PIC16F1825-E/ML【PDF 17/208 页】MCU PIC 14K FLASH 1K RAM 16QFN

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138

ATmega8515(L)

2512K–AVR–01/10

Signal description:

txclk

Transmitter clock. (Internal Signal)

rxclk

Receiver base clock. (Internal Signal)

xcki

Input from XCK pin (internal Signal). Used for synchronous slave operation.

xcko

Clock output to XCK pin (Internal Signal). Used for synchronous master

operation.

fosc

XTAL pin frequency (System Clock).

Internal Clock Generation –

The Baud Rate Generator

Internal clock generation is used for the asynchronous and the synchronous master

modes of operation. The description in this section refers to Figure 65.

The USART Baud Rate Register (UBRR) and the down-counter connected to it function

as a programmable prescaler or baud rate generator. The down-counter, running at sys-

tem clock (fosc), is loaded with the UBRR value each time the counter has counted

down to zero or when the UBRRL Register is written. A clock is generated each time the

cou n ter rea c hes zero. Th is clock is the bau d r a te gen erato r clock outp u t

(= fosc/(UBRR+1)). The Transmitter divides the baud rate generator clock output by 2,

8, or 16 depending on mode. The baud rate generator output is used directly by the

Receiver’s clock and data recovery units. However, the recovery units use a state

machine that uses 2, 8, or 16 states depending on mode set by the state of the UMSEL,

U2X, and DDR_XCK bits.

Table 60 contains equations for calculating the baud rate (in bits per second) and for

calculating 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)

OSC

System Oscillator 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

68 (see page 160).

Table 60. 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

OSC

16 UBRR

()

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

UBRR

OSC

16BAUD

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

–

BAUD

OSC

8 UBRR

()

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

UBRR

OSC

8BAUD

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

–

BAUD

OSC

2 UBRR

()

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

UBRR

OSC

2BAUD

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

–

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