Datasheet

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SBAS124C − DECEMBER 1999 − REVISED DECEMBER 2005
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28
Serial Interface Control Register
The Serial Interface Control register (ADDR = 24), see Figure 29, allows certain aspects of the serial interface
to be controlled by the user. It controls whether data is presented MSB or LSB first and whether the serial
interface is configured for 2-wire or 3-wire operation, and it determines proper timing control for 8051-type
microprocessor interfaces.
The information in this register is formatted with the information 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 microcontroller
communication code.
The instruction byte to write this configuration data to Register 24 is itself symmetric. From Figure 14, a register
mode write instruction of 8 bits to address 24 is 0001 1000 in binary form. Therefore, this command to write
to this register is valid under all conditions.
SERIAL INTERFACE CONTROL REGISTER
ADDR D7 (MSB) D6 D5 D4 D3 D2 D1 D0
24 LSB 2W/3W 8051 0 0 8051 2W/3W LSB
ADDR = 7
BIT SYMBOL NAME VALUE FUNCTION
D7 LSB LSB or MSB first 0
1
Serial interface receives and transmits MSB first.
Serial interface receives and transmits LSB first.
D6 2W/3W 2 Wire or 3 Wire 0
1
3-Wire mode
2-Wire mode
D5 8051 Serial Interface 0
1
DIN high impedance on the next inactive edge or when CS goes inactive.
DIN pin is high impedance on last active SCLK edge of the bye of data transfer
D4−D3 — — 0 These bits are reserved and must always be set 0.
D2 8051 Serial Interface 0
1
DIN high impedance on the next inactive edge or when CS goes inactive.
DIN pin is high impedance on last active SCLK edge of the byte of data transfer
D1 2W/3W 2 Wire or 3 Wire 0
1
3-Wire mode
2-Wire mode
D0 LSB LSB or MSB first 0
1
Serial interface receives and transmits MSB first.
Serial interface receives and transmits LSB first.
Bold items are power-up default conditions.
Figure 29. Serial Interface Control Register (ADDR = 24)
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/3W bit configures the ADS7870 for 2-wire or 3-wire mode. In two-wire mode (2W/3W = 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/3W = 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.