Rainbow Electronics MAX1231 - Manual (Page 12)

MAX1227/MAX1229/MAX1231
12-Bit 300ksps ADCs with FIFO,
Temp Sensor, Internal Reference
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Applications Information
Register Descriptions
The MAX1227/MAX1229/MAX1231 communicate
between the internal registers and the external circuitry
through the SPI/QSPI-compatible serial interface. Table
1 details the registers and the bit names. Tables 2–7
show the various functions within the conversion regis-
ter, setup register, averaging register, reset register,
unipolar register, and bipolar register.
Conversion Time Calculations
The conversion time for each scan is based on a num-
ber of different factors: conversion time per sample,
samples per result, results per scan, if a temperature
measurement is requested, and if the external refer-
ence is in use.
Use the following formula to calculate the total conver-
sion time for an internally timed conversion in clock
modes 00 and 10 (see the Electrical Characteristics
section as applicable):
total conversion time = t cnv x n avg x n result + t TS + t RP
where:
t cnv = t acq (max) + t conv (max)
n avg = samples per result (amount of averaging)
n result = number of FIFO results requested; determined
by number of channels being scanned or by NSCAN1,
NSCAN0
t TS = time required for temperature measurement; set
to zero if temp measurement is not requested
t RP = internal reference wake up; set to zero if internal
reference is already powered up or external reference
is being used
In clock mode 01, the total conversion time depends on
how long CNVST is held low or high, including any time
required to turn on the internal reference. Conversion
time in externally clocked mode (CKSEL1, CKSEL0 = 11)
depends on the SCLK period and how long CS is held
high between each set of eight SCLK cycles. In clock
mode 01, the total conversion time does not include the
time required to turn on the internal reference.
Conversion Register
Select active analog input channels, scan modes, and
a single temperature measurement per scan by writing
to the conversion register. Table 2 details channel
selection, the four scan modes, and how to request a
temperature measurement. Request a scan by writing
to the conversion register when in clock mode 10 or 11,
or by applying a low pulse to the CNVST pin when in
clock mode 00 or 01.
A conversion is not performed if it is requested on a
channel that has been configured as CNVST or REF-.
Do not request conversions on channels 8–15 on the
MAX1227 and channels 12–15 on the MAX1229. Set
CHSEL3:CHSEL0 to the lower channel’s binary values.
If the last two channels are configured as a differential
pair and one of them has been reconfigured as CNVST
or REF-, the pair is ignored.
Select scan mode 00 or 01 to return one result per sin-
gle-ended channel and one result per differential pair
within the requested range, plus one temperature result if
selected. Select scan mode 10 to scan a single input
channel numerous times, depending on NSCAN1 and
NSCAN0 in the averaging register (Table 6). Select scan
mode 11 to return only one result from a single channel.
Setup Register
Write a byte to the setup register to configure the clock,
reference, and power-down modes. Table 3 details the
REGISTER NAME BIT 7 BIT 6 BIT 5 BIT 4 BIT 3 BIT 2 BIT 1 BIT 0
Conversion 1 CHSEL3 CHSEL2 CHSEL1 CHSEL0 SCAN1 SCAN0 TEMP
Setup 0 1 CKSEL1 CKSEL0 REFSEL1 REFSEL0 DIFFSEL1 DIFFSEL0
Averaging 0 0 1 AVGON NAVG1 NAVG0 NSCAN1 NSCAN0
Reset 0 0 0 1 RESET X X X
Unipolar mode (setup) UCH0/1 UCH2/3 UCH4/5 UCH6/7 UCH8/9* UCH10/11 UCH12/138** UCH14/15**
Bipolar mode (setup) BCH0/1 BCH1/2 BCH4/5 BCH6/7 BCH8/9* BCH10/11 BCH12/13** BCH14/15**
Table 1. Input Data Byte (MSB First)
*Unipolar/bipolar channels 8–15 are only valid on the MAX1229 and MAX1231.
**Unipolar/bipolar channels 12–15 are only valid on the MAX1231.
X = Don’t care.