Rules For Correct Setting Of Baud Rate - NEC UPD703116 User Manual

11.12 Rules for Correct Setting of Baud Rate

The CAN protocol limit values for ensuring correct operation of FCAN are described below. If these limit values are
exceeded, a CAN protocol violation may occur, which can result in operation faults. Always make sure that settings
are within the range of limit values.
(a) 5 × BTL ≤ SPT (sampling point) ≤ 17 × BTL [4 ≤ SPT4 to SPT0 set values ≤ 16]
(b) 8 × BTL ≤ DBT (data bit time) ≤ 25 × BTL [7 ≤ DBT4 to DBT0 set values ≤ 24]
(c) SJW (synchronization jump width) ≤ DBT − SPT
(d) 2 × (DBT − SPT) ≤ 8
Remark BTL = 1/f
BTL
SPT4 to SPT0 (Bits 9 to 5 of CAN1 synchronization control register (C1SYNC))
DBT4 to DBT0 (Bits 4 to 0 of CAN1 synchronization control register (C1SYNC))
(1) Example of FCAN baud rate setting (when C1BRP register's TLM bit = 0)
The following is an example of how correct settings for the C1BRP register and C1SYNC register can be
calculated.
Conditions from CAN bus:
<1> CAN base clock frequency (f
<2> CAN bus baud rate: 83 kbps
<3> Sampling point: 80% or more
<4> Synchronization jump width: 3 BTL
First, calculate the ratio between the CAN base clock frequency and the CAN bus baud rate frequency as
shown below.
/CAN bus baud rate = 16 MHz/83 kHz ≠ 192.77 ≠ 2
f
MEM
Set an even number between 2 and 128 to the C1BRP register's bits BRP5 to BRP0 as the setting for the
prescaler (CAN protocol layer base system clock: f
register's bits DBT4 to DBT0 as the data bit time.
Since it is assumed that the SJW (synchronization jump width) value is 3, the maximum setting for SPT
(sampling point) is 3 less than the data bit time setting and is 17.
(SPT ≤ DBT – 3 and SPT = 17)
CHAPTER 11 FCAN CONTROLLER
(f : CAN protocol layer base system clock)
BTL
): 16 MHz
MEM
User's Manual U14492EJ5V0UD
× 3
6
), then set a value between 8 and 25 to the C1SYNC
BTL
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