Z80 Datasheet, PDF (293/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

Z80 CPU Peripherals
User Manual
273
characteristics of the channels. With the exception of WR0, programming
the write registers requires two bytes. The first byte contains three bits (D2-
D0) that point to the selected register; the second byte is the actual control
word that is written into the register to configure the Z80 SIO.
After pointing to the selected register, a programmer can either read to test
the read register or write to initialize the write register. The Z80 SIO
software can be initialized in either a modular or structured mode, allowing
the programmer to use powerful block I/O instructions.
WR0 functions as a unique register because all the basic commands
(CM02-CMDO) can be accessed with a single byte. Reset (internal or
external) initializes the pointer bits D0-D2 to point to WR0.
The basic commands (CMD2-CMD0) and the CRC controls (CRC0,
CRC1) are contained in the first byte of any write register access. This
allows maximum flexibility and system control. Each channel contains
the following control registers. These registers are addressed as
commands, not data.
Write Register 0
WR0, (Figure 114) is the command register; however, it is also used for
CRC reset codes and points to the other registers.
Table 12. Write Register 0
D7
D6
D5
D4
D3
D2
D1
DO
CRC CRC CMD CMD CMD PTR PTR PTR
Reset Reset 2
1
0
2
1
0
Code Code
1
0
UM008101-0601
Serial Input/Output