Transceivers for CAN-bus networks. Reference data

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Transceivers for CAN-bus networks. Reference data

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The industrial network CAN-bus (Controller Area Network) was created by Robert Bosch for use in distributed control systems operating in real time with transmission speeds of up to 2 Mbit/s and was initially used exclusively in the automotive industry. High noise immunity and reliability, as well as a large selection of components from major manufacturers (Fujitsu, Maxim, Motorola, NEC, Siemens, Texas Instruments, etc.) have turned CAN into a set of software, circuitry and algorithmic solutions for a wide range of applications - into CAN technology. An important place in the CAN network is given to transceivers (TCs) that connect the CAN controller and the actual wires of the CAN bus. Let's look at MAXIM's PPs, which may be of interest to consumers due to the traditionally high quality of all the company's products, low cost (about 2,5 USD) and a number of technological features.

The MAX3050/3057 transceivers (Figure 1) were originally intended for the automotive industry, where data rates up to 2 Mbps and short-circuit protection are required in systems with high supply voltages. PPs are characterized by a unipolar power supply +5 V, a current consumption of 56 mA in the active state and 3,6 mA in the passive state.

MAX3050/3057 instruments operate in four modes:

high performance;
adjustable speed;
reduced consumption;
shutdown.

Transceivers for CAN-bus networks. Reference data. MAX3050/3057 transceivers

Rice. 1. MAX3050/3057 transceivers

The mode is determined by a signal of a certain level at the RS pin of the microcircuit. The transmitter then converts the unipolar signal from the CAN controller into a differential signal for the CAN bus (CANH, CANL). The high-speed mode (the KB output is connected to a common wire) allows you to achieve a transfer rate of 2 Mbps. In this mode, there is an increased level of electromagnetic interference, which can be reduced by using a shielded pair.

To reduce the level of interference without the use of shielded twisted pair, you can use the adjustable speed mode. In this mode, the baud rate (40 to 500 kbps) is determined by a resistor connected between the RS pin and ground. Resistor resistance is calculated by the formula:

To switch to low power mode, you must apply a high level to the RS pin of the microcircuit. In this case, the transmitters are turned off, and the receivers are in a low power state. In this mode, it is possible to lose the first message transmitted at high speed.

The MAX3057 transceiver turns off when a low level signal is applied to the SHDN pin. The MAX3050 software implements the AutoShutdown mode developed by MAXIM, in which the microcircuit is turned off if there is no reception or transmission for a specified time. The value of this time is set by an external capacitor connected to the SHDN pin and is determined by the formula:

where v_SHDN - SHDN threshold.

Applying a high level to the SHDN pin puts the MAX3050 into operation.

The receiver reads the differential signal from the bus (CANH, CANL) and converts it to unipolar (RXD) for the CAN controller. The comparator at the receiver input compares the voltage difference ΔV = (CANH-CANL) with an internal threshold of 0,7 V. If the difference is positive, then a low level signal is generated at the RXD pin, if negative, then a high one.

In the MAX3050/MAX3057 software, the "echo" mode is implemented for the receiver, i.e. the transmitted data is repeated.

The common-mode signal range for the CANH and CANL lines is from ~7 to +12 V. In the event of a short circuit or open circuit in the CANH-CANL circuit, and also if the input common-mode voltage is less than 0,5 V, a high level signal is generated at the RXD pin. If the input voltage is more than 0,9 V, then RXD is low.

MAX3050/3057 microcircuits have two types of protection. The first type is thermal protection, which disables the chip and sets the CANH and CANL lines to the third state if the crystal temperature exceeds +160°C. This is possible, for example, due to a short circuit on the bus. The hysteresis is 20°C, i.e. the reverse switching will occur when the temperature drops to +140°C. The second type of protection is limiting the output stage current during a short circuit.

A more simplified version of the MAX3050 chip is the MAX3053, which does not have a low power mode. Otherwise, it is a complete analogue of the MAX3050.

MAXIM also produces MAX3054/3055/3056 microcircuits (their block diagram is shown in Fig. 2), intended for use in automotive systems (including those with an on-board voltage of 42 V). Their main feature is the transition to single-wire transmission mode when various failures occur. The reliability of microcircuits is determined by thermal and current protection. Thermal protection is similar to MAX3050 (hysteresis -15°C). Current protection protects the transmitter output in case of a short circuit on the bus. Additionally, the CANH and CANL lines are protected from impulse noise typical of automotive electronics.

