DOC7707 datasheet(169/307 Pages) ETC1

169 / 307 page

169

7707F–AVR–11/10

AT90USB82/162

17.10.5

USART Control and Status Register n D – UCSRnD

• Bits 1 – CTSEN : USART CTS Enable

Set this bit to one by firmware to enable the transmission flow control (CTS). Transmission is

allowed if CTS = 0.

Set this bit to zero by firmware to disable the transmission flow control (CTS). Transmission is

always allowed.

• Bits 0 – RTSEN : USART RTS Enable

Set this bit to one by firmware to enable the receive flow control (RTS).

Set this bit to zero by firmware to disable the receive flow control (RTS).

17.10.6

USART Baud Rate Registers – UBRRLn and UBRRHn

• 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 – UBRR11:0: USART Baud Rate Register

This is a 12-bit register which contains the USART baud rate. The UBRRH contains the four

most significant bits, and the UBRRL 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 UBRRL will trigger an immediate update of the baud rate prescaler.

17.11 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 UBRR settings in Table 17-9 to Table 17-12.

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 161). The error values are calculated using the following equation:

Bit

5432

----

CTSEN

RTSEN

UCSRnD

Read/Write

R/W

Initial Value

0000

Bit

1514

1312

1110

––––

UBRR[11:8]

UBRRHn

UBRR[7:0]

UBRRLn

7654

Read/Write

RRRR

R/W

R/WR/W

Initial Value

0000

Error[%]

BaudRate

Closest Match

BaudRate

--------------------------------------------------------

–

⎝⎠

⎛⎞

100%

•

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