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MB96300 Super Series Hardware Manual
20.7.1
Operation in Asynchronous Mode (Op. Modes 0 and 1)
When USART is used in operation mode 0 (normal mode) or operation mode 1
(multiprocessor mode), the asynchronous transfer mode is selected.
■ Operation in asynchronous mode
●
Transfer data format
Generally each data transfer in the asynchronous mode operation begins with the start bit (low-level on bus)
and ends with at least one stop bit (high-level). The direction of the bit stream (LSB first or MSB first) is
determined by the BDS bit of the Serial Status Register (SSRn). The parity bit (if enabled) is always placed
between the last data bit and the (first) stop bit.
In operation mode 0 the length of the data frame can be 7 or 8 bits, with or without parity, and 1 or 2 stop
bits.
In operation mode 1 the length of the data frame can be 7 or 8 bits with a following address-/data-selection
bit instead of a parity bit. 1 or 2 stop bits can be selected.
The calculation formula for the bit length of a transfer frame is:
Length = 1 + d + p + s
(d = number of data bits [7 or 8], p = parity [0 or 1], s = number of stop bits [1 or 2]
Figure 20.7-1 Transfer data format (operation modes 0 and 1))
Operation mode 0
Operation mode 1
* D7 (bit 7) if parity is not provided and data length is 8 bits
** only if SBL bit of SCR is set to 1
ST: Start Bit
Note:
If BDS bit of the Serial Status Register (SSRn) is set to "1" (MSB first), the bit stream processes as: D7,
D6, ..., D1, D0, (P).
During Reception both stop bits are detected, if selected. But the Reception data register full (RDRF) flag
will go "1" at the first stop bit. The bus idle flag (RBI of ECCRn) goes "1" after the second stop bit if no
further start bit is detected. (The second stop bit belongs to "bus activity", although it is just mark level.)
ST
D0
D1
ST
D0
D1
P (parity) if parity is provided and data length is 7 bits
SP: Stop Bit
A/D: Address/data selection bit in mode 1 (multiprocessor mode)
*1
D2
D3
D4
D5
D6
D7/P
SP SP
D2
D3
D4
D5
D6
D7 A/D
CHAPTER 20 USART
*2
SP
513
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