# 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 BAUDxCON register selects 16-bit mode.

The SPxBRGH, SPxBRGL 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 TXxSTA register and the BRG16 bit of the BAUDxCON register. In Synchronous mode, the BRGH bit is ignored.

Table 1 contains the formulas for determining the baud rate. Figure 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 and are shown in Table 2. 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. The BRGH bit is used to achieve very high baud rates.

Writing a new value to the SPxBRGH, SPxBRGL 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.

Figure 1. Calculating Baud Rate Error

For a device with Fosc of 16 MHz, desired baud rate of 9600, Asynchronous mode, 8-bit BRG:

DesiredBaudrate=FOSC(64×SPxBRG)+1

Solving for SPxBRG:

SPxBRG=FOSC64×DesiredBaudrate1

SPxBRG=1600000064×96001

SPxBRG=25.04225

CalculatedBaudrate=1600000064×(25+1)

CalculatedBaudrate=9615

Error=CalculatedBaudrateDesiredBaudrateDesiredBaudrate

Error=961596009600

Error=0.16%

Table 1. Baud Rate Formulas
Configuration Bits BRG/EUSART Mode Baud Rate Formula
SYNC BRG16 BRGH
0 0 0 8-bit/Asynchronous FOSC/[64 (n+1)]
0 0 1 8-bit/Asynchronous FOSC/[16 (n+1)]
0 1 0 16-bit/Asynchronous
0 1 1 16-bit/Asynchronous FOSC/[4 (n+1)]
1 0 x 8-bit/Synchronous
1 1 x 16-bit/Synchronous
Note: x = Don’t care, n = value of SPxBRGH:SPxBRGL register pair.
 BAUD RATE SYNC = 0, BRGH = 0, BRG16 = 0 FOSC = 32.000 MHz FOSC = 20.000 MHz FOSC = 18.432 MHz FOSC = 11.0592 MHz Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) 300 — — — — — — — — — — — — 1200 — — — 1221 1.73 255 1200 0.00 239 1200 0.00 143 2400 2404 0.16 207 2404 0.16 129 2400 0.00 119 2400 0.00 71 9600 9615 0.16 51 9470 -1.36 32 9600 0.00 29 9600 0.00 17 10417 10417 0.00 47 10417 0.00 29 10286 -1.26 27 10165 -2.42 16 19.2k 19.23k 0.16 25 19.53k 1.73 15 19.20k 0.00 14 19.20k 0.00 8 57.6k 55.55k -3.55 3 — — — 57.60k 0.00 7 57.60k 0.00 2 115.2k — — — — — — — — — — — —
 BAUD RATE SYNC = 0, BRGH = 0, BRG16 = 0 Fosc = 8.000 MHz Fosc = 4.000 MHz Fosc = 3.6864 MHz Fosc = 1.000 MHz Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) 300 — — — 300 0.16 207 300 0.00 191 300 0.16 51 1200 1202 0.16 103 1202 0.16 51 1200 0.00 47 1202 0.16 12 2400 2404 0.16 51 2404 0.16 25 2400 0.00 23 — — — 9600 9615 0.16 12 — — — 9600 0.00 5 — — — 10417 10417 0.00 11 10417 0.00 5 — — — — — — 19.2k — — — — — — 19.20k 0.00 2 — — — 57.6k — — — — — — 57.60k 0.00 0 — — — 115.2k — — — — — — — — — — — —

 BAUD RATE SYNC = 0, BRGH = 1, BRG16 = 0 Fosc = 32.000 MHz Fosc = 20.000 MHz Fosc = 18.432 MHz Fosc = 11.0592 MHz Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) 300 — — — — — — — — — — — — 1200 — — — — — — — — — — — — 2400 — — — — — — — — — — — — 9600 9615 0.16 207 9615 0.16 129 9600 0.00 119 9600 0.00 71 10417 10417 0.00 191 10417 0.00 119 10378 -0.37 110 10473 0.53 65 19.2k 19.23k 0.16 103 19.23k 0.16 64 19.20k 0.00 59 19.20k 0.00 35 57.6k 57.14k -0.79 34 56.82k -1.36 21 57.60k 0.00 19 57.60k 0.00 11 115.2k 117.64k 2.12 16 113.64k -1.36 10 115.2k 0.00 9 115.2k 0.00 5

 BAUD RATE SYNC = 0, BRGH = 1, BRG16 = 0 Fosc = 8.000 MHz Fosc = 4.000 MHz Fosc = 3.6864 MHz Fosc = 1.000 MHz Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) 300 — — — — — — — — — 300 0.16 207 1200 — — — 1202 0.16 207 1200 0.00 191 1202 0.16 51 2400 2404 0.16 207 2404 0.16 103 2400 0.00 95 2404 0.16 25 9600 9615 0.16 51 9615 0.16 25 9600 0.00 23 — — — 10417 10417 0.00 47 10417 0.00 23 10473 0.53 21 10417 0.00 5 19.2k 19231 0.16 25 19.23k 0.16 12 19.2k 0.00 11 — — — 57.6k 55556 -3.55 8 — — — 57.60k 0.00 3 — — — 115.2k — — — — — — 115.2k 0.00 1 — — —

