Data Byte (Read Cycle)
A read cycle must be initiated to obtain conversion
results. Read cycles begin with the bus master issuing
a START condition followed by seven address bits and
a read bit (R/W = 1). If the address byte is successfully
received, the MAX1236–MAX1239 (slave) issues an
acknowledge. The master then reads from the slave.
The result is transmitted in two bytes; first four bits of
the first byte are high, then MSB through LSB are con-
secutively clocked out. After the master has received
the byte(s), it can issue an acknowledge if it wants to
continue reading or a not-acknowledge if it no longer
wishes to read. If the MAX1236–MAX1239 receive a not-
acknowledge, they release SDA, allowing the master to
generate a STOP or a repeated START condition. See
the
Clock Modes
and
Scan Mode
sections for detailed
information on how data is obtained and converted.
Clock Modes
The clock mode determines the conversion clock and
the data acquisition and conversion time. The clock
mode also affects the scan mode. The state of the set-
up byte’s CLK bit determines the clock mode (Table 1).
At power-up, the MAX1236–MAX1239 are defaulted to
internal clock mode (CLK = 0).
Internal Clock
When configured for internal clock mode (CLK = 0), the
MAX1236–MAX1239 use their internal oscillator as the con-
version clock. In internal clock mode, the MAX1236–
MAX1239 begin tracking the analog input after a valid
address on the eighth rising edge of the clock. On the
falling edge of the ninth clock, the analog signal is
acquired and the conversion begins. While converting the
analog input signal, the MAX1236–MAX1239 holds SCL
low (clock stretching). After the conversion completes, the
results are stored in internal memory. If the scan mode is
set for multiple conversions, they all happen in succession
with each additional result stored in memory. The
MAX1236/MAX1237 contain four 12-bit blocks of memory,
and the MAX1238/ MAX1239 contain twelve 12-bit blocks
of memory. Once all conversions are complete, the
MAX1236–MAX1239 release SCL, allowing it to be pulled
high. The master can now clock the results out of the mem-
ory in the same order the scan conversion has been done
at a clock rate of up to 1.7MHz. SCL is stretched for a max-
imum of 8.3µs per channel (see Figure 10).
The device memory contains all of the conversion
results when the MAX1236–MAX1239 release SCL. The
converted results are read back in a first-in-first-out
MAX1236–MAX1239
2.7V to 3.6V and 4.5V to 5.5V, Low-Power,
4-/12-Channel, 2-Wire Serial, 12-Bit ADCs
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