Maxim Integrated High-Speed Microcontroller Users Guide: DS80C390 Supplement User Manual
Page 155
High-Speed Microcontroller User’s Guide: DS80C390 Supplement
155 of 158
maximum bit time = t
SYNC_SEG
+ t
TSEG1
+ t
TSEG2
+ t
SJW
=
OSC
F
)]
SJW
(
)
LEN
_
2
TS
(
)
LEN
_
1
TS
(
1
)[
CCD
)(
BRPV
(
+
+
+
CAN baud rate
=
)]
LEN
_
2
TS
(
)
LEN
_
1
TS
(
1
)[
CCD
)(
BRPV
(
F
OSC
+
+
Threefold Bit Sampling:
The DS80C390 supports the ability perform one or three samplings of each bit, based on the SMP bit
(CxBT1.7). The single sample mode (SMP=0) is available in all settings and takes one sample during
each bit time. The triple sampling mode (SMP=1) samples each bit three times for increased noise
immunity. This mode can only be used when the baud rate prescale value (BPRV) is greater than 3.
Bus Rate Timing Example:
The following table shows a few example bit timing settings for common oscillator frequency and baud
rate selections. Because of the large number of variables, there are many combinations not shown that can
achieve a desired baud rate. There are a number of approaches to determining all the bit timing factors,
but this utilizes the most common, i.e., the oscillator frequency and baud rate have already been
determined by system constraints.
Additional Bit Timing Examples:
F
OSC
Baud
rate
BRPV
CCD t
QU
t
QU
per bit
TS1_LEN TS2_LEN SJW
SMP=1
Permitted?
40 MHz
1 Mbps
2
2
100 ns
10
5
4
3
NO
500 kbps
4
2
200 ns
10
5
4
3
YES
250 kbps
5
2
250 ns
16
10
5
4
YES
125 kbps
10
2
500 ns
16
10
5
4
YES
16 MHz
1 Mbps
1
2
125 ns
8
4
3
4
NO
500 kbps
1
2
125 ns
16
10
5
4
NO
250 kbps
2
2
250 ns
16
10
5
4
NO
125 kbps
4
2
500 ns
16
10
5
4
YES
8 MHz
1 Mbps
1
1
125 ns
8
4
3
2
NO
500 kbps
1
1
125 ns
16
10
5
4
NO
250 kbps
1
1
250 ns
16
10
5
4
NO
125 kbps
2
2
500 ns
16
10
5
4
NO
As an aid to understanding, the following is an explanation of how the table row illustrating an oscillator
frequency of 16 MHz and a CAN baud rate of 125kbps is derived.
Various combinations of BRPV are selected until one is located that meets the “t
QU
per bit” criteria, i.e.,
an integer value less than 25. Selecting BRPV=4, the previously described equations state that there
should be 16 t
QU
per bit. That leaves 16-1 or 15 t
QU
remaining for TS1_LEN and TS2_LEN, which are
arbitrarily assigned as shown. Because BRPV > 3, the triple sampling feature (SMP=1) may be used if
desired.