National CP3BT26 Minimum Clock Frequency Requirements Baud Rate, Acceptance Filter Considerations

Table 65 CAN Module Internal Timing

Task	Cycle	Occurrence/
	Count	Frame
Copy hidden buffer to receive	17	0–1
message buffer
Update status from TX_RTR	3	0–15
to TX_ONCE_RTR
Schedule a message for	2	0–1
transmission

The critical path derives from receiving a remote frame, which triggers the transmission of one or more data frames. There are a minimum of four bit times in-between two con- secutive frames. These bit times start at the validation point of received frame (reception of 6th EOF bit) and end at the earliest possible transmission start of the next frame, which is after the third intermission bit at 100% burst bus load.

These four bit times have to be set in perspective with the timing requirements of the CAN module.

The minimum duration of the four CAN bit times is deter- mined by the following Bit Time Logic settings:

PSC = PSCmin = 2

TSEG1 = TSEG1min = 2

TSEG2 = TSEG2min = 1

Bit time = Sync + Time Segment 1 + Time Segment 2

=(1 + 2 + 1) tq = 4 tq

=(4 tq × PSC) clock cycles

=(4 tq × 2) clock cycles = 8 clock cycles

For these minimum BTL settings, four CAN bit times take 32 clock cycles.

The following is an example that assumes typical case:

„Minimum BTL settings

„Reception and copy of a remote frame

„Update of one buffer from TX_RTR

„Schedule of one buffer from transmit

As outlined in Table 65, the copy process, update, and scheduling the next transmission gives a total of 17 + 3 + 2

=22 clock cycles. Therefore under these conditions there is no timing restriction.

The following example assumes the worst case:

„Minimum BTL settings

„Reception and copy of a remote frame

„Update of the 14 remaining buffers from TX_RTR

„Schedule of one buffer for transmit

All these actions in total require 17 + (14 × 3) + 2 = 61 clock cycles to be executed by the CAN module. This leads to the limitation of the Bit Time Logic of 61 / 4 = 15.25 clock cycles per CAN bit as a minimum, resulting in the minimum clock frequencies listed below. (The frequency depends on the desired baud rate and assumes the worst case scenario can occur in the application.)

Table 66 gives examples for the minimum clock frequency in order to ensure proper functionality at various CAN bus speeds.

Table 66 Minimum Clock Frequency Requirements

Baud Rate	Minimum Clock
	Frequency
1 Mbit/sec	15.25 MHz
500 kbit/sec	7.625 MHz
250 kbit/sec	3.81 MHz

19.11.4 Bit Time Logic Calculation Examples

The calculation of the CAN bus clocks using CKI = 16 MHz is shown in the following examples. The desired baud rate for both examples is 1 Mbit/s.

Example 1

PSC = PSC[5:0] + 2 = 0 + 2 = 2

TSEG1 = TSEG1[3:0] + 1 = 3 + 1 = 4

TSEG2 = TSEG2[2:0] + 1 = 2 + 1 = 3

SJW = TSEG2 = 3

„Sample point positioned at 62.5% of bit time

„Bit time = 125 ns × (1 + 4 + 3 ± 3) = (1 ± 0.375) µs

„Bus Clock = 16 MHz / (2 × (1 + 4 + 3)) = 1 Mbit/s (nomi- nal)

Example 2

PSC = PSC[5:0] + 1 = 2 + 2 = 4

TSEG1 = TSEG1[3:0] + 1 = 1 + 1 = 2

TSEG2 = TSEG2[2:0] + 1 = 0 + 1 = 1

SJW = TSEG2 = 1

„Sample point positioned at 75% of bit time

„Bit time = 250 ns × (1 + 2 + 1 ± 1) = (1 ± 0.25) µs

„Bus Clock = 16 MHz / (2 × (1 + 4 + 3)) = 1Mbit/s (nominal)

19.11.5 Acceptance Filter Considerations

The CAN module provides two acceptance filter masks GMSK and BMSK, as described in “Acceptance Filtering” on page 119, “Global Mask Register (GMSKB/GMSKX)” on page 136, and “Basic Mask Register (BMSKB/BMSKX)” on page 137. These masks allow filtering of up to 32 bits of the message object, which includes the standard identifier, the extended identifier, and the frame control bits RTR, SRR, and IDE.

19.11.6 Remote Frames

Remote frames can be automatically processed by the CAN module. However, to fully enable this feature, the RTR/ XRTR bits (for both standard and extended frames) within the BMSK and/or GMSK register need to be set to “don’t care”. This is because a remote frame with the RTR bit set should trigger the transmission of a data frame with the RTR bit clear and therefore the ID bits of the received message need to pass through the acceptance filter. The same ap- plies to transmitting remote frames and switching to receive the corresponding data frames.

CP3BT26

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