The serial port circuit uses MAX3232 chip, can also use MAX323 chip, 3232 is +3.3v power supply, 323 is +5v power supply. The capacitance value in the circuit is 0.1uf on the MAX3232 chip manual. In the experiment, the serial port can not work, and the communication is normal after the capacitance parameter is modified. Among them: add a 10nf patch capacitor between pins 4 and 5, and the value of 2 capacitors between pins 2 and 6 is 1uf. SMD capacitors can also work normally, and the polarity of the capacitor depends on the space and needs.

The definition of each pin of the DB9 header:

1. Carrier detection DCD;

2. Receive data RXD;

3. Send data TXD;

4. Data terminal ready for DTR;

5. Signal ground SG;

6. Data ready for DSR;

7. Request to send RTS;

8. Clear to send CTS ;

9. Ringing prompts RI

3.3V serial port driver MAX3232 digital protection meter

Using a proprietary low-dropout transmitter output stage, using dual charge pumps to achieve true RS-232 performance when powered by a 3.0V to 5.5V power supply, the device only requires four 0.1UF external small-size charge pump capacitors.

Max3232 ensures a data rate of 120kbps while maintaining the RS-232 output level. The max3232 has two receivers and two drivers, and provides 1UA shutdown mode, which effectively reduces power efficiency and delays the battery life of portable products. In shutdown mode F, the receiver maintains an active state and monitors external devices. Only the pins, packages and functions of max3232 are compatible with the industry standard MAX242 and 1UA power supply current, and MAX232 is compatible. Even when working at a high data rate, max3232 can still maintain the RS-232 standard requirement as long as the input voltage is within the range of 3.0V to 5.5V, plus or minus 5.0V minimum transmitter output voltage. Can provide +5.5V (voltage double charge pump) and -5.5V (inverting charge pump) output voltage, the charge pump works in discontinuous mode, once the output voltage is lower than 5.5V, the charge pump will be turned on; the output voltage exceeds 5.5 V, that is, each charge pump needs a flying container and an energy storage capacitor to generate voltages of V+ and V-. Turn off the charge pump,

Max3232 can guarantee a data rate of 120kbps under the worst working conditions. Under normal circumstances, it can work at a data rate of 235kbps, and the transmitter can drive multiple receivers and mice in parallel.

MAX3232 Typical Operating Circuits