Rainbow Electronics MAX1139 - Manual (Page 17)

External Clock
When configured for external clock mode (CLK = 1),
the MAX1136–MAX1139 use the SCL as the conversion
clock. In external clock mode, the MAX1136–MAX1139
begin tracking the analog input on the ninth rising clock
edge of a valid slave address byte. Two SCL clock
cycles later the analog signal is acquired and the con-
version begins. Unlike internal clock mode, converted
data is available immediately after the first four empty
high bits. The device will continuously convert input
channels dictated by the scan mode until given a not
acknowledge. There is no need to re-address the
device with a read command to obtain new conversion
results (see Figure 11).
The conversion must complete in 1ms or droop on the
track-and-hold capacitor will degrade conversion
results. Use internal clock mode if the SCL clock period
exceeds 60µs.
The MAX1136–MAX1139 must operate in external clock
mode for conversion rates from 40ksps to 94.4ksps.
Below 40ksps internal clock mode is recommended
due to much smaller power consumption.
Scan Mode
SCAN0 and SCAN1 of the configuration byte set the
scan mode configuration. Table 5 shows the scanning
configurations. If AIN_/REF is set to be a reference
input or output (SEL1 = 1, Table 6), AIN_/REF will be
excluded from a multichannel scan. The scanned
results are written to memory in the same order as the
conversion. Read the results from memory in the order
they were converted. Each result needs a 2-byte trans-
mission, the first byte begins with six empty bits during
which SDA is left high. Each byte has to be acknowl-
edged by the master or the memory transmission will
be terminated. It is not possible to read the memory
independently of conversion.
Applications Information
Power-On Reset
The configuration and setup registers (Tables 1 and 2)
will default to a single-ended, unipolar, single-channel
conversion on AIN0 using the internal clock with V
DD as
the reference and AIN_/REF configured as an analog
input. The memory contents are unknown after power-up.
Automatic Shutdown
SEL[2:0] of the setup byte (Table 1 and Table 6) control
the state of the reference and AIN_/REF. If automatic
shutdown is selected (SEL[2:0] = 100), shutdown will
occur between conversions when the MAX1136–
MAX1139 are idle. When operating in external clock
mode, a STOP, not-acknowledge or repeated START,
condition must be issued to place the devices in idle
mode and benefit from automatic shutdown. A STOP
condition is not necessary in internal clock mode to
benefit from automatic shutdown because power-down
occurs once all contents are written to memory (Figure
10). All analog circuitry is inactive in shutdown and
supply current is less than 0.5µA (typ). The digital con-
version results are maintained in memory during shut-
down and are available for access through the serial
interface at any time prior to a STOP or a repeated
START condition.
When idle the MAX1136–MAX1139 continuously wait
for a START condition followed by their slave address
(see Slave Address section). Upon reading a valid
address byte the MAX1136–MAX1139 power-up. The
internal reference requires 10ms to wake up, so when
using the internal reference it should be powered up
MAX1136–MAX1139
2.7V to 3.6V and 4.5V to 5.5V, Low-Power,
4-/12-Channel, 2-Wire Serial 10-Bit ADCs
______________________________________________________________________________________ 17
SCAN1 SCAN0 SCANNING CONFIGURATION
0 0
Scans up from AIN0 to the input selected by CS3–CS0. When CS3–CS0 exceeds 11, the scanning will
stop at AIN11. When AIN_/REF is set to be a REF in/out, scanning will stop at AIN3 and AIN10.
0 1 *Converts the input selected by CS3–CS0 eight times. (See Tables 3 and 4)
Scans up from AIN2 to the input selected by CS1 and CS0. When CS1 and CS0 are set for AIN0–AIN2
the scanning will stop at AIN2 (MAX1136/MAX1137). When AIN_/REF is set to be a REF in/out,
scanning will stop at AIN3 and AIN10.
1 0
Scans up from AIN6 to the input selected by CS3–CS0. When CS3–CS0 is set for AIN0–AIN6 scanning
will stop at AIN6 (MAX1138/MAX1139). When AIN_/REF is set to be a REF in/out, scanning will stop at
AIN3 and AIN10.
1 1 *Converts channel selected by CS3–CS0.
*When operating in external clock mode there is no difference between SCAN[1:0] = 01 and SCAN[1:0] = 11 and converting will occur
perpetually until not acknowledge occurs.
Table 5. Scanning Configuration