Z85233 Datasheet, PDF (245/317 Pages) Zilog, Inc. – The Zilog SCC Serial Communication Controller

Application Note
Using SCC with Z8000 in SDLC Protocol
INITIALIZATION
The SCC can be initialized for use in different modes by
setting various bits in its write registers. First, a hardware
reset must be performed by setting bits 7 and 6 of WR9 to
one; the rest of the bits are disabled by writing a logic zero.
SDLC protocol is established by selecting a SDLC mode,
sync mode enable, and a x1 clock in WR4. A data rate of
9600 baud, NRZ encoding, and a character length of eight
bits are among the other options that are selected in this
example (Table 2).
Note that WR9 is accessed twice, first to perform a
hardware reset and again at the end of the initialization
sequence to enable the interrupts. The programming
sequence depicted in Table 2 establishes the necessary
parameters for the receiver and transmitter so that they are
ready to perform communication tasks when enabled.
Table 2. Programming Sequence for Initialization
Register
WR9
WR4
WR10
WR6
WR7
WR2
WR11
WR12
WR13
WR14
WR15
WR5
WR3
WR1
WR9
Value
(Hex)
C0
20
80
AB
7E
20
16
CE
0
03
00
60
C1
08
09
Effect
Hardware reset
x1 clock, SDLC mode,
sync mode enable
NRZ, CRC preset to one
Any station address e.g. “AB”
SDLC flag (01111110) = “7E”
Interrupt vector “20”
Tx clock from BRG output, /TRxC pin
= BRG out
Lower byte of time constant = “CE” for
9600 baud
Upper byte = 0
BRG source bit =1 for PCKL as input,
BRG enable
External Interrupt Disable
Transmit 8 bits/character SDLC CRC
Rx 8 bits/character, Rx enable
(Automatic Hunt mode)
ext int. disable
MIE, VIS, status Low
The Z8002 CPU must be operated in System mode to
execute privileged I/O instructions. So the Flag and
Control Word (FCW) should be loaded with system normal
(S//N), and the Vectored Interrupt Enable (VIE) bits set.
The Program Status Area Pointer (PSAP) is loaded with
address %4400 using the Load Control Instruction
(LDCTL). If the Z8000 Development Module is intended to
be used, the PSAP need not be loaded by the programmer
because the development module’s monitor loads it
automatically after the NMI button is pressed.
Since VIS and Status Low are selected in WR9, the
vectors listed in Table 3 will be returned during the
Interrupt Acknowledge cycle. Of the four interrupts listed,
only two, Ch A Receive Character Available and Ch A
Special Receive Condition, are used in the example given
here.
Table 3. Interrupt Vectors
Vector
(Hex)
28
2A
2C
2E
PS
Address
446E
4472
4476
447A
Interrupt
Ch A Transmit Buffer Empty
Ch A External Status Change
Ch A Receive Char. Available
Ch A Special Receive Condition
* Assuming that PSAP has been set to 4400 hex, “PS
Address” refers to the location in the Program Status Area
where the service routine address is stored for that
particular interrupt.
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