LM12434CIWM NSC [National Semiconductor], LM12434CIWM Datasheet - Page 55

• Manufacturer Part Number: LM12434CIWM
• Description: Sign Data Acquisition System with Serial I/O and Self-Calibration
• Manufacturer: NSC [National Semiconductor]

7 0 Digital Interface
7 2 8051 INTERFACE MODE
The 8051 interface mode is designed to work directly with
the 8051 family of microcontrollers’ mode 0 serial interface
This interface mode is a simple shift register type of serial
data transfer The serial clock synchronizes the transfer of
data to and from the LM12434 and LM12 L 438 The inter-
face uses 3 lines a bidirectional data line (RXD) a serial
clock line (TXD) and a chip-select (CS) line More than one
device can share the data and serial clock lines provided
that each device has its own chip-select line
The 8051 mode is selected when the MODESEL1 and
MODESEL2 pins have the logic state of ‘‘00’’ Figure 14
shows a typical connection diagram for the 8051 mode seri-
al interface The CS RXD and TXD lines are respectively
assigned to interface pins P3 through P5 The P1 and P2
pins are not used in this mode and should be left open or
connected to logic ‘‘1’’ In this interface the idle state of the
serial clock TXD is logic ‘‘1’’ The data is stable at both
edges of the TXD clock and is shifted after its rising edge
The interface has a bidirectional RXD data line The
LM12434 and LM12 L 438 leaves the RXD line in a high
impedance state whenever it is not outputting any data
Data transfer in this mode is byte oriented As mentioned
the LM12434 and LM12 L 438 has three different commu-
nication cycles write cycle read cycle and burst read cycle
At the start of each data transfer cycle ‘‘command byte’’ is
written to the LM12434 and LM12 L 438 followed by write
or read data The command byte informs the LM12434 and
LM12 L 438 about the communication cycle and carries
the following information
what type of data transfer (communication cycle) is start-
ed
which device register is to be accessed
(Continued)
55
Figure 13 shows the timing diagrams for different communi-
The command byte has the following format
The first bit is the LSB of the byte based on the 8051 mode
0 serial interface protocol
cation cycles Figure 13a shows a write cycle Figure 13b
shows a read cycle Figure 13c shows a burst read cycle
Note that these timing diagrams depict general relationships
between the SCLK edges the data bits and CS These dia-
grams are not meant to show guaranteed timing perform-
ance (See specification tables for parametric switching
characteristics )
Write cycle A write cycle begins with the falling edge of the
CS Then a command byte is written to the DAS on the RXD
line synchronized by TXD clock The command byte has the
R W and B bits equal to zero Following the command byte
16 bits of data (2 bytes) is shifted in on the RXD line The
data is written to the register addressed in the command
byte (A3 A2 A1 A0) The data is always LSB first in this
interface CS will go high after the transfer of the last bit
thus completing the write cycle
Read cycle A read cycle starts the same way as a write
cycle except that the command bytes R W bit is equal to
one Following the command byte the DAS outputs the
data on the RXD line synchronized with the microcontrol-
ler’s TXD clock The data is read from the register ad-
dressed in the command byte Data is shifted in LSB first
Again CS will go high after the transfer of the last data bit
thus completing the read cycle
TL H 11879– 53