MCP2510-SLASHST datasheet(8/80 Pages) MICROCHIP

8 / 80 page

MCP2510

DS21291F-page 8

sequence actively violates the bit stuffing rule. All other

stations recognize the resulting bit stuffing error and in

turn generate error frames themselves, called error

echo flags. The error flag field, therefore, consists of

between six and twelve consecutive dominant bits

(generated by one or more nodes). The error delimiter

field completes the error frame. After completion of the

error frame, bus activity returns to normal and the inter-

rupted node attempts to resend the aborted message.

If an error-passive node detects a bus error then the

node transmits an error-passive flag followed by the

error delimiter field. The error-passive flag consists of

six consecutive recessive bits, and the error frame for

an error-passive node consists of 14 recessive bits.

From this, it follows that unless the bus error is

detected by the node that is actually transmitting, the

transmission of an error frame by an error-passive

node will not affect any other node on the network. If

the transmitting node generates an error-passive flag

then this will cause other nodes to generate error

frames due to the resulting bit stuffing violation. After

transmission of an error frame, an error-passive node

must wait for six consecutive recessive bits on the bus

before attempting to rejoin bus communications.

The error delimiter consists of eight recessive bits and

allows the bus nodes to restart bus communications

cleanly after an error has occurred.

2.5

Overload Frame

An Overload Frame, shown in Figure 2-5, has the

same format as an active error frame. An overload

frame, however can only be generated during an lnter-

frame space. In this way an overload frame can be dif-

ferentiated from an error frame (an error frame is sent

during the transmission of a message). The overload

frame consists of two fields, an overload flag followed

by an overload delimiter. The overload flag consists of

six dominant bits followed by overload flags generated

by other nodes (and, as for an active error flag, giving

a maximum of twelve dominant bits). The overload

delimiter consists of eight recessive bits. An overload

frame can be generated by a node as a result of two

conditions. First, the node detects a dominant bit during

the interframe space which is an illegal condition. Sec-

ond, due to internal conditions the node is not yet able

to start reception of the next message. A node may

generate a maximum of two sequential overload

frames to delay the start of the next message.

2.6

Interframe Space

The lnterframe Space separates a preceeding frame

(of any type) from a subsequent data or remote frame.

The interframe space is composed of at least three

recessive bits called the Intermission. This is provided

to allow nodes time for internal processing before the

start of the next message frame. After the intermission,

the bus line remains in the recessive state (bus idle)

until the next transmission starts.

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