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PIC18F23K22-E/SP Datasheet(PDF) 275 Page - Microchip Technology |
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PIC18F23K22-E/SP Datasheet(HTML) 275 Page - Microchip Technology |
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275 / 496 page  2010 Microchip Technology Inc. Preliminary DS41412C-page 275 PIC18(L)F2X/4XK22 16.3 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 BAUDCONx register selects 16-bit mode. The SPBRGHx:SPBRGx 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 TXSTAx register and the BRG16 bit of the BAUDCONx register. In Synchronous mode, the BRGH bit is ignored. Table contains the formulas for determining the baud rate. Example 16-1 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 16-5. 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 SPBRGHx, SPBRGx 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 16-1: CALCULATING BAUD RATE ERROR For a device with FOSC of 16 MHz, desired baud rate of 9600, Asynchronous mode, 8-bit BRG: Solving for SPBRGHx:SPBRGx: X FOSC Desired Baud Rate --------------------------------------------- 64 --------------------------------------------- 1 – = Desired Baud Rate FOSC 64 [SPBRGHx:SPBRGx] 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% == TABLE 16-3: BAUD RATE FORMULAS Configuration Bits BRG/EUSART Mode Baud Rate Formula SYNC BRG16 BRGH 00 0 8-bit/Asynchronous FOSC/[64 (n+1)] 00 1 8-bit/Asynchronous FOSC/[16 (n+1)] 01 0 16-bit/Asynchronous 01 1 16-bit/Asynchronous FOSC/[4 (n+1)] 10 x 8-bit/Synchronous 11 x 16-bit/Synchronous Legend: x = Don’t care, n = value of SPBRGHx, SPBRGx register pair. |
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