Rainbow Electronics ATmega64L - Manual (Page 160)

160
ATmega16(L)
2466B–09/01
• Bit 11:0 - UBRR11:0: USART Baud Rate Register
This is a 12-bit register which contains the USART baud rate. The UBRRH contains the
4 most significant bits, and the UBRRL contains the 8 least significant bits of the USART
baud rate. Ongoing transmissions by the transmitter and receiver will be corrupted if the
baud rate is changed. Writing UBRRL will trigger an immediate update of the baud rate
prescaler.
Examples of Baud Rate
Setting For standard crystal and resonator frequencies, the most commonly used baud rates for
asynchronous operation can be generated by using the UBRR settings in Table 68.
UBRR values which yield an actual baud rate differing less than 0.5% from the target
baud rate, are bold in the table. Higher error ratings are acceptable, but the receiver will
have less noise resistance when the error ratings are high, especially for large serial
frames (see “Asynchronous Operational Range” on page 151). The error values are cal-
culated using the following equation:
Error[%]
BaudRate
Closest Match
BaudRate
-------------------------------------------------------- 1 – и
ж ц 100%
•
=
Table 68. Examples of UBRR Settings for Commonly Used Oscillator Frequencies
Baud
Rate
(bps)
f
osc
= 1.0000 MHz f
osc
= 1.8432 MHz f
osc
= 2.0000 MHz
U2X = 0 U2X = 1 U2X = 0 U2X = 1 U2X = 0 U2X = 1
UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error UBRR Error
2400 25 0.2% 51 0.2% 47 0.0% 95 0.0% 51 0.2% 103 0.2%
4800 12 0.2% 25 0.2% 23 0.0% 47 0.0% 25 0.2% 51 0.2%
9600 6 -7.0% 12 0.2% 11 0.0% 23 0.0% 12 0.2% 25 0.2%
14.4k 3 8.5% 8 -3.5% 7 0.0% 15 0.0% 8 -3.5% 16 2.1%
19.2k 2 8.5% 6 -7.0% 5 0.0% 11 0.0% 6 -7.0% 12 0.2%
28.8k 1 8.5% 3 8.5% 3 0.0% 7 0.0% 3 8.5% 8 -3.5%
38.4k 1 -18.6% 2 8.5% 2 0.0% 5 0.0% 2 8.5% 6 -7.0%
57.6k 0 8.5% 1 8.5% 1 0.0% 3 0.0% 1 8.5% 3 8.5%
76.8k - - 1 -18.6% 1 -25.0% 2 0.0% 1 -18.6% 2 8.5%
115.2k - - 0 8.5% 0 0.0% 1 0.0% 0 8.5% 1 8.5%
230.4k - - - - - - 0 0.0% - - - -
250k - - - - - - - - - - 0 0.0%
Max (1) 62.5kbps 125kbps 115.2kbps 230.4Mbps 125kpbs 250kbps
1. UBRR = 0, Error = 0.0%