LM12434CIWM NSC [National Semiconductor], LM12434CIWM Datasheet - Page 42

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

Figure 11 shows the timing diagrams for different communi-
7 0 Digital Interface
In both cases the data transfer is insensitive to idle state of
the SCLK SCLK can stay at either logic level high or low
when not clocking (see Figure 11 )
Data transfer in this mode is basically byte-oriented This is
compatible with the serial interface of the target microcon-
trolIers and microprocessors As mentioned the LM12434
and LM12 L 438 have three different communication cy-
cles write cycle read cycle and burst read cycle At the
start of each data transfer cycle ‘‘command byte’’ is written
to the serial DAS followed by write or read data The com-
mand byte informs the LM12434 and LM12 L 438 about
the communication cycle The command byte carries the
following information
The command byte has the following format
Note that the first bit may be either the MSB or the LSB of
the byte depending on the processor type but it must be the
first bit transmitted to the LM12434 and LM12 L 438
cation cycles Figures 11a b c d show write cycles for
various combinations of R F pin logic level and SCLK idle
state Figures 11e f g h show read cycles for similar sets
of conditions Figure 11i shows a burst read cycle for the
case of R F
timing diagrams depict general relationships between the
SCLK edges the data bits and CS These diagrams are not
meant to show guaranteed timing (See specification tables
for parametric switching characteristics )
Write cycle A write cycle begins with the falling edge of
CS Then a command byte is written to the DAS on the DI
line synchronized by SCLK 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 same DI line
what type of data transfer (communication cycle) is start-
ed
which device register to be accessed
e
0 and low SCLK idle state Note that these
(Continued)
TL H 11879– 52
42
This data is written to the register addressed in the com-
mand byte (A3 A2 A1 A0) The data is interpreted as MSB
or LSB first based on the logic level of the 7th bit (MSB
LSB) in the command byte There is no activity on the DO
line during write cycles and the DAS leaves the DO line in
the high impedance state 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 byte’s R W bits equal to
one Following the command byte the DAS outputs the
data on the DO line synchronized with the microcontroller’s
SCLK The data is read from the register addressed in the
command byte Data is shifted out MSB or LSB first de-
pending on the logic level of the MSB LSB bit The logic
state of the Dl line is ‘‘don’t care’’ after the command byte
CS will go high after the transfer of the last data bit then
completing the read cycle
Burst read cycle A burst read cycle starts the same way
as a single read cycle but the B bit in the command byte is
set to one indicating a burst read cycle Following the com-
mand byte the data is output on the DO line as long as the
DAS receives SCLK from the system To tell the DAS when
a burst read cycle is completed pull CS high after the 8th
and before the 15th SCLK cycle during the last data byte
transfer (see Figure 11i ) After CS high is detected and the
last data bit is transferred the DAS is ready for a new com-
munication cycle to begin
The timing diagrams in Figure 11 show the transfer of data
in packets of 8 bits (bytes) This represents the way the
serial ports of most microcontrollers and microprocessors
produce serial clock and data The DAS does not require a
gap between the first and second byte of the data 16 con-
tinuous clock cycles will transfer the data word However
there should be a gap equal to 3 CLK (the DAS main clock
input not the SCLK) cycles between the end of the com-
mand byte and the start of the data during a read cycle This
is not a concern in most systems for two reasons First the
processor generally has some inherent gap between byte
transfers Second the SCLK frequency is usually signifi-
cantly slower than the CLK frequency For example a
68HC11 processor with an 8 MHz crystal generates a maxi-
mum SCLK frequency of 1 MHz If the DAS is running with a
6 MHz CLK there are 6 cycles of CLK within each cycle of
SCLK and the requirement is satisfied even if SCLK oper-
ates continuously during and after the command byte