CDP1854 Intersil Corporation, CDP1854 Datasheet - Page 8

• Manufacturer Part Number: CDP1854
• Description: Programmable Universal Asynchronous Receiver/Transmitter (UART)
• Manufacturer: Intersil Corporation

Description of Mode 1 Operation
CDP1800-Series Microprocessor
Compatible (Mode Input = V
Initialization and Controls
In the CDP1800-series microprocessor compatible mode,
the CDP1854A is configured to receive commands and send
status via the microprocessor data bus. The register
connected to the transmitter bus or the receiver bus is
determined by the RD/WR and RSEL inputs as follows:
In this mode the CDP1854A is compatible with a
bidirectional bus system. The receiver and transmitter buses
are connected to the bus. CDP1800-series microprocessor
I/O control output signals can be connected directly to the
CDP1854A inputs as shown in Figure 2. The CLEAR input is
pulsed, resetting the Control, Status, and Receiver Holding
Registers and setting SERIAL DATA OUT (SDO) high. The
Control Register is loaded from the Transmitter Bus in order
to determine the operating configuration for the UART. Data
is transferred from the Transmitter Bus inputs to the Control
Register during TPB when the UART is selected (CS1 CS2
H, RD/WR = L). The CDP1854A also has a Status Register
which can be read onto the Receiver Bus (R BUS 0 - R BUS
7) in order to determine the status of the UART. Some of
these status bits are also available at separate terminals as
indicated in Table 2.
Transmitter Operation
Before beginning to transmit, the TRANSMlT REQUEST
(TR) bit in the Control Register (see bit assignment, Table 4)
is set. Loading the Control Register with TR = 1 (bit 7 = high)
inhibits changing the other control bits. Therefore two loads
are required: one to format the UART, the second to set TR.
When TR has been set, a TRANSMlTTER HOLDING REG-
ISTER EMPTY (THRE) interrupt will occur, signalling the
microprocessor that the Transmitter Holding Register is
empty and may be loaded. Setting TR also causes assertion
of a low-level on the REQUEST TO SEND (RTS) output to
the peripheral. It is not necessary to set TR for proper opera-
tion for the UART. If desired, it can be used to enable THRE
interrupts and to generate the RTS signal. The Transmitter
Holding Register is loaded from the bus by TPB during exe-
cution of an output instruction. The CDP1854A is selected
by CS1
selected by RSEL = L and RD/WR = L. When the CLEAR
TO SEND (CTS) input, which can be connected to a periph-
eral device output, goes low, the Transmitter Shift Register
CS3 = 1) and the Control Register is designated (RSEL =
RSEL
High
High
Low
Low
TABLE 3. REGISTER SELECTION SUMMARY
CS2
RD/WR
High
High
Low
Low
CS3 = 1, and the Holding Register is
Load Transmitter Holding Register from
Transmitter Bus
Read Receiver Holding Register from
Receiver Bus
Load Control Register from Transmitter
Bus
Read Status Register from Receiver Bus
FUNCTION
DD
)
CDP1854A, CDP1854AC
5-49
will be loaded from the Transmitter Holding Register and
data transmission will begin. If CTS is always low, the Trans-
mitter Shift Register will be loaded on the first high-to-low
edge of the clock which occurs at least 1/2 clock period after
the trailing edge of TPB and transmission of a start bit will
occur 1/2 clock period later (see Figure 3). Parity (if pro-
grammed) and stop bit(s) will be transmitted following the
last data bit. If the word length selected is less than 8 bits,
the most significant unused bits in the transmitter shift regis-
ter will not be transmitted.
One transmitter clock period after the Transmitter Shift Reg-
ister is loaded from the Transmitter Holding Register, the
THRE signal will go low and an interrupt will occur (INT goes
low). The next character to be transmitted can then be
loaded into the Transmitter Holding Register for transmission
with its start bit immediately following the last stop bit of the
previous character. This cycle can be repeated until the last
character is transmitted, at which time a final THRE
interrupt will occur. This interrupt signals the microprocessor
that TR can be turned off. This is done by reloading the orig-
inal control byte in the Control Register with the TR bit 0,
thus terminating the REQUEST TO SEND (RTS) signal.
SERIAL DATA OUT (SDO) can be held low by setting the
BREAK bit in the Control Register (see Table 4). SDO is held
low until the BREAK bit is reset.
Receiver Operation
The receive operation begins when a start bit is detected at
the SERlAL DATA IN (SDl) input. After detection of the first
high-to-low transition on the SDl line, a valid start bit is
verified by checking for a low-level input 7-1/2 receiver clock
periods later. When a valid start bit has been verified, the fol-
lowing data bits, parity bit (if programmed) and stop bit(s) are
shifted into the Receiver Shift Register by clock pulse 7-1/2
FIGURE 2. RECOMMENDED CDP1800-SERIES CONNECTION,
CPU
CLEAR
MODE 1 (NON-INTERRUPT DRIVEN SYSTEM)
MRD
BUS
TPB
EF
EF
EF
EF
INT
N0
N1
N2
X
X
X
X
(8)
V
V
DD
SS
T CLOCK R CLOCK
RSEL
CS1
CS2
CS3
RD/WR
TPB
INT
THRE
DA
FE
PE/OE
T BUS
R BUS
CLEAR
CDP1854A
UART
MODE
V
DD
SDO
CTS
RTS
SDI
PSI
ES
TSRE