Users Guide: Network Microcontroller Supplement User Manual

High-Speed Microcontroller User’s
Guide: Network Microcontroller
Supplement
164
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 uses the most common (i.e., the oscillator frequency and baud rate have
already been determined by system constraints.)
Additional Bit Timing Examples
The following is an explanation of how the table row illustrating an oscillator frequency of 16MHz 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
remain-
ing for TS1_LEN and TS2_LEN, which are arbitrarily assigned as shown. Because BRPV > 3, the triple sampling feature (SMP = 1) can
be used, if desired.
FOSC
CCD BRPV TQU
BAUD
RATE
T
QU
PER BIT
TS1_LEN
TS2_LEN
SJW
SMP = 1
PERMITTED?
2 2 100ns 1Mbps 10 5 4 3 NO
2 4 200ns 500kbps 10 5 4 3 YES
2 5 250ns 250kbps 16 10 5 4 YES
40MHz
2 10 500ns 125kbps 16 10 5 4 YES
0.5 2 125ns 1Mbps 16 10 5 4 NO
1 1 125ns 1Mbps 16 10 5 4 NO
2 1 125ns 1Mbps 8 4 3 2 NO
2 1 125ns 500kbps 16 10 5 4 NO
2 2 250ns 250kbps 16 10 5 4 NO
16MHz
2 4 500ns 125kbps 16 10 5 4 YES
1 1 125ns 1Mbps 8 4 3 2 NO
1 1 125ns 500kbps 16 10 5 4 NO
1 1 250ns 250kbps 16 10 5 4 NO
8MHz
2 2 500ns 125kbps 16 10 5 4 NO
Maxim Integrated