C8051F045 View Datasheet(PDF) - Silicon Laboratories

Part Name

Description

Manufacturer

C8051F045

Mixed Signal ISP Flash MCU Family

Silicon Laboratories 

C8051F040/1/2/3/4/5/6/7

18.1.1. CAN Controller Timing

The CAN controller’s system clock (fsys) is derived from the CIP-51 system clock (SYSCLK). Note that an

external oscillator (such as a quartz crystal) is typically required due to the high accuracy requirements for

CAN communication. Refer to Section “4.10.4 Oscillator Tolerance Range” in the Bosch CAN User’s Guide

for further information regarding this topic.

18.1.2. Example Timing Calculation for 1 Mbit/Sec Communication

This example shows how to configure the CAN contoller timing parameters for a 1 Mbit/Sec bit rate.

Table 18.1 shows timing-related system parameters needed for the calculation.

Table 18.1. Background System Information

Parameter

CIP-51 system clock (SYSCLK)

CAN Controller system clock

(fsys)

CAN clock period (tsys)

CAN time quantum (tq)

CAN bus length

Propagation delay time3

Value

22.1184 MHz

22.1184 MHz

45.211 ns

45.211 ns

10 m

400 ns

Description

External oscillator in ‘Crystal Oscillator Mode’. A

22.1184 MHz quartz crystal is connected between

XTAL1 and XTAL2.

Derived from SYSCLK.

Derived from 1/fsys.

Derived from tsys x BRP1,2

5 ns/m signal delay between CAN nodes.

2 x (transceiver loop delay + bus line delay)

Notes:

1. The CAN time quantum (tq) is the smallest unit of time recognized by the CAN contoller. Bit timing parameters

are often specified in integer multiples of the time quantum.

2. The Baud Rate Prescaler (BRP) is defined as the value of the BRP Extension Register plus 1. The BRP

Extension Register has a reset value of 0x0000; the Baud Rate Prescaler has a reset value of 1.

3. Based on an ISO-11898 compliant transceiver. CAN does not specify a physical layer.

Each bit transmitted on a CAN network has 4 segments (Sync_Seg, Prop_Seg, Phase_Seg1, and

Phase_Seg2), as shown in Figure 18.3. The sum of these segments determines the CAN bit time (1/bit

rate). In this example, the desired bit rate is 1 Mbit/sec; therefore, the desired bit time is 1000 ns.

CAN Bit Time (4 to 25 tq)

Sync_Seg

Prop_Seg

Phase_Seg1 Phase_Seg2

1tq

1 to 8 tq

1 to 8 tq

1 to 8 tq

1tq

Sample Point

Figure 18.3. Four Segments of a CAN Bit Time

Rev. 1.5

229

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