Datasheet

193
2490R–AVR–02/2013
ATmega64(L)
UBRRnL and UBRRnH
– USART Baud Rate
Registers
(1)
Note: 1. UBRRH is not available in mega103 compatibility mode
• Bit 15:12 – Reserved Bits
These bits are reserved for future use. For compatibility with future devices, these bit must be
written to zero when UBRRH is written.
• Bit 11:0 – UBRRn11:0: USART Baud Rate Register
This is a 12-bit register which contains the USART baud rate. The UBRRnH contains the four
most significant bits, and the UBRRnL contains the eight 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 UBRRnL 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 asyn-
chronous operation can be generated by using the UBRRn settings in Table 82 to Table 85.
UBRRn 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 “Asyn-
chronous Operational Range” on page 186). The error values are calculated using the following
equation:
Bit 151413121110 9 8
– – – – UBRRn[11:8] UBRRnH
UBRRn[7:0] UBRRnL
76543210
Read/Write R R R R R/W R/W R/W R/W
R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
00000000
Error[%]
BaudRate
Closest Match
BaudRate
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