LM12434CIWM NSC [National Semiconductor], LM12434CIWM Datasheet - Page 56

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

7 0 Digital Interface
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 RXD line as long as the
DAS receives TXD clock from the system To tell the DAS
when a burst read cycle is completed CS should be set high
after the 8th and before the 15th SCLK cycle during the last
data byte transfer (see Figure 13c ) After CS high is detect-
ed and the last data bit is transferred the DAS is ready for a
new communication cycle to begin
The timing diagrams in Figure 13 show the transfer of data
in packets of 8 bits (bytes) This represents the way the
serial ports of the 8051 family of microcontrollers produce
the serial clock and data The DAS does not require a gap
between the first and second bytes of the data
FIGURE 13 Timing Diagrams for LM12434 and LM12 L 438 8051 Serial Interface Mode
Idle State of SCLK
Idle State of SCLK
(Continued)
e
e
1 Data Shifted at the Rising Edge of the SCLK
1 Data Shifted at the Rising Edge of the SCLK
(a) Write Cycle
(b) Read Cycle
56
16 continuous clock cycles will transfer the data word How-
ever there should be a gap equal to 3 CLK (the DAS main
clock input not the TXD clock) cycles between the end of
the command byte and the start of the data during a read
cycle This is not concerned in most systems for two rea-
sons First the processor generally has some inherent gap
between byte transfers Second the TXD frequency is usu-
ally significantly slower than the CLK frequency For exam-
ple an 8051 processor with 12 MHz crystal generates a
TXD of 1 MHz If the DAS is running with 6 MHz CLK there
are 6 cycles of CLK within each cycle of TXD and the re-
quirement is satisfied even if TXD comes continuously after
command byte The user should pay attention to this re-
quirement if running the TXD with a speed near or higher
than CLK
TL H 11879 – 40
TL H 11879 – 41