28.3 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 operations. By default, the BRG operates in 8-bit mode. Setting the BRG16 bit 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 and the BRG16 bits. In Synchronous mode, the BRGH bit is ignored.

Table 28-1 contains the formulas for determining the baud rate. Equation 28-1 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 the table below. 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, check the status of the Receive Idle Flag (RCIDL) bit to make sure the receive operation is idle before changing the system clock.

Equation 28-1. Calculating Baud Rate Error

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

D e s i r e d B a u d r a t e = \frac{F_{O S C}}{64 \times (S P x B R G + 1)}

Solving for SPxBRG:

S P x B R G = \frac{F_{O S C}}{64 \times D e s i r e d B a u d r a t e} - 1

S P x B R G = \frac{16000000}{64 \times 9600} - 1

S P x B R G = 25.042 ≃ 25

C a l c u l a t e d B a u d r a t e = \frac{16000000}{64 \times (25 + 1)}

C a l c u l a t e d B a u d r a t e = 9615

E r r o r = \frac{C a l c u l a t e d B a u d r a t e - D e s i r e d B a u d r a t e}{D e s i r e d B a u d r a t e}

E r r o r = \frac{9615 - 9600}{9600}

E r r o r = 0.16 %

Table 28-1. Baud Rate Formulas
Configuration Bits			BRG/EUSART Mode	Baud Rate Formula
SYNC	BRG16	BRGH	BRG/EUSART Mode	Baud Rate Formula
`0`	`0`	`0`	8-bit/Asynchronous	F_OSC/[64 (n+1)]
`0`	`0`	`1`	8-bit/Asynchronous	F_OSC/[16 (n+1)]
`0`	`1`	`0`	16-bit/Asynchronous	F_OSC/[16 (n+1)]
`0`	`1`	`1`	16-bit/Asynchronous	F_OSC/[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.

Table 28-2. Sample Baud Rates for Asynchronous Modes
Baud Rate	SYNC = `0`, BRGH = `0`, BRG16 = `0`
	F_OSC = 32.000 MHz			F_OSC = 20.000 MHz			F_OSC = 18.432 MHz			F_OSC = 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`
	F_OSC = 8.000 MHz			F_OSC = 4.000 MHz			F_OSC = 3.6864 MHz			F_OSC = 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`
	F_OSC = 32.000 MHz			F_OSC = 20.000 MHz			F_OSC = 18.432 MHz			F_OSC = 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`
	F_OSC = 8.000 MHz			F_OSC = 4.000 MHz			F_OSC = 3.6864 MHz			F_OSC = 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`
	F_OSC = 32.000 MHz			F_OSC = 20.000 MHz			F_OSC = 18.432 MHz			F_OSC = 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`
	F_OSC = 8.000 MHz			F_OSC = 4.000 MHz			F_OSC = 3.6864 MHz			F_OSC = 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`
	F_OSC = 32.000 MHz			F_OSC = 20.000 MHz			F_OSC = 18.432 MHz			F_OSC = 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`
	F_OSC = 8.000 MHz			F_OSC = 4.000 MHz			F_OSC = 3.6864 MHz			F_OSC = 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	—	—	—