The MAX3054/3055/3056 transceivers operate in three modes, the transition to each of which (and the output too) is set by applying control signals to the STB and EN pins. The INH pin is used to turn off the external power regulator.

Each of the microcircuits of this family is designed for a certain data transfer rate: MAX3054 - 250 kb / s, MAX55 - 125 kb / s, MAX3056 - 40 kb / s. High noise immunity is provided by filters at the input of the receivers, and the built-in variable speed circuit in the MAX3055 and MAX3056 microcircuits eliminates the need for a shielded cable.

The operation of the receiver and transmitter in normal mode is similar to that of the MAX3050/3057. We only note their differences.

To operate from a car battery (including a 42-volt one), the microcircuit has a WATT output, through which power is supplied (up to +80 V).

In order to reduce consumption in standby mode, the MAX3054-MAX3056 microcircuits implement the function of controlling an external power source. When entering the standby mode, the potential at the INH output changes from low to high in three cases: during a “cold” start, along the edge or fall of the signal at the WAKE pin, and if the duration of the dominant signal is more than 38 μs at low levels at the EN and STB pins. Upon power-up, the INH pin goes high and the internal power-on flag is set. The value of this flag can be read in standby mode at the ERR pin (STB = 1, EN = 0) and reset when returning to normal operating mode.

To have information about the state of the microcircuits, they provide an ERR pin. The signal on this pin is set to "0" upon power-up, exit from standby mode, and during a fault condition. In other cases, the ERR pin is high.

Refusals and what to do with them

The failure detection circuitry is fully enabled in normal operating mode. When a failure is detected, it switches to the appropriate state, as shown in Table. 1. Typical threshold voltage is -3,2V (Vcc = 5V) to ensure correct data reception with marginal noise immunity in normal mode or faults 1, 2, 5, and 9.

Table 1

Refusal No.	Description	Mode	Status of internal keys (0 - disabled, 1 - enabled)	Output status (0 - disable, 1 - enable)
Refusal No.	Description	Mode	Status of internal keys (0 - disabled, 1 - enabled)	SOUP	CANL
1	CANH line break	Normal	RTH, RTL-1	1	1
2	CANL line break	Normal	RTH, RTL-1	1	1
3	Short circuit CANH to BATT bus	All	RTH-0	0	1
4	Short circuit CANL to common bus	All	RTH or RTL-0	1	0
5	Short circuit CANH to common bus	Normal	RTH,RTL-1	1	1
6	Short circuit CANL to BATT bus	Normal	RTH-1, RTL-0	1	1
7	Short circuit CANL line to CANH	All	RTH or RTL-0	1	0
8	Short circuit CANH on the power bus	All	RTH-0	0	1
9	Short circuit CANL to the power bus	Normal	RTH,RTL-1	1	1

Low Power Modes

The MAX3054-MAX3056 transceivers can operate in one of three low power modes, which are selected by applying signals to the STB and EN pins (Table 2).

Table 2

Mode	Pin levels						Buses to which the RTL pin is connected
	ETC.	EN	ERR		RXD
	ETC.	EN	0	1	0	1
Transition to duty	0	1	Interrupt exit	-	Interrupt exit	-	V_WATT
Duty	0	0
Expectations	0	0
Waiting for power on	1	0	Enable flag V_WATT	-	Interrupt exit	-	V_WATT
Normal	1	1	Error Flag	no flag	Dominant received data	Recessive received data	Vcc

Standby (sleep) mode is a mode with minimal consumption. In this mode, the external voltage stabilizer is turned off by switching the INH output to the third state, the CANL output is biased from the power source through the RTL output. If power is applied, an interrupt is generated on the RXD and ERR pins to start the system.

The standby mode can be used when an external voltage regulator is needed to keep the active state at low consumption (similar to the standby mode, but the INH output is set high).

Power-on standby mode - when the microcircuit switches from standby mode to power-on standby mode, the INH output is set to a high level, and from standby - to a low level.

Features of the application in the network

The MAX3054/3055/3056 can network with up to 32 total transceivers on the same bus. These circuit breakers are designed to operate with a total of 100 ohm terminating resistors (for each of the CANH and CANL lines). The value of the terminating resistors RTL and RTH varies according to the size of the particular system. But if you do not have a desire to do calculations, then you can put the same resistors, it is important that their total resistance does not exceed 100 ohms.

Publication: cxem.net

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