MikroElektronika PIC Microcontrollers PIC18 Instrukcja Użytkownika Pobierz pdf (Strona 13)

device can have its own different clock frequency. One message bit consists of four

nonoverlapping time segments:



Synchronization segment (Sync_Seg)



Propagation time segment (Prop_Seg)



Phase buffer segment 1 (Phase_Seg1)



Phase buffer segment 2 (Phase_Seg2)

The Sync_Seg segment is used to synchronize various nodes on the bus, and an edge

is expected to lie within this segment. The Prop_Seg segment compensates for

physical delay times within the network. The Phase_Seg1 and Phase_Seg2 segments

compensate for edge phase errors. These segments can be lengthened or shortened by

synchronization. The sample point is the point in time where the actual bit value is

located and occurs at the end of Phase_Seg1. A CAN controller can be configured

to sample three times and use a majority function to determine the actual bit value.

Each segment is divided into units known as time quantum, or T

. A desired bit

timing can be set by adjusting the number of T

’s that comprise one message bit

and the number of T

’s that comprise each segment in it. The T

is a fixed unit

derived from the oscillator period, and the time quantum of each segment can vary

from 1 to 8. The lengths of the various time segments are:



Sync_Seg is 1 time quantum long



Prop_Seg is programmable as 1 to 8 time quanta long



Phase_Seg1 is programmable as 1 to 8 time quanta long



Phase_Seg2 is programmable as 2 to 8 time quanta long

By setting the bit timing, a sampling point can be set so multiple units on the bus can

sample messages with the same timing.

The nominal bit time is programmable from a minimum of 8 time quanta to a maximum

of 25 time quanta. By definition, the minimum nominal bit time is 1ms, corresponding

to a maximum 1Mb/s rate. The nominal bit time (T

BIT

) is given by:

BIT

¼ T

ðSync Seg þ Prop Seg þ Phase Seg1 þ Phase Seg2Þð9:1Þ

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