Intel 386 User Manual page 410

If the transmitter is disabled while a data value in the shift register is being shifted out, it continues
running until the last bit is shifted out. Then the shift register stops and the data and clock pins
(SSIOTX and STXCLK) are three-stated; the contents of the buffer register are not loaded into
the shift register.
If the transmitter is disabled then re-enabled before the current value has been shifted out, it con-
tinues as if it were never disabled.
If you enable the transmitter while the baud-rate generator clock is high, the data and clock pin
states are as shown in Figure 13-10. If you enable the transmitter while the baud-rate generator
clock is low, the data and clock pin states are as shown in Figure 13-11. These figures show master
mode, single word transfers. At the end of transmission, STXCLK and SSIOTX are three-stated
and require external pull-up resistors. For single word transfers, you must enable the transmitter,
which starts the shifting process, then disable the transmitter before 16 bits are shifted out. For
high baud rates use the Autotransmit mode.
Baud-rate
Generator Clock
Transmitter Enable
Float
STXCLK
Float
SSIOTX
Figure 13-10. Transmitter Master Mode, Single Word Transfer
Baud-rate
Generator Clock
Transmitter Enable
Float
STXCLK
Float
SSIOTX
Figure 13-11. Transmitter Master Mode, Single Word Transfer
TB15 TB14 TB13
(Enabled when Clock is High)
TB15 TB14
(Enabled when Clock is Low)
SYNCHRONOUS SERIAL I/O UNIT
TB1 TB0
TB1 TB0
Float
Float
A2445-01
Float
Float
A2444-01
13-11