Zilog Z08470 - Manual (Page 282)

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UM008101-0601 Serial Input/Output
262
In all modes, characters are sent with the least-significant bits first. This
requires right justification of data to be transmitted when the word length is
less than eight bits. If the word length is five bits or less, use the special
technique described in
“Write Register 5” on page 289 .
Because the number of bits/character can be changed on-the-fly, the data
field can be filled with any number of bits. When used in conjunction with
the Receiver Residue codes, the Z80 SIO can receive a message that has a
variable I-field and retransmit it exactly as received with no previous infor-
mation about the character structure of the I-field (if any). A change in the
number of bits does not affect the character being shifted out. Characters
are sent with the number of bits programmed at the time that the character
is loaded from the transmit buffer to the transmitter.
If the External/Status Interrupt Enable is set, transmitter conditions, such as
“starting to send CRC characters,” “starting to send flag characters,” and CTS
changing state, cause interrupts, having a unique vector if Status Affects
Vector is set. All interrupts can be disabled for operation in a polled mode.
Table 9 describes the typical program steps that implement the half-duplex
SDLC Transmit mode.
Table 9. SDLC Transmit Mode
Function Typical Program Steps Comments
Register Information loaded:
Initialize WR0 Channel Reset Reset SIO
WR0 Pointer 2
WR2 Interrupt Vector Channel B only
WR0 Pointer 3
WR3 Auto Enables Transmitter sends data only after CTS is
detected
WR0 Pointer 4, Reset External/status
Interrupts