 BAUD RATE SYNC = 0, BRGH = 0, BRG16 = 1 Fosc = 32.000 MHz Fosc = 20.000 MHz Fosc = 18.432 MHz Fosc = 11.0592 MHz Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) 300 300.0 0.00 6666 300.0 -0.01 4166 300.0 0.00 3839 300.0 0.00 2303 1200 1200 -0.02 3332 1200 -0.03 1041 1200 0.00 959 1200 0.00 575 2400 2401 -0.04 832 2399 -0.03 520 2400 0.00 479 2400 0.00 287 9600 9615 0.16 207 9615 0.16 129 9600 0.00 119 9600 0.00 71 10417 10417 0.00 191 10417 0.00 119 10378 -0.37 110 10473 0.53 65 19.2k 19.23k 0.16 103 19.23k 0.16 64 19.20k 0.00 59 19.20k 0.00 35 57.6k 57.14k -0.79 34 56.818 -1.36 21 57.60k 0.00 19 57.60k 0.00 11 115.2k 117.6k 2.12 16 113.636 -1.36 10 115.2k 0.00 9 115.2k 0.00 5

 BAUD RATE SYNC = 0, BRGH = 0, BRG16 = 1 Fosc = 8.000 MHz Fosc = 4.000 MHz Fosc = 3.6864 MHz Fosc = 1.000 MHz Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) 300 299.9 -0.02 1666 300.1 0.04 832 300.0 0.00 767 300.5 0.16 207 1200 1199 -0.08 416 1202 0.16 207 1200 0.00 191 1202 0.16 51 2400 2404 0.16 207 2404 0.16 103 2400 0.00 95 2404 0.16 25 9600 9615 0.16 51 9615 0.16 25 9600 0.00 23 — — — 10417 10417 0.00 47 10417 0.00 23 10473 0.53 21 10417 0.00 5 19.2k 19.23k 0.16 25 19.23k 0.16 12 19.20k 0.00 11 — — — 57.6k 55556 -3.55 8 — — — 57.60k 0.00 3 — — — 115.2k — — — — — — 115.2k 0.00 1 — — —

 BAUD RATE SYNC = 0, BRGH = 1, BRG16 = 1 or SYNC = 1, BRG16 = 1 Fosc = 32.000 MHz Fosc = 20.000 MHz Fosc = 18.432 MHz Fosc = 11.0592 MHz Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) 300 300.0 0.00 26666 300.0 0.00 16665 300.0 0.00 15359 300.0 0.00 9215 1200 1200 0.00 6666 1200 -0.01 4166 1200 0.00 3839 1200 0.00 2303 2400 2400 0.01 3332 2400 0.02 2082 2400 0.00 1919 2400 0.00 1151 9600 9604 0.04 832 9597 -0.03 520 9600 0.00 479 9600 0.00 287 10417 10417 0.00 767 10417 0.00 479 10425 0.08 441 10433 0.16 264 19.2k 19.18k -0.08 416 19.23k 0.16 259 19.20k 0.00 239 19.20k 0.00 143 57.6k 57.55k -0.08 138 57.47k -0.22 86 57.60k 0.00 79 57.60k 0.00 47 115.2k 115.9k 0.64 68 116.3k 0.94 42 115.2k 0.00 39 115.2k 0.00 23

 BAUD RATE SYNC = 0, BRGH = 1, BRG16 = 1 or SYNC = 1, BRG16 = 1 Fosc = 8.000 MHz Fosc = 4.000 MHz Fosc = 3.6864 MHz Fosc = 1.000 MHz Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) Actual Rate % Error SPBRG value (decimal) 300 300.0 0.00 6666 300.0 0.01 3332 300.0 0.00 3071 300.1 0.04 832 1200 1200 -0.02 1666 1200 0.04 832 1200 0.00 767 1202 0.16 207 2400 2401 0.04 832 2398 0.08 416 2400 0.00 383 2404 0.16 103 9600 9615 0.16 207 9615 0.16 103 9600 0.00 95 9615 0.16 25 10417 10417 0 191 10417 0.00 95 10473 0.53 87 10417 0.00 23 19.2k 19.23k 0.16 103 19.23k 0.16 51 19.20k 0.00 47 19.23k 0.16 12 57.6k 57.14k -0.79 34 58.82k 2.12 16 57.60k 0.00 15 — — — 115.2k 117.6k 2.12 16 111.1k -3.55 8 115.2k 0.00 7 — — —