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PIC16F946-E/MLQTP Datasheet(PDF) 134 Page - Microchip Technology |
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PIC16F946-E/MLQTP Datasheet(HTML) 134 Page - Microchip Technology |
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134 / 330 page  PIC16F913/914/916/917/946 DS41250F-page 132 © 2007 Microchip Technology Inc. 9.2 AUSART Baud Rate Generator (BRG) The Baud Rate Generator (BRG) is an 8-bit timer that is dedicated to the support of both the asynchronous and synchronous AUSART operation. The SPBRG register determines the period of the free running baud rate timer. In Asynchronous mode the multiplier of the baud rate period is determined by the BRGH bit of the TXSTA register. In Synchronous mode, the BRGH bit is ignored. Table 9-3 contains the formulas for determining the baud rate. Example 9-1 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 9-3. It may be advantageous to use the high baud rate (BRGH = 1), to reduce the baud rate error. Writing a new value to the SPBRG register causes the BRG timer to be reset (or cleared). This ensures that the BRG does not wait for a timer overflow before out- putting the new baud rate. EXAMPLE 9-1: CALCULATING BAUD RATE ERROR TABLE 9-3: BAUD RATE FORMULAS TABLE 9-4: REGISTERS ASSOCIATED WITH THE BAUD RATE GENERATOR For a device with FOSC of 16 MHz, desired baud rate of 9600, Asynchronous mode: Solving for SPBRG: X FOSC Desired Baud Rate --------------------------------------------- 64 --------------------------------------------- 1 – = Desired Baud Rate FOSC 64 SPBRG 1 + () --------------------------------------- = 16000000 9600 ------------------------ 64 ------------------------1 – = 25.042 [] 25 == Calculated Baud Rate 16000000 64 25 1 + () --------------------------- = 9615 = Error Calc. Baud Rate Desired Baud Rate – Desired Baud Rate -------------------------------------------------------------------------------------------- = 9615 9600 – () 9600 ---------------------------------- 0.16% == Configuration Bits AUSART Mode Baud Rate Formula SYNC BRGH 00 Asynchronous FOSC/[64 (n+1)] 01 Asynchronous FOSC/[16 (n+1)] 1x Synchronous FOSC/[4 (n+1)] Legend: x = Don’t care, n = value of SPBRG register Name Bit 7Bit 6Bit 5Bit 4Bit 3Bit 2Bit 1Bit 0 Value on POR, BOR Value on all other Resets RCSTA SPEN RX9 SREN CREN ADDEN FERR OERR RX9D 0000 000x 0000 000x SPBRG BRG7 BRG6 BRG5 BRG4 BRG3 BRG2 BRG1 BRG0 0000 0000 0000 0000 TXSTA CSRC TX9 TXEN SYNC —BRGH TRMT TX9D 0000 -010 0000 -010 Legend: x = unknown, - = unimplemented read as ‘0’. Shaded cells are not used for the Baud Rate Generator. |
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