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ADS7870 Datasheet(HTML) 20 Page - Burr-Brown (TI)

[Old version datasheet] Texas Instruments acquired Burr-Brown Corporation. Click here to check the latest version.
Part No. ADS7870
Description  12-Bit ADC, MUX, PGA and Internal Reference DATA ACQUISITION SYSTEM
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Maker  BURR-BROWN [Burr-Brown (TI)]
Homepage  http://www.burr-brown.com
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ADS7870 Datasheet(HTML) 20 Page - Burr-Brown (TI)

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®
ADS7870
Selecting the Reference Voltage
When the RBG bit is set to “1” the voltage on the VREF pin
is 1.15V and the R2V bit has no effect. When this bit is set
to “0” (power-up default condition), the R2V bit determines
the value of the reference voltage.
When R2V = 0 and RBG = 0 (power-up default condition), the
voltage at the VREF pin is 2.5V. When R2V = 1 and RBG =
0, the reference voltage is 2.048V.
A 12-bit bipolar input A/D converter has 4096 states and
each state corresponds to 1.22mV with the 2.500V refer-
ence. With a 2.048V reference, each A/D bit corresponds to
1.0mV.
SERIAL INTERFACE CONTROL REGISTER
The Serial Interface Control register (ADDR = 24), see
Table XI, allows certain aspects of the serial interface to be
controlled by the user. It controls whether data is presented
MSB or LSB first, whether the serial interface is configured
for 2-wire or 3-wire operation and determines proper timing
control for 8051-type microprocessor interfaces.
The information in this register is formatted with the infor-
mation symmetric about its center. This is done so that it
may be read or written either LSB (bit D7) or MSB (bit D0)
first. Each control bit has two locations in the register. If
either of the two is set, the function is activated. This
arrangement can potentially simplify micro-controller com-
munication code.
The instruction byte to write this configuration data to
Register 24 is itself symmetric. From Table I, a register
mode write instruction of 8 bits to address 24 is “0001 1000”
in binary form. Therefore, this command will be valid under
all conditions.
LSB or MSB
The LSB bit determines whether the serial interface receives
and transmits either LSB or MSB first. Setting the LSB bit
(“1”) configures the interface to expect all bytes LSB first as
opposed to the default MSB first (LSB = “0”).
2-Wire or 3-Wire Operation
The 2W bit configures the ADS7870 for two-wire or three-
wire mode. In two-wire mode (2W = 1), the DIN pin is
enabled as an output during the data output portion of a read
instruction. The DIN pin accepts data when the ADS7870
is receiving and it outputs data when the ADS7870 is
transmitting. When data is being sent out of the DIN pin, it
also appears on the DOUT pin as well. In three-wire mode
(2W = 0), data to the ADS7870 is received on the DIN pin
and is transmitted on the DOUT pin. The power-up default
condition is three-wire mode.
Serial Interface Timing (8051 Bit)
The 8051 bit will change the timing of when the DIN pin will
go to high impedance at the end of a operation. When the bit
is a “1” the pin goes to high impedance on the last active
SCLK edge of the last byte of data transfer instead of waiting
for the next inactive edge or CS to go inactive. This allows the
ADS7870 to disconnect from the data lines soon enough to
avoid contention with an 80C51-type interface. The 80C51
drives data four CPU cycles before an inactive SCLK edge
and for two CPU cycles after an active SCLK edge. When the
bit is a “0” the DIN pin goes high impedance on the next
inactive SCLK edge or when CS goes inactive (“1”).
BIT
SYMBOL
NAME
VALUE
FUNCTION
D7
LSB
LSB or MSB first
0
Serial Interface receives and transmits MSB first.
1
Serial Interface receives and transmits LSB first.
D6
2W
2-Wire or 3-Wire
0
3-Wire Mode
1
2-Wire Mode
D5
8051
Serial Interface
0
DIN high impedance on the next inactive edge or when CS
goes inactive.
1
DIN pin is high impedance on last active SCLK edge of the last byte
of data transfer.
D4-D3
0
These bits are reserved and must always be set 0.
D2
8051
Serial Interface
0
DIN high impedance on the next inactive edge or when CS
goes inactive
1
DIN pin is high impedance on last active SCLK edge of the last
byte of data transfer
D1
2W
2-Wire or 3-Wire
0
3-Wire Mode
1
2-Wire Mode
D0
LSB
LSB or MSB first
0
Serial Interface receives and transmits MSB first.
1
Serial Interface receives and transmits LSB first.
Bold items are power-up default conditions.
SERIAL INTERFACE CONTROL REGISTER
ADDR
D7 (MSB)
D6
D5
D4
D3
D2
D1
D0
24
LSB
2W
8051
0
0
8051
2W
LSB
TABLE XI. Serial Interface Control Register (ADDR = 24).
ADDR = 24


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