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COP87L88EB Datasheet(PDF) 32 Page - National Semiconductor (TI) |
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COP87L88EB Datasheet(HTML) 32 Page - National Semiconductor (TI) |
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32 / 72 page  Frame Formats (Continued) ERROR MANAGEMENT AND DETECTION There are multiple mechanisms in the CAN protocol, to de- tect errors and to inhibit erroneous modules from disabling all bus activities. The following errors can be detected: • Bit Error A CAN device that is sending also monitors the bus. If the monitored bit value is different from the bit value that is sent, a bit error is detected. The reception of a “dominant” bit in- stead of a “recessive” bit during the transmission of a pas- sive error flag, during the stuffed bit stream of the arbitration field or during the acknowledge slot, is not interpreted as a bit error. • Stuff error A stuff error is detected, if the bit level after 6 consecutive bit times has not changed in a message field that has to be coded according to the bit stuffing method. • Form Error A form error is detected, if a fixed frame bit (e.g., CRC delim- iter, ACK delimiter) does not have the specified value. For a receiver a “dominant” bit during the last bit of End of Frame does NOT constitute a form error. • Bit CRC Error A CRC error is detected if the remainder of the CRC calcula- tion of a received CRC polynomial is non-zero. • Acknowledgment Error An acknowledgment error is detected whenever a transmit- ting node does not get an acknowledgment from any other node (i.e., when the transmitter does not receive a “domi- nant” bit during the ACK frame). The device can be in one of three states with respect to error handling: • Error active An error active unit can participate in bus communication and sends an active (“dominant”) error flag. • Error passive An error passive unit can participate in bus communication. However, if the unit detects an error it is not allowed to send an active error flag. The unit sends only a passive (“reces- sive”) error flag. A device is error passive when the transmit error counter is greater than 127 or when the receive error counter is greater than 127. A device becoming error passive sends an active error flag. An error passive device becomes error active again when both transmit and receive error counter are less than 128. • Bus off A unit that is “bus off” has the output drivers disabled, i.e., it does not participate in any bus activity. A device is bus off when the transmit error counter is greater than 255. A bus off device will become error active again in one of two ways de- pending on which mode is selected by the user through the Fault Confinement Mode select bit (FMOD) in the CAN Bus Control Register (CBUS). Setting the FMOD bit to “0” (de- fault after power on reset) will select the Standard Fault Con- finement mode. In this mode the device goes from “bus off” to “error active” after monitoring 128*11 recessive bits (in- cluding bus idle) on the bus. This mode has been imple- mented for compatibility reasons with existing solutions. Set- ting the FMOD bit to “1” will select the Enhanced Fault Confinement mode. In this mode the device goes from “bus off” to “error active” after monitoring 128 “good” messages, as indicated by the reception of 11 consecutive “recessive” bits including the End of Frame. The enhanced mode offers the advantage that a “bus off” device (i.e., a device with a se- rious fault) is not allowed to destroy any messages on the bus until other devices can transmit at least 128 messages. This is not guaranteed in the standard mode, where a defec- tive device could seriously impact bus communication. When the device goes from “bus off” to “error active”, both error counters will have the value “0”. In each CAN module there are two error counters to perform a sophisticated error management. The receive error counter (REC) is 7 bits wide and switches the device to the error passive state if it overflows. The transmit error counter (TEC) is 8 bits wide. If it is greater than 127, the device is switched to the error passive state. As soon as the TEC overflows, the device is switched bus-off, i.e., it does not par- ticipate in any bus activity. DS100044-31 FIGURE 26. Order of Bit Transmission within a CAN Frame www.national.com 32 |
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