Rainbow Electronics ATmega128L - Manual (Page 170)

170
ATmega128(L)
2467B–09/01
plete frame is transmitted, it can be directly followed by a new frame, or the
communication line can be set to an idle (high) state.
Figure 81 illustrates the possible
combinations of the frame formats. Bits inside brackets are optional.
Figure 81. Frame Formats
St Start bit, always low.
(n) Data bits (0 to 8).
P Parity bit. Can be odd or even.
Sp Stop bit, always high.
IDLE No transfers on the communication line (RxD or TxD). An IDLE line must be
high.
The frame format used by the USART is set by the UCSZ2:0, UPM1:0 and USBS bits in
UCSRB and UCSRC. The receiver and transmitter use the same setting. Note that
changing the setting of any of these bits will corrupt all ongoing communication for both
the receiver and transmitter.
The USART Character SiZe (UCSZ2:0) bits select the number of data bits in the frame.
The USART Parity Mode (UPM1:0) bits enable and set the type of parity bit. The selec-
tion between one or two stop bits is done by the USART Stop Bit Select (USBS) bit. The
receiver ignores the second stop bit. An FE (Frame Error) will therefore only be detected
in the cases where the first stop bit is zero.
Parity Bit Calculation The parity bit is calculated by doing an exclusive-or of all the data bits. If odd parity is
used, the result of the exclusive or is inverted. The relation between the parity bit and
data bits is as follows::
P
even
Parity bit using even parity
P
odd
Parity bit using odd parity
d
n
Data bit n of the character
If used, the parity bit is located between the last data bit and first stop bit of a serial
frame.
USART Initialization The USART has to be initialized before any communication can take place. The initial-
ization process normally consists of setting the baud rate, setting frame format and
enabling the transmitter or the receiver depending on the usage. For interrupt driven
USART operation, the global interrupt flag should be cleared (and interrupts globally dis-
abled) when doing the initialization.
Before doing a re-initialization with changed baud rate or frame format, be sure that
there are no ongoing transmissions during the period the registers are changed. The
TXC flag can be used to check that the transmitter has completed all transfers, and the
RXC flag can be used to check that there are no unread data in the receive buffer. Note
1 0 2 3 4 [5] [6] [7] [8] [P] St Sp1 [Sp2] (St / IDLE) (IDLE)
FRAME
P
even
d
n 1 –
… d
3
d
2
d
1
d
0
0
P
o dd
⊕ ⊕ ⊕ ⊕ ⊕ ⊕
d
n 1 –
… d
3
d
2
d
1
d
0
1 ⊕ ⊕ ⊕ ⊕ ⊕ ⊕
=
=