figure 1 DIP and SOIC package (left) Figure 2 SSOP package (right)

Functional features of SP2328
SP2328 has four UART serial ports, including one master serial port and three sub-serial ports. The master serial port and all sub-serial ports are in full-duplex working mode, and all serial ports are allowed to receive and send serial data at the same time. Serial port 0~serial port 2 are sub-serial ports, and the working rate of each sub-serial port is 75bps~4800bps. Serial port 3 is the master serial port, and its data transmission rate is 4 times that of the sub-serial port. After serial port 3 receives the serial data, the SP2328 chip sends the data through the corresponding sub-serial port according to the received address signal. After the sub-serial port receives the serial data, it sends the data through serial port 3 and gives the corresponding sub-serial port address. Setting the baud rate of serial port 0~serial port 2 is very simple and does not require software setting. Just change the frequency of the input clock. The baud rate is calculated according to the following formula:

K=2400*f(osc)/8.0(bps)

Among them, f(osc)<=16.0MHZ
The DC operating voltage of SP2328 is 2.4V~5.5V, the typical current is 3.7mA, and it has a power-saving mode.
Figure 1 and Figure 2 are the pin arrangement diagrams of the SP2328 chip.
ADRI0 and ADRI1 are the serial port address lines when serial port 3 receives downstream data. "00", "01" and "10" correspond to the addresses of serial port 0, serial port 1 and serial port 2 respectively; "11" is the address of serial port 3. When the host computer wants to send data to the lower computer connected to a certain sub-serial port, it must first set ADRI0 and ADRI1 to the responding address state, and then send the data to serial port 3.
ADRO0 and ADRO1 are the serial port address lines when serial port 3 sends upstream data. "00", "01" and "10" correspond to the addresses of serial port 0, serial port 1 and serial port 2 respectively. When the serial port of the host computer receives the data sent from serial port 3, it immediately reads the status of the address lines ADRO0 and ADRO1. Based on the status of the address lines ADRO0 and ADRO1, it can be determined which serial port the received data was uploaded from.

Figure 3 Interface circuit between SP2328 and microcontroller

Example of the interface between SP2328 and the microcontroller.
Figure 3 is the interface circuit between SP2328 and the microcontroller in the data acquisition circuit of the microcontroller. The serial port of AT89C51 is connected to the serial port 3 of SP2328. P1.0, P1.1, P1.2, and P1.3 are connected to ADRI0, ADRI1, ADRO0, and ADRO1 respectively, which are used to set and read the address of the sub-serial port. The three sub-serial ports are connected to the serial ports of three slave computers respectively. The baud rate of serial port 0, serial port 1 and serial port 2:
 K=(2400*16)/8=4800(bps)
The baud rate of serial port 3:
 K3=4*K=4*4800=19200(bps)
AT89C51 The baud rate of the serial port must also be set to 19200bps.
Since SP2328 automatically enters the working state after power on and does not need to be set by a program, it is relatively simple and convenient to write a serial port program.

Issues that need attention
: When selecting the crystal oscillator frequency, the baud rate of the microcontroller serial port must be strictly consistent with the baud rate of SP2328. The crystal oscillator frequency must be stable, otherwise it is easy for the data received by the SP2328 chip and the data sent out to be incorrect. of.
Since the speed of the main serial port 3 is 4 times that of other sub-serial ports, that is, the main serial port can send 4 bytes in the time it takes the sub-serial port to send one byte. Therefore, if the serial port 3 sends a byte to the sub-serial port, the serial port 3 does not send a byte to the other sub-serial ports. The sub-serial port sends another 4 bytes, and immediately sends data to the atomic serial port, the data of the sub-serial port will overflow. Therefore, more than 4 bytes must be added between them for delay (the master serial port sends data to other sub-serial ports, which is equivalent to a delay of one byte).
After the host computer receives the upstream data from the master serial port, it must read the address information of the upstream data in time (the address information of the upstream data can be maintained for 6 bits after the stop bit of the upstream data ends). This enables the host computer to accurately receive the serial data from the sub-serial port.
Before the host computer sends data to the master serial port, it must first send the address signal of the sub-serial port to SP2328.

Conclusion
: The SP2328 chip is used to expand the serial port of the microcontroller, which provides a good solution for the multi-serial communication of the microcontroller. Compared with solutions such as using dual serial port microcontrollers and using multiple pieces of AT89C2051 to expand serial ports, it has simple programming, high reliability and low cost.

References
1 Yu Yongquan. "Principles and Applications of Flash Microcontrollers". Beijing: Electronic Industry Press. 1997,10
2 SP2328 Data Application Manual. Chengdu Shipu Technology Co., Ltd.