AT90PWM2 datasheet(188/365 Pages) ATMEL

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4317K–AVR–03/2013

AT90PWM2/3/2B/3B

Table 18-1 contains equations for calculating the baud rate (in bits per second) and for calculat-

ing the UBRR value for each mode of operation using an internally generated clock source.

Note:

1. The baud rate is defined to be the transfer rate in bit per second (bps)

BAUD Baud rate (in bits per second, bps).

fclk

System I/O Clock frequency.

UBRR Contents of the UBRRH and UBRRL Registers, (0-4095).

Some examples of UBRR values for some system clock frequencies are found in Table 18-9

(see page 209).

18.3.2

Double Speed Operation (U2X)

The transfer rate can be doubled by setting the U2X bit in UCSRA. Setting this bit only has effect

for the asynchronous operation. Set this bit to zero when using synchronous operation.

Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively doubling

the transfer rate for asynchronous communication. Note however that the Receiver will in this

case only use half the number of samples (reduced from 16 to 8) for data sampling and clock

recovery, and therefore a more accurate baud rate setting and system clock are required when

this mode is used. For the Transmitter, there are no downsides.

18.3.3

External Clock

External clocking is used by the synchronous slave modes of operation. The description in this

section refers to Figure 18-2 for details.

External clock input from the XCK pin is sampled by a synchronization register to minimize the

chance of meta-stability. The output from the synchronization register must then pass through

an edge detector before it can be used by the Transmitter and Receiver. This process intro-

duces a two CPU clock period delay and therefore the maximum external XCK clock frequency

is limited by the following equation:

Table 18-1.

Equations for Calculating Baud Rate Register Setting

Operating Mode

Equation for Calculating Baud

Rate(1)

Equation for Calculating UBRR

Value

Asynchronous Normal mode

(U2X = 0)

Asynchronous Double Speed

mode (U2X = 1)

Synchronous Master mode

BAUD

CLKio

16 UBRRn



------------------------------------------

UBRRn

CLKio

16BAUD

------------------------

–

BAUD

CLKio

8 UBRRn



---------------------------------------

UBRRn

CLKio

8BAUD

--------------------

–

BAUD

CLKio

2 UBRRn



---------------------------------------

UBRRn

CLKio

2BAUD

--------------------

–

XCKn

CLKio

----------------



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