[En-Nut-Discussion] AT90CAN128 CPU and Nut/OS

[Henrik Maier]

Michael,

you certainly can use both UARTs and CAN at the same time. The accuracy 
of the baud rates depends on the crystal frequency used. The following 
table has been calculated for the AT90CAN128 for typical crystal values.

At 14.7456 the CAN baud rate error rate is greater than the 0.5% allowed 
by the CAN spec for typical CAN baud rates.

At 16.0 MHz the UART baud rates are acceptable for up to 57600 bps. 
115200 has an error of 2.12% which above the 1.5% mark recommended by 
Atmel but still below the 3% mark usually mentioned in the literature. 
However if a low baud rate error for 115200 bps is required then I 
recommend to use a 12 MHz crystal as the table below shows. The UART 
baud rate error is only 0.16% and the CAN baud rate error 0%.

UART

MHz     14.7456 12.0000 12.2880 14.0000 14.3182 16.0000
9600    0.00%   0.16%   0.00%   0.16%   0.23%   0.16%
38400   0.00%   0.16%   0.00%   0.93%   0.83%   0.16%
57600   0.00%   0.16%   1.23%   1.27%   0.23%   0.79%
115200  0.00%   0.16%   2.56%   1.27%   2.90%   2.12%
230400  0.00%   6.99%   4.76%   5.06%   2.90%   3.55%


CAN

MHz     14.7456 12.0000 12.2880 14.0000 14.3182 16.0000
100k    1.70%   0.00%   2.40%   0.00%   0.57%   0.00%
125k    1.70%   0.00%   0.31%   0.00%   0.48%   0.00%
250k    1.70%   0.00%   1.70%   0.00%   2.27%   0.00%
500k    1.70%   0.00%   2.40%   0.00%   2.27%   0.00%
1000k   5.33%   0.00%   2.40%   0.00%   2.27%   0.00%


Henrik Maier

Michael Fischer wrote:
> Hello Henrik,
> 
> can you use the CAN and the UART from the AT90CAN128?
> 
> I think for a working CAN bus, 16Mhz is needed.
> But how can set a baud rate of 115200 for the UART if the cpu use 16Mhz?
> 
> Best regards,
> 
> Michael
> 
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