Principle Application of Physical Interface Device MC33399 for Automotive LIN Bus

Keywords: MC33399LIN Bus Automotive Electronics

1 Overview

LIN (Local Interconnection Network) is a low-cost bus network. The purpose of its initial development is to make up for the deficiency of CAN bus. It is mainly used in some occasions in automobiles where the communication rate is not high. Using LIN bus as an auxiliary network or subnetwork of CAN bus can solve many problems caused by excessive wires in automobiles.

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In addition to the microcontroller, a simple LIN node requires two chips, a LIN interface chip and a 5V voltage regulator. Freescale MC33399 chips are single-line physical interface devices designed for LIN. The power consumption of the device is very low, the external regulator can be controlled, the safety conforms to the LIN specification, and the anti-jamming capability is strong. It is a high-performance analog device, suitable for cars with complex working environment. The main features of MC33399 are as follows:

The communication rate ranges from 1 to 20 kb/s;

Rated operating voltage: 8V-18V, normal voltage: 7V-27V;

Reactive node does not affect bus state;

/ There are two working modes, normal and sleep, and the static current in sleep mode is only 20. μ A;

/ LIN bus wake-up, MCU command wake-up and high voltage switch input wake-up outside the interface;

Interface with the MCU through a compatible CMSI/O foot;

With external regulator control function;

Built-in pull-up resistance;

The ESD voltage of the LIN pin can reach 4kV;

Good electromagnetic compatibility;

The operating temperature ranges from -40 to 125 C.
2 pin function

MC33399 is packed with SO8 patch. Figure 1 shows the pin arrangement, and the functional description of each pin is listed in Table 1.

Table 1 Pin function of MC33399

Pin Number Pin Name Function

1RX Receive Data Output
2EN Enabler
3Wake wake-up end for waking chips from sleep
4TX Send Data Input
5GND power source
6LINLIN Bus Interface End
7Vsup Power Supply
8INH External Regulator Control End

3 Internal structure and working principle

The internal structure of MC33399 is shown in Figure 2. Now its main working principle is described as follows:

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3.1 Main Functions

The working mode of MC33399 is controlled by enabler EN. When EN is 1, the circuit is in normal operation, and the two transmission paths from TX pin to LIN bus and from LIN bus to RX pin are activated. When EN is 0, the chip goes to sleep or low power mode, and the two transmission paths are prohibited. In sleep mode, the LIN pin maintains a high level Vsup through pull-up resistance and upper current source, and provides a static current to the chip from the power supply through the Vsup pin (typical value is 20) μ A).

The LIN pin of MC33399 is used to complete the single-bus transceiver function. The TX pins are used to control the output status of the LIN pins, which are always in the same state when working. The RX pin can be used to display the status of the LIN bus, which is high when the LIN bus is at a high level (recessive). Conversely, when the LIN bus is low (dominant), the RX is low. The RX output is a typical CMOS push-pull output structure.

Due to its specific internal structure, when local drift or power connection fails, no reverse current will enter the chip from the LIN bus. MC33399 allows the Wake pin to enter a high voltage wake-up, and can also wake up with two jumps of the Wake pin from high to low or from low to high. When the chip goes to sleep, the chip will record the current Wake pin status. When the wake-up state is detected by the circuit, the wake-up event is considered to have occurred. In addition, the chip has a built-in noise filter, which can suppress high frequency bus noise interference and prevent error wake-up.

The Wake Pin External Switch enables high voltage input to the Wake Pin to wake MC33399 from sleep mode. When the input voltage of the Wake pin exceeds 14V, to prevent excessive input current, a series resistor must be connected to limit the instantaneous pulse current (the resistance value is generally 33k). At the same time, it must be noted that the Wake pin should not be suspended. If the pin is not used, it must be grounded to prevent false wakeup.

5 Concluding remarks

MC33399 circuit is widely used in automobile body control system with its perfect performance. The external regulator used with this circuit can optionally use the 8-pin circuit LT1121. In fact, these two chips will be replaced by Freescale's new System Base Chip (SBC).