38.3.3.8 Bit Time Configuration Example
The following tables illustrate the configuration of the CAN FD Bit Time registers,
assuming there is a CAN FD network in an automobile with the following parameters:
- 500 kbps NBR – sample point at 80%
- 2 Mbps DBR – sample point at 80%
- 40 meters – minimum bus length
Table 38-3 and Table 38-4 illustrate how the bit time parameters are calculated. Since the parameters depend on multiple constraints and equations, and are calculated using an iterative process, it is recommended to enter the equations in a spreadsheet.
Table 38-5 translates the calculated values into
register values. It is recommended to let the CAN FD Protocol module measure the
Transmitter Delay Compensation Value (TDCV). This is accomplished by setting TDCMOD[1:0]
(C1TDCH[1:0]) = 10
(Automatic mode). To set the SSP to 80%, the
TDCO[6:0] bits are set to 1 + DTSEG1.
Parameter | Constraint | Value | Unit | Equations and Comments |
---|---|---|---|---|
NBT | NBT ≥ µs | 2 | µs | Equation 38-1 |
FSYSCLK | FSYSCLK ≤ 40 MHz | 40 | MHz | CAN clock frequency = 40 MHz |
NBRP | 1 to 256 | 1 | - | Select smallest possible BRP value to maximize resolution. |
NTQ | NBT, FSYSCLK | 25 | ns | Equation 38-3 |
NBT/NTQ | 4 to 385 | 80 | - | Equation 38-5 |
NSYNC | Fixed | 1 | NTQ | Defined in ISO11898-1:2015 |
NPRSEG | NPRSEG > TPROP | 47 | NTQ | Equation 38-9; TPROP = 910 ns, minimum NPRSEG = TPROP/NTQ = 72.8 NTQ. Selecting 95 will allow up to a 60m bus length |
NTSEG1 | 2 to 256 NTQ | 64 | NTQ | Equation 38-7. Select NTSEG1 to achieve 80% NSP. |
NTSEG2 | 1 to 128 NTQ | 16 | NTQ | There are 32 NTQ left to reach NBT/NTQ = 160 |
NSJW | 1 to 128 NTQ; SJW ≤ min(NPHSEG1, NPHSEG2) | 16 | NTQ | Maximizing NSJW lessens the requirement for the oscillator tolerance. |
Parameter | Constraint | Value | Unit | Equations and Comments |
---|---|---|---|---|
DBT | DBT ≥ 125 ns | 500 | ns | Equation 38-2 |
DBRP | 1 to 256 | 1 | - | Selecting the same prescaler as for NBT ensures that to the TQresolution does not change during the Bit Rate Switching |
DTQ | DBT, FSYSCLK | 25 | ns | Equation 38-4 |
DBT/DTQ | 3 to 49 | 20 | - | Equation 38-6 |
DSYNC | Fixed | 1 | DTQ | Defined in ISO11898-1:2015. |
DTSEG1 | 1 to 32 DTQ | 16 | DTQ | Equation 38-8 |
DTSEG2 | 1 to 16 DTQ | 4 | DTQ | There are 8 DTQ left to reach DBT/DTQ = 40 |
DSJW | 1 to 16 DTQ; SJWmin (DPHSEG1, DPHSEG2) | 4 | DTQ | Maximizing DSJW lessens the requirement for the oscillator tolerance |
Oscillator Tolerance Conditions 1-5 | Minimum of Conditions 1-5 | .78 | % | Equation 38-11 through Equation 38-16 |
CxNBTCFG | Value | CxDBTCFG | Value | CxTDC | Value |
---|---|---|---|---|---|
BRP[7:0] | 0 | BRP[7:0] | 0 | TDCMOD[1:0] | 2 |
TSEG[7:0] | 63 | TSEG[7:0] | 15 | TDCO[6:0] | 31 |
TSEG2[6:0] | 15 | TSEG2[6:0] | 3 | TDCV[5:0] | 0 |
SJW[6:0] | 15 | SJW[6:0] | 3 | - | - |