Rainbow Electronics DS1667 - Manual (Page 3)

DS1667
021492 3/11
Information is written to and read from the wiper 0 and
wiper 1 registers and the stack select bit via the 17-bit
I/O shift register. The I/O shift register is serially loaded
by a 3 wire serial port consisting of RST, DQ, and CLK. It
is updated by transferring all 17 bits (Figure 2). Data can
be entered into the 17 bit shift register only when the
RST input is at a high level. While at a high level, the
RST function allows serial entry of data via the D/Q pin.
The potentiometers always maintain their previous val-
ue until RST is taken to a low level, which terminates
data transfer. While RST input is low, the DQ and CLK
inputs are ignored.
Valid data is entered into the I/O shift register while RST
is high on the low-to-high transition of the CLK input.
Data input on the DQ pin can be changed while the clock
input is high or low, but only data meeting the setup re-
quirements will enter the shift register. Data is always
entered starting with the value of the stack select bit.
The next 8 bits to be entered are those specifying the
wiper 1 setting. The MSB of these 8 bits is sent first. The
next 8 bits to be entered are those specifying the wiper 0
setting, sent MSB first. The 17th bit to be entered, there-
fore, will be the least significant bit of the wiper 0 setting.
If fewer than 17 bits are entered, the value of the poten-
tiometer settings will result from the number of bits that
were entered plus the remaining bits of the old value
shifted over by the number of bits sent. If more than 17
bits are sent, only the last 17 bits are left in the shift regis-
ter. Therefore, sending other than 17 bits can produce
indeterminate potentiometer settings.
As bits are entered into the shift register, the previous
value is shifted out bit by bit on the cascade serial port
pin (COUT). By connecting the COUT pin to the DQ pin
of a second DS1667, multiple devices can be daisy
chained together as shown in Figure 3.
When connecting multiple devices, the total number of
bits sent is always 17 times the number of DS1667s in
the daisy chain. In applications where it is desirable to
read the settings of potentiometers, the COUT pin of the
last device connected in a daisy chain must be con-
nected back to the DQ input of the first device through a
resistor with a value of 1K to 10K. This resistor provides
isolation between COUT and DQ when writing to the de-
vice (see Figure 3).
When reading data, the DQ line is left floating by the
reading device. When RST is held low, bit 17 is always
present on the COUT pin, which is fed back to the input
DQ pin through the resistor (see Figure 4).This data bit
can now be read by the reading device. The RST pin is
then transitioned high to initiate a data transfer