Design Considerations for CAN Bus and Asynchronous Serial

Introduction

When designing a system that requires both CAN bus and asynchronous serial interfaces on a DS80C390 or DS80C400 processor, the designer can encounter problems choosing a CPU crystal that allows use of standard bit rates on both interfaces. The CAN bus requires less than 0.5% error, while an asynchronous serial interface requires 2.5% error or less. Common serial baud-rate crystals such as 11.0592MHz, 14.7456MHz, and 18.432MHz will not allow high-bit-rate CAN transmission without violating CAN bus bit-error guidelines. To find the compromise for both interfaces, the designer must first understand how each I/O block generates its bit clock.

CAN Bus Timing

Bit rates for the CAN controller are generated from the microcontroller system clock and are affected by the on-board crystal multiplier (doubling and quadrupling do affect the input clock to the CAN controller). The clock signal is first divided by two and then by the baud-rate prescaler (BPR), which determines the time quantum (t_qu). The CAN specification requires that each bit time be composed of between 8t_qu and 25t_qu. After passing through the BPR, the clock signal is divided once again by the user-specified divisor, which is composed of t_SEG1 and t_SEG2 plus t_{SYNC_SEG} of 1t_qu. For more detailed information, refer to Section 19 of the High-Speed Microcontroller User's Guide: DS80C390 Supplement.

The bit rate is determined by the following equation:

Now, we calculate the reload and actual bit rate:

All useful frequencies will have an error of less than 0.005.

Example of unusable crystal for 1Mbps CAN bit rate:

reload = 36.864E6 / (2*1E6) = 18.432

actual bit rate = 36.864E6 / (2*18) = 1024000

error = (1024000 - 1000000) / 1000000 = 0.024

Example of acceptable crystal for 1Mbps CAN bit rate:

reload = 36E6 / (2*1E6) = 18

actual bit rate = 36E6 / (2*18) = 1000000

error = (1000000 - 1000000) / 1000000 = 0.0

(t_SEG1 + t_SEG2 + t_{SYNC_SEG}) = 18 / BPR

Remembering that t_{SYNC_SEG} is always 1, and choosing 1 for BPR gives:

(t_SEG1 + t_SEG2) = 17

The designer picks the appropriate TSEG values from this constraint.

Async Serial Using Timer 2

Bit rates for Serial Port 0 using Timer 2 are calculated while Timer 2 is in 16-bit autoreload mode. Timer 2 runs off the crystal inputs and is not affected by any crystal multiplier settings. For more detailed information, refer to Section 12 of the High-Speed Microcontroller User's Guide.

The bit rate is determined by the following equations:

All useful frequencies will have an error of less than 0.025.

Example of unusable crystal for 115200 bit/s:

reload = 16E6 / (32 x 115200) = 4.340

actual = 16E6 / (32 x 4) = 125000

error = (125000 - 115200) / 115200 = 0.0851

Example of acceptable crystal for 115200 bit/s:

reload = 18E6 / (32 x 115200) = 4.883

actual = 18E6 / (32 x 5) = 112500

error = (112500 - 115200) / 115200 = 0.0234

Async Serial Using Timer 1

Bit rates for Serial Ports 0 and 1 using Timer 1 are calculated while Timer 1 is in 8-bit autoreload mode. An added feature of this process is the ability to generate baud rates based on divide-by-4 or divide-by-12 from the external crystal or the system clock. In divide-by-4 mode, the input clock is generated from the system clock (f _MULT), while in divide-by-12, the base frequency of the external crystal will be used (f _osc). You can also enable a serial baud-rate doubler by setting the appropriate SMOD (SMOD_0 or SMOD_1) bit. For more detailed information, refer to Section 12 of the High-Speed Microcontroller User's Guide.

Timer 1 in Divide-by-4 Mode

The bit rate is determined by the following equations:

All useful frequencies will have an error of less than 0.025

Example of unusable crystal:

reload = 20E6 / (64 x 115200) = 2.713

actual = 20E6 / (64 x 3) = 104167

error = (104167 - 115200) / 115200 = 0.096

Example of acceptable crystal:

reload = 22E6 / (64 x 115200) = 2.984

actual = 22E6 / (64 x 3) = 114583

error = (114583 - 115200) / 115200 = 0.0054

Timer 1 in Divide-by-12 Mode

All useful frequencies will have an error of less than 0.025

Example of unusable crystal for 115200 bit/s:

reload = 18E6 / (192 x 115200) = 0.814

actual = 18E6 / (192 x 1) = 93750

error = (93750 - 115200) / 115200 = 0.186

Example of acceptable crystal for 115200 bit/s:

reload = 22E6 / (192 x 115200) = 0.995

actual = 22E6 / (192 x 1) = 114583

error = (114583 - 115200) / 115200 = 0.0054

Suggested Crystal Values for 115200 Serial and CAN Bus

The following table lists crystal frequencies that allow 115,200bps asynchronous serial using Serial 0 and Timer 2 and a 1Mb CAN bus.

Note: These values assume that the designer is not using the system clock multiplier.