ST72521 datasheet(150/211 Pages) STMICROELECTRONICS

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ST72521

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CONTROLLER AREA NETWORK (Cont’d)

The worst case is when the abort request is done

when the transmission has just started. In this

case the LOCK bit cannot be reset as long as the

BUSY bit is set, this means until the end of the

frame. So the application will wait for READY to be

reset during the whole frame and in this case the

worst case will be the longest frame the applica-

tion is expected to transmit.

Figure 80. Abort with the software work-around

- by NRTX, BUSY and LOCK

Using the software work-around the worst case

occurs in the arbitration lost case. If the abort is re-

quested just after pCAN has lost the arbitration

then the application has to wait for the next falling

edge of the BUSY bit before the LOCK bit can be

reset. If the next arbitration is won by pCAN then

the BUSY bit will be reset by the end of the suc-

cessful transmission. The longest time the applica-

tion has to wait in this case is the time of the long-

est message expected on the bus (minus identifi-

er) plus the longest message expected to be trans-

mitted by the application. This roughly double the

time the application may have to wait before the

abort sequence is performed.

10.8.5.4 WKPS Functionality

Due to a fix implemented to solve the “Unexpected

Message

Transmission”

issue

(see

Section

10.8.5.3) the WKPS functionality has been modi-

fied as follows in Flash ST72F521 devices:

10.8.5.5 Bus-off state not entered

Symptom:

pCAN does not enter bus-off state under certain

conditions. This is fixed in flash ST72F521 Rev R .

Details:

According to the CAN standard, pCAN is expected

to enter bus-off state when TEC (Transmit Error

Counter) is greater than 255.

But if REC (Receive Error Counter) is greater than

127 (Error Passive State) pCAN does not enter

bus-off and the BOFF bit of the CSR register is not

set. To enter bus-off, REC must decrease to a val-

ue lower than 128, this is the case with any correct

reception even if the message is filtered out.

As bus-off state is not entered and pCAN still at-

tempts to transmit its message, after the overflow

the TEC register continues to increment as long as

transmission errors occur.

Impact on the application:

The application will not stop attempting to transmit

CAN messages, even when the bus-off conditions

have been reached, until the transmission has

been successful or the value of REC becomes

lower than 128. However the application will not

disturb the communication of the other nodes on

the CAN network as pCAN is in Error Passive

State.

CAN TX

CAN RX

LOCK

READY

BUSY

TX RQST

NRTX

ABORT RQST

Device

Modification

Flash

ST72F521

Rev R

WKPS bit does not generate a wakeup

pulse. It is used to synchronize the re-

set of the LOCK bit (see “Software

Work-around - Devices with Hardware

Fix (ST72F521 rev “R”):” on page 146)

ROM

ST72521 All

revisions

WKPS bit functions according to the

datasheet description.

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