Filter
Top Products
730
Chapter 34 CAN Controller
4.CAN Application
The Status Interrupt has the highest priority. Among the message interrupts, the Message Object’ s interrupt
priority decreases with increasing message number.
A message interrupt is cleared by clearing the Message Object’s IntPnd bit. The Status Interrupt is cleared by
reading the Status Register.
The interrupt identifier IntId in the Interrupt Register indicates the cause of the interrupt. When no interrupt is
pending, the register will hold the value zero. If the value of the Interrupt Register is different from zero, then
there is an interrupt pending and, if IE is set, the interrupt line to the CPU is active. The interrupt line remains
active until the Interrupt Register is back to value zero (the cause of the interrupt is reset) or until IE is reset.
The value 0x8000 indicates that an interrupt is pending because the CAN Core has updated (not necessarily
changed) the Status Register (Error Interrupt or Status Interrupt). This interrupt has the highest priority. The
CPU can update (reset) the status bits RxOk, TxOk and LEC, but a write access of the CPU to the Status
Register can never generate or reset an interrupt.
All other values indicate that the source of the interrupt is one of the Message Objects, IntId points to the
pending message interrupt with the highest interrupt priority.
The CPU controls whether a change of the Status Register may cause an interrupt (bits EIE and SIE in the
CAN Control Register) and whether the interrupt line becomes active when the Interrupt Register is different
from zero (bit IE in the CAN Control Register). The Interrupt Register will be updated even when IE is reset.
The CPU has two possibilities to follow the source of a message interrupt. First it can follow the IntId in the
Interrupt Register and second it can poll the Interrupt Pending Register (see section 2.6 Message Handler
Registers (Page No.714).
An interrupt service routine reading the message that is the source of the interrupt may read the message and
reset the Message Object’s IntPnd at the same time (bit ClrIntPnd in the Command Mask Register). When
IntPnd is cleared, the Interrupt Register will point to the next Message Object with a pending interrupt.
4.17 Bit Time and Bit Rate
The timing parameter of the bit time (i.e. the reciprocal of the bit rate) can be configured individually for each
CAN node, creating a common bit rate even though the CAN nodes’ oscillator periods (f
osc
) may be different.
The frequencies of these oscillators are not absolutely stable, small variations are caused by changes in
temperature or voltage and by deteriorating components. As long as the variations remain inside a specific
oscillator tolerance range (df), the CAN nodes are able to compensate for the different bit rates by
resynchronising to the bit stream.
According to the CAN specification, the bit time is divided into four segments (see Figure 4-5). The
Synchronisation Segment, the Propagation Time Segment, the Phase Buffer Segment 1, and the Phase
Buffer Segment 2. Each segment consists of a specific, programmable number of time quanta (see Table 1).
The length of the time quantum (t
q
) , which is the basic time unit of the bit time, is defined by the CAN
controller’s system clock f
sys
and the Baud Rate Prescaler (BRP) : t
q
= BRP / f
sys
. The CAN’s system clock
f
sys
is the frequency of its CAN_CLK input.
The Synchronisation Segment Sync_Seg is that part of the bit time where edges of the CAN bus level are
expected to occur; the distance between an edge that occurs outside of Sync_Seg and the Sync_Seg is called
the phase error of that edge. The Propagation Time Segment Prop_Seg is intended to compensate for the
physical delay times within the CAN network. The Phase Buffer Segments Phase_Seg1 and Phase_Seg2
surround the Sample Point. The (Re-)Synchronisation Jump Width (SJW) defines how far a resynchronisation
may move the Sample Point inside the limits defined by the Phase Buffer Segments to compensate for edge
phase errors.