Datasheet

LTC2488
17
2488fa
state. However, the data output state may be aborted by
pulling
C
S HIGH any time between the 1st falling edge
and the 24th falling edge of SCK (see Figure 5). On the
rising edge of
C
S, the device aborts the data output state
and immediately initiates a new conversion. In order to
program a new input channel, 8 SCK clock pulses are
required. If the data output sequence is aborted prior to
the 8th falling edge of SCK, the new input data is ignored
and the previously selected input channel remains valid.
If the rising edge of
C
S occurs after the 8th falling edge
of SCK, the new input channel is loaded and valid for the
next conversion cycle.
External Serial Clock, 3-Wire I/O
This timing mode uses a 3-wire serial I/O interface. The
conversion result is shifted out of the device by an exter-
nally generated serial clock (SCK) signal (see Figure 6).
C
S is permanently tied to ground, simplifying the user
interface or isolation barrier.
The external serial clock mode is selected at the end of
the power-on reset (POR) cycle. The POR cycle typically
concludes 4ms after V
CC
exceeds 2V. The level applied to
SCK at this time determines if SCK is internally generated
or externally applied. In order to enter the external SCK
mode, SCK must be driven LOW prior to the end of the
POR cycle.
Since
C
S is tied LOW, the end-of-conversion (
E
O
C) can be
continuously monitored at the SDO pin during the convert
and sleep states.
E
O
C may be used as an interrupt to an
external controller.
E
O
C = 1 while the conversion is in
progress and
E
O
C = 0 once the conversion is complete.
On the falling edge of
E
O
C, the conversion result is load-
ing into an internal static shift register. The output data
can now be shifted out the SDO pin under control of the
externally applied SCK signal. Data is updated on the fall-
ing edge of SCK. The input data is shifted into the device
through the SDI pin on the rising edge of SCK. On the
24th falling edge of SCK, SDO goes HIGH, indicating a
new conversion has begun. This data now serves as
E
O
C
for the next conversion.
APPLICATIONS INFORMATION
V
CC
F
O
SCK
SDI
GND
6
REFERENCE
VOLTAGE
0.1V TO V
CC
ANALOG
INPUTS
= EXTERNAL OSCILLATOR
= INTERNAL OSCILLATOR
LTC2488
4-WIRE
SPI INTERFACE
REF
+
REF
CH0
CH1
CH2
CH3
COM
CS
SDO
12 1
13
14
8
9
10
11
7
3
4
5
2
2.7V TO 5.5V
0.1μF
10μF
Hi-Z
2488 F05
CS
SCK
(EXTERNAL)
SDI
SDO
CONVERSION SLEEP DATA INPUT/OUTPUT SLEEPCONVERSION
EOC
BIT 20 BIT 19 BIT 18 BIT 17 BIT 16 BIT 15BIT 21BIT 22BIT 23
12345678
1 0 EN SGL A2 A1 A0ODD
DON'T CAREDON'T CARE
MSBSIG“0”
Figure 5. External Serial Clock, Reduced Output Data Length and Valid Channel Selection