AS1520/AS1521
Data Sheet - Detailed Description
Control Register
The control register on the AS1520/AS1521 is a 8-bit, write-only register. Data is written to this register using the CSN,
DIN and SCLK pins. The control register format is shown in Table 7 and the function of the bits are defined in Table 8.
The AS1520/AS1521 operating modes are selected by sending an 8-bit data word to the internal shift register via pin
DIN. After pin CSN is pulled low, the first logic 1 on pin DIN is interpreted as a start bit. A start bit is defined as one of
the following:
ꢀ
The first logic 1 bit clocked into pin DIN (with CSN low) any time the AS1520/AS1521 is idle, e.g., after VDD1 and
VDD2 are applied.
ꢀ
The first logic 1 bit clocked into pin DIN after bit 6 of a conversion in progress is clocked out of pin DOUT.
Figure 22 on page 17 shows the serial-interface timing necessary to perform a conversion every 16 SCLK cycles. If
CSN is tied low and SCLK is continuous, guarantee a start bit by first clocking in sixteen 0s. The fastest speed at which
the devices can operate is 16 clocks per conversion (with CSN held low between conversions).
Table 7. Control Byte Format
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
START(MSB)
SEL2
SEL1
SEL0
RANGE
MODE
PD1
PD0 (LSB)
Table 8. Bit Descriptions
Bit
Name
Description
7
START
The first logic 1 bit after CSN goes low signifies the start of a control byte.
These three bits select which of the eight channels and pin COM are used for
the conversion (see Table 10 and Table 11).
6:4
SEL2:SEL0
This bit selects the analog input range of the AS1520/AS1521.
0 = The analog input range extends from -VREF/2 to +VREF/2.
1= The analog input range extends from 0V to VREF.
3
RANGE
This bit in conjunction with bit RANGE changes the analog input configuration.
0 = The voltage difference between two selectable channels is converted. This
setting selects two's complement coding (see Table 10 on page 16 and
Table 11 on page 16).
2
MODE
1 = One of the eight input channels is referenced to COM. This setting also
selects binary coding.
Selects the AS1520/AS1521 operating mode:
PD1
PD0
Mode
0
0
1
1
0
1
0
1
Full power-down mode.
Reduced-power mode.
Reduced-power mode.
Normal operation.
1:0
PD1:PD0
Analog Input Configuration
Table 9. Analog Input Configuration
Analog Input Configuration
Mode Range
Coding
Comments
AIN+ from 0 to VREF.
8-Channel Single-Ended
1
1
1
1
1
0
Binary
Binary
Binary
COM should be tied to GND.
8-Channel Pseudo Differential
referenced to COM
AIN+ from COM to COM + VREF
8-Channel Pseudo Differential
referenced to COM
AIN+ from -VREF/2+COM to + VREF/2+COM
4-Channel Pseudo Differential
4-Channel Pseudo Differential
0
0
1
0
Two's Complement
Two's Complement
AIN+ - AIN- from 0 to VREF
AIN+ - AIN- from -VREF/2 to +VREF/2
AIN+ - AIN- from -VREF/2 to +VREF/2,
fully differential input signal.
4-Channel Fully Differential
0
0
Two's Complement
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