MT8880 usage tips and interface circuit

The advantage of this IC is that it can form a dialing and receiving circuit with a few components. The only disadvantage is its price, which is 30 yuan per piece.

This chip is rarely used in products with fierce price competition in the market.

The external telephone line and MT8880 are not directly connected, but indirectly connected through isolation. Generally, isolation can be solved by isolating coils or rectifier bridges.

Relatively speaking, I prefer a bridge to take on this role.

MT8880 is a dual-audio voice dial chip. It uses two audio signals to form a single-bit data signal to send out, one for low frequency and one for high frequency.

High-frequency signal. The receiving party receives the two audio signals separately and knows which data is received by looking up the table.

From the table, we can see that when sending 1, it is 0001, when sending 2, it is 0010, and so on. Haha, you have to read it carefully, the 0 in the phone number cannot be sent

0000. I made this mistake the first time. Why can I dial some numbers but not others? I compared multiple numbers and found the number

I didn't get any phone numbers with 0 in the digits, so I looked it up in the table and realized that 0 corresponds to 1010. I was stunned.

The standard of 1 second for dial-up ringback tone and 4 seconds for ringback tone makes detection difficult, so this function is not used.

MT8880 must be initialized before sending data. The following is a process of initializing it.

void MT8880_Rst()
{
// MT8880_RS();
MT8880_WC(0);
MT8880_WC(0);
MT8880_WC(8);
MT8880_WC(0);
MT8880_RS();
}

When sending and receiving data, you must first go off-hook. That is, the telephone line is voltage-reduced and current-divided. See the figure below:

Dual-tone multi-frequency (DTMF) codecs are widely used in remote control, telemetry, data transmission, etc., especially when used with some microprocessors, they are more flexible to use. MT8880 is a complete DTMF transceiver. It integrates the receiver and transmitter into one, and has the data bus and control bus characteristics of the microprocessor interface. It can be easily connected to the single-chip microcomputers of MOTOROLA, INTEL, etc. to form a complete receiving and sending communication. . The bidirectional I/O port of the AT89C2051 single-chip microcomputer is directly connected to the MT8880, and its rich bit operation instructions are used to simulate the timing of the MT8880, so as to realize the read and write operation control of the MT8880. This method occupies up to 9 I/O lines (if working in interrupt mode), and at least 6 I/O lines can be selected, such as expanding serial EEPROM, keyboard, display, etc. Its biggest advantages are low cost, small size, simple circuit connection, and high program confidentiality. The hardware circuit of its communication part is shown in Figure 1. In the figure, the data lines D0-D3 of MT8880 correspond to P1.0-P1.3 of AT89C2051, the read and write lines (WR*), chip select line (CS*), register input select line (RSI) and clock line (CP) correspond to P1.7-P1.4 of AT89C2051 respectively, and the interrupt line (IRQ*) is connected to the external interrupt 0 (P3.2) of AT89C2051. The interface circuit is as follows: