16F1829 datasheet(309/420 Pages) MICROCHIP

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 2010 Microchip Technology Inc.

Preliminary

DS41440A-page 311

PIC16F/LF1825/1829

26.3

EUSART Baud Rate Generator

(BRG)

The Baud Rate Generator (BRG) is an 8-bit or 16-bit

timer that is dedicated to the support of both the

asynchronous and synchronous EUSART operation.

By default, the BRG operates in 8-bit mode. Setting the

BRG16 bit of the BAUDCON register selects 16-bit

mode.

The SPBRGH, SPBRGL register pair determines the

period of the free running baud rate timer. In

Asynchronous mode the multiplier of the baud rate

period is determined by both the BRGH bit of the TXSTA

Synchronous mode, the BRGH bit is ignored.

Table 27-3 contains the formulas for determining the

baud rate. Example 27-1 provides a sample calculation

for determining the baud rate and baud rate error.

Typical baud rates and error values for various

asynchronous modes have been computed for your

convenience and are shown in Table 27-3. It may be

advantageous to use the high baud rate (BRGH = 1),

or the 16-bit BRG (BRG16 = 1) to reduce the baud rate

error. The 16-bit BRG mode is used to achieve slow

baud rates for fast oscillator frequencies.

Writing a new value to the SPBRGH, SPBRGL register

pair causes the BRG timer to be reset (or cleared). This

ensures that the BRG does not wait for a timer overflow

before outputting the new baud rate.

If the system clock is changed during an active receive

operation, a receive error or data loss may result. To

avoid this problem, check the status of the RCIDL bit to

make sure that the receive operation is Idle before

changing the system clock.

EXAMPLE 26-1:

CALCULATING BAUD

RATE ERROR

For a device with FOSC of 16 MHz, desired baud rate

of 9600, Asynchronous mode, 8-bit BRG:

Solving for SPBRGH:SPBRGL:

FOSC

Desired Baud Rate

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

--------------------------------------------- 1

–

Desired Baud Rate

FOSC

64 [SPBRGH:SPBRGL]



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

16000000

9600

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

------------------------1

–

25.042



Calculated Baud Rate

16000000

64 25



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

9615

Error

Calc. Baud Rate Desired Baud Rate

–

Desired Baud Rate

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

9615 9600

–



9600

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

0.16%

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