Rainbow Electronics DS3904 - Manual (Page 7)

DS3904
DS3904 Triple 128-Position Nonvolatile
Variable Digital Resistor/Switch
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Detailed Description
The DS3904 contains three, 128-position, NV, low tem-
perature coefficient variable digital resistors. It is con-
trolled through a 2-wire serial interface, and it serves as
a low-cost replacement for designs using conventional
trimming resistors. Furthermore, the address pin allows
two DS3904s to be placed on the same 2-wire bus.
With its low cost and small size, the DS3904 is well tai-
lored to replace larger mechanical trimming variable
resistors. This allows the automation of calibration in
many instances because the 2-wire interface can easily
be adjusted by test/production equipment.
Variable Resistor
Memory Organization
The variable resistors of the DS3904 are addressed by
communicating with the registers in Table 1.
*Writing a value greater than 7Fh to any of the resistor registers
sets the high-impedance mode control bit (RHIZ, the MSB of
the resistor register) resulting in the resistor going into high-
impedance mode.
Device Operation
Clock and Data Transitions
The SDA pin is normally pulled high with an external
resistor or device. Data on the SDA pin can only change
during SCL low time periods. Data changes during SCL
high periods indicates a start or stop condition depend-
ing on the conditions discussed below. See the timing
diagrams for further details (Figures 2 and 3).
Start Condition
A high-to-low transition of SDA with SCL high is a start
condition, which must precede any other command. See
the timing diagrams for further details (Figures 2 and 3).
Stop Condition
A low-to-high transition of SDA with SCL high is a stop
condition. After a read or write sequence, the stop
command places the DS3904 into a low-power mode.
See the timing diagrams for further details (Figures 2
and 3).
Acknowledge
All address and data bytes are transmitted through a
serial protocol. The DS3904 pulls the SDA line low dur-
ing the ninth clock pulse to acknowledge that it has
received each byte.
Pin Description
PIN NAME FUNCTION
1 SDA
2-Wire Serial Data. Open-drain
input/output for 2-wire data.
2 SCL
2-Wire Serial Clock. Input for 2-wire
clock.
3 V CC Supply Voltage Terminal
4 GND Ground Terminal
5, 6, 7 H2, H1, H0 Resistor High Terminals
8 A0 Address-Select Input
2-WIRE
INTERFACE
RHIZ CONTROL
EEPROM
RES 0
20kΩ
H0
F8h
MSB 7 LSB
DATA
GND
SCL
SDA
A0
V CC
V CC
DS3904
RESISTOR 0
RHIZ CONTROL
RES 1
20kΩ
H1
F9h
MSB LSB
RESISTOR 1
RHIZ CONTROL
RES 2
20kΩ
H2
FAh
MSB LSB
RESISTOR 2
7
7
Figure 1. DS3904 Block Diagram
ADDRESS
VARIABLE
RESISTOR POSITION 7Fh
RESISTANCE
NUMBER OF
POSITIONS*
F8h Resistor 0 20k
Ω
128 (00h to
7Fh) + Hi-Z
F9h Resistor 1 20k
Ω
128 (00h to
7Fh) + Hi-Z
FAh Resistor 2 20k Ω
128 (00h to
7Fh) + Hi-Z
Table 1. Variable Resistor Registers