MCP2021 datasheet(7/52 Pages) MICROCHIP

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DS22018F-page 7

MCP2021/2/1P/2P

1.3.5

TRANSMITTER-OFF MODE

Whenever the FAULT/TXE signal is low, or permanent

dominant on TXD/LBUS is detected, the LBUS

transmitter is off.

The transmitter may be re-enabled whenever the

FAULT/TXE signal returns high, either by removing the

internal fault condition or when the CPU returning the

FAULT/TXE high. The transmitter will not be enabled if

the FAULT/TXE pin is brought high when the internal

fault is still present.

If TX-OFF mode is caused by TXD/LBUS permanent

dominant level, the transmitter can recover when the

permanent dominant status disappears.

The transmitter is also turned off whenever the voltage

regulator is unstable or recovering from a fault. This

prevents unwanted disruption of the bus during times of

uncertain operation.

1.3.6

WAKE-UP

The Wake-Up sub-module observes the LBUS in order

to detect bus activity. Bus activity is detected when the

voltage on the LBUS stays below a threshold of approx-

imately 0.4VBB for at least a typical duration of 20 µs.

The MCP2021/2 device is level sensitive to LBUS. Dom-

inant level longer than 20 µs will cause the device to

leave the Power-Down mode. The MCP2021P/2P

device is falling-edge sensitive to LBUS. Only the LBUS

transition from recessive to dominant followed by at

least 20 µs dominant level can wake up the device.

Putting CS/LWAKE to high level also wakes up the

device. Refer to Figure 1-2 and Figure 1-3.

1.3.7

DIFFERENCE DETAILS BETWEEN

MCP2021/2 AND MCP2021P/2P

The MCP202xP is a minor variation of the MCP202x

device that adds improved state machine control as

well as the ability to disconnect the internal 30k

Ω pull-

up between LIN and VBB in all modes except normal

operation. These changes allow the system designer

to better handle fault conditions and reduce the overall

system current consumption. The differences between

the two device versions are as follows:

Switchable LIN-VBB Pull-Up Resistor:

On the MCP202xP device, the internal 30k

pull-up resistor is disconnected in all modes

except Operation Mode.

On the MCP202x

device, this pull-up resistor is always connected.

(See the MCP2021/2 Block Diagram and the

MCP2021P/2P Block Diagram for details.)

Power Down Wake-up on LIN Traffic:

The MCP202xP device requires a LIN falling

edge to generate a valid Wake condition due to

bus traffic. The MCP202x device will generate a

Wake anytime LIN is at a valid dominant level.

Because of this, if the LIN bus becomes perma-

nently shorted, it becomes impossible to place

the MCP202x in a low power state.

State Machine Options:

The MCP202xP device is able to enter Trans-

mitter Off Mode from Ready Mode without tran-

sitioning

through

Operation

Mode.

The

MCP202x device must enter Operation Mode

from Ready Mode. (See State Machine Dia-

grams, Figure 1-2 and Figure 1-3 for details).

This capability allows the system designer to

monitor the bus in Ready Mode to determine if

the system should transition to normal operation

and connect the internal pull-up or if Ready

Mode was reached due to an invalid condition.

In the case of an invalid condition, the

MCP202xP device can be placed into Power

Down mode without connecting the internal pull-

up and waking other nodes on the LIN Bus net-

work.

To properly take advantage of the device differences

will require the system designer to implement some

microcontroller code to the power-up routine. This code

will monitor the status of the LIN bus to determine how

the dominant signal should be responded to. It will also

determine if the local LIN node needs to respond or can

‘Listen Only’. If the local LIN node does not need to

respond, it can enter Transmitter Off Mode, disconnect-

ing the 30k

Ω pull-up, reducing module current while still

maintaining the ability to properly receive all valid LIN

messages.

Note:

To enter Transmitter Off, the system must

set TXE ‘low’ before pulling CS high (see

Figure 1-5). Otherwise, if CS is pulled high

first, the MCP202xP will enter Operation

Mode due to the internal pull-up on TXE.

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