
MAX1220/MAX1257/MAX1258
12-Bit, Multichannel ADCs/DACs with FIFO,
Temperature Sensing, and GPIO Ports
36
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DIN
CONVERSION BYTE
MSB1
MSB2
LSB1
tDOV
(UP TO 514 INTERNALLY CLOCKED ACQUISITIONS AND CONVERSIONS)
EOC
DOUT
SCLK
CS
Figure 8. Clock Mode 10—The command byte to the conversion register begins the acquisition (CNVST is not required).
Set CNVST high to begin a conversion. Sampling is
completed approximately 500ns after CNVST goes
high. After the conversion is complete, the ADC shuts
down and pulls EOC low. EOC stays low until CS or
CNVST is pulled low again. Wait until EOC goes low
before pulling CS or CNVST low. The number of CNVST
signals must equal the number of conversions request-
ed by the scan and averaging registers to correctly
update the FIFO. Wait until all conversions are com-
plete before reading the FIFO. SPI communications to
the DAC and GPIO registers are permitted during
conversion. However, coupled noise may result in
degraded ADC SNR.
If averaging is turned on, multiple CNVST pulses need to
be performed before a result is written to the FIFO. Once
the proper number of conversions has been performed
to generate an averaged FIFO result (as specified to the
averaging register), the scan logic automatically switch-
es the analog input multiplexer to the next requested
channel. If a temperature measurement is programmed,
it is performed after the first rising edge of CNVST follow-
ing the command byte written to the conversion register.
The temperature-conversion result is available on DOUT
once EOC has been pulled low.
Internally Timed Acquisitions and
Conversions Using the Serial Interface
ADC Conversions in Clock Mode 10
In clock mode 10, the wake-up, acquisition, conversion,
and shutdown sequence is initiated by writing a com-
mand byte to the conversion register, and is performed
automatically using the internal oscillator. This is the
default clock mode upon power-up. See Figure 8 for
clock mode 10 timing.
Initiate a scan by writing a command byte to the conver-
sion register. The MAX1220/MAX1257/MAX1258 then
power up, scan all requested channels, store the results
in the FIFO, and shut down. After the scan is complete,
EOC is pulled low and the results are available in the
FIFO. If a temperature measurement is requested, the
temperature result precedes all other FIFO results. EOC
stays low until CS is pulled low again. Wait until all con-
versions are complete before reading the FIFO. SPI
communications to the DAC and GPIO registers are per-
mitted during conversion. However, coupled noise may
result in degraded ADC SNR.
Externally Clocked Acquisitions and
Conversions Using the Serial Interface
ADC Conversions in Clock Mode 11
In clock mode 11, acquisitions and conversions are ini-
tiated by writing a command byte to the conversion
register and are performed one at a time using SCLK
as the conversion clock. Scanning, averaging and the
FIFO are disabled, and the conversion result is avail-
able at DOUT during the conversion. Output data is
updated on the rising edge of SCLK in clock mode 11.
See Figures 9a and 9b for clock mode 11 timing.
Initiate a conversion by writing a command byte to the
conversion register followed by 16 SCLK cycles. If CS
is pulsed high between the eighth and ninth cycles, the
pulse width must be less than 100s. To continuously
convert at 16 cycles per conversion, alternate 1 byte of
zeros (NOP byte) between each conversion byte. If 2
NOP bytes follow a conversion byte, the analog cells
power down at the end of the second NOP. Set the
FBGON bit to one in the reset register to keep the inter-
nal bias block powered.