AT90CAN128-16AE ATMEL Corporation, AT90CAN128-16AE Datasheet - Page 232

• Manufacturer Part Number: AT90CAN128-16AE
• Description: 8-bit Avr Microcontroller With 128K Bytes of Isp Flash And CAN Controller.flash (Kbytes) 128 Vcc (V) 2.7-5.5 EEPROM (Kbytes) 4 SRAM (bytes) 4K CAN (mess. Obj.) 15
• Manufacturer: ATMEL Corporation

Phase Segment 1
Sample Point
Phase Segment 2
Bit Lengthening
Bit Shortening
Synchronization Jump Width
Programming the Sample Point
Arbitration
232
AT90CAN128
Phase Segment 1 is used to compensate for edge phase errors.
This segment may be lengthened during resynchronization.
The sample point is the point of time at which the bus level is read and interpreted as the
value of the respective bit. Its location is at the end of Phase Segment 1 (between the
two Phase Segments).
This segment is also used to compensate for edge phase errors.
This segment may be shortened during resynchronization, but the length has to be at
least as long as the information processing time and may not be more than the length of
Phase Segment 1.
As a result of resynchronization, Phase Segment 1 may be lengthened or Phase Seg-
ment 2 may be shortened to compensate for oscillator tolerances. If, for example, the
transmitter oscillator is slower than the receiver oscillator, the next falling edge used for
resynchronization may be delayed. So Phase Segment 1 is lengthened in order to
adjust the sample point and the end of the bit time.
If, on the other hand, the transmitter oscillator is faster than the receiver one, the next
falling edge used for resynchronization may be too early. So Phase Segment 2 in bit N
is shortened in order to adjust the sample point for bit N+1 and the end of the bit time
The limit to the amount of lengthening or shortening of the Phase Segments is set by the
Resynchronization Jump Width.
This segment may not be longer than Phase Segment 2.
Programming of the sample point allows "tuning" of the characteristics to suit the bus.
Early sampling allows more Time Quanta in the Phase Segment 2 so the Synchroniza-
tion Jump Width can be programmed to its maximum. This maximum capacity to
shorten or lengthen the bit time decreases the sensitivity to node oscillator tolerances,
so that lower cost oscillators such as ceramic resonators may be used.
Late sampling allows more Time Quanta in the Propagation Time Segment which allows
a poorer bus topology and maximum bus length.
Figure 111. Bus Arbitration
The CAN protocol handles bus accesses according to the concept called “Carrier Sense
Multiple Access with Arbitration on Message Priority”.
CAN bus
node A
TXCAN
node B
TXCAN
SOF
SOF
ID10 ID9 ID8 ID7 ID6 ID5 ID4 ID3 ID2 ID1 ID0
Arbitration lost
Node A loses the bus
Node B wins the bus
RTR IDE
- - - - - - - - -
4250C–CAN–03/04