NEC V850E/CA1 ATOMIC, Preliminary User'S Manual

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Chapter 13 FCAN Interface Function
Preliminary User’s Manual U14913EE1V0UM00
13.4.2 Example for baudrate setting of CAN module
To illustrate how to calculate the correct setting of the registers CxBRP and CxSYNC the following
example is given to 3:
Requirements from CAN bus:
- FCAN system global frequency f
MEM
= 16 MHz
- CAN bus baud rate f
CANBUS
= (83
1
/
3
) kHz
- Sampling point 75% or above
- Synchronization jump width 3 TQ
First the frequency ratio between the global frequency and the CAN bus baud rate is calculated:
The register descriptions show that the prescaler must be an even number between 2 and 128, the data
bit time must be a value in the range 8 to 25.
As the synchronization jump width (SJW) is defined as 3 TQ, the maximum setting for the sampling
point (SPT) can be only 3 TQ less than the setting for the data bit time (DBT) and also less than 17 TQ:
Based on the restrictions and assumptions above, the four settings are basically possible:
Prescaler
(BRP)
Data Bit Time
(DBT)
Max. Sampling Point
(SPT)
Calculated Sampling
Point
24 8 TQ 5 TQ 5/8 = 62.5%
16 12 TQ 9 TQ 9/12 = 75%
12 16 TQ 13 TQ 13/16 = 81.25%
8 24 TQ 17 TQ 17/24 = 71%
f
MEM
f
CANBUS
=
16 MHz
(83
1
/
3
) KHz
=
192 = 3 * 2
6
SPT min
≤
DBT 1 – 17
{ , }