Fujitsu FR81S User Manual
CHAPTER 41: CAN
5. Operation
FUJITSU SEMICONDUCTOR LIMITED
CHAPTER
: CAN
FUJITSU SEMICONDUCTOR CONFIDENTIAL
81
5.6. Bit Timing and CAN System Clock (fsys) Generation
Bit timing and CAN system clock (fsys) generation is shown.
This section explains the overview of bit timing and its role in the CAN controller.
Each CAN node of the CAN network has a clock oscillator (normally a crystal oscillator). Time parameter
of bit time can be configured individually for each CAN node. A common bit rate can be produced even if
the oscillation cycle (fosc) of each CAN node is different.
Frequency of these oscillators differ slightly by temperature/voltage change or component deterioration.
CAN node can compensate different bit rates by resynchronizing to the bit stream, as long as this
fluctuation falls within the tolerance range (df) of the oscillator.
The bit time is divided into the following four segments (see Figure 5-3 Bit timing) according to the CAN
specification: synchronization segment (Sync_Seg), transmission time segment (Prop_Seg), phase buffer
segment 1 (Phase_Seg1) and phase buffer segment 2 (Phase_Seg2). Each segment consists of a
programmable time quantum (see Table 5-3 CAN Bit Time Parameters). Basic unit time (tq) of the bit time
is defined by the system clock fsys of the CAN controller and baud rate prescaler (BRP).
tq = BRP / fsys
CAN system clock fsys will be generated as shown in the figure below. Sync_Seg of the synchronization
segment will be the timing within the bit time expecting the edge of the CAN bus. Prop_Seg of the
transmission time segment compensates the physical delay time in the CAN network. Phase_Seg1 and
Phase_Seg2 of the phase buffer segment specify the sampling point. Resynchronization jump width (SJW)
defines the displacement of the sampling point at resynchronization in order to compensate the edge phase
error.
Figure 5-3 Schematic Diagram of CAN System Clock (fsys) Generation
Figure 5-4 Bit timing
( Set with CANPRE[3:0])
/(1 to 12)
CAN prescaler clock
(See "CHAPTER5 CLOCK".)
CAN system clock (fsys)
Prescaler
divide (1 to 12)
(See CHAPTER of "CLOCK")
1 bit time (BT)
Sample point
1 unit time
(tq)
Sync
_Seg
Prop_Seg
Phase_Seg1
Phase_Seg2
MB91520 Series
MN705-00010-1v0-E
1774