HD64570 Datasheet, PDF (203/469 Pages) Hitachi Semiconductor – Serial Communications Adaptor

Break Transmission and Detection: When the transmitter must suspend data transmission, it
transmits a break (space or low level).
Normally, the transmitter issues a break transmission request after completing the current
character transmission. The transmitter must continue to send the break signal for one or more
character cycles. Break transmission is controlled by the BRK bit of CTL. When this bit is set to
1, the TXD line goes low at the falling edge of the next transmit clock pulse. When this bit is
cleared, the TXD line goes high at the falling edge of the next transmit clock pulse to cancel break
transmission. At break transmission cancellation, the transmitter guarantees one or more bit
cycles of high level to next start bit.
When break transmission is requested, the output data in the transmit shift register is lost, but the
transmit buffer is not affected.
The receiver detects a break in the following procedure:
On receiving data whose data bits and parity bits are all 0s and contain a framing error, the
receiver assumes it to be the start of a break, and sets the BRKD bit of ST1 to 1. Here, the null
character containing the framing error is discarded (not transferred to the receive buffer).
If break transmission starts during character transmission, the break transmission must continue
for two or more character cycles. The reason for this can be seen from figure 5.21, which shows
break detection by the receiver.
On detecting a mark (high level) of a half-bit cycle or longer after detecting the start of a break,
the receiver assumes it to be the end of a break, and sets the BRKE bit of ST1 to 1. (In 1/1 clock
mode, detection of the first mark causes the break to end.)
When the BRKD or BRKE bit is set to 1, an interrupt request is generated (if enabled).
D₀ D₁
Stop
D₀ D₁
Stop
Data with framing
error received
Start bit detected
Start bit search started
and space detected
Break start detected
(The received null
character is not sent
to the receive buffer)
Figure 5.21 Break Detection by the Receiver
Break end detected
Rev. 0, 07/98, page 187 of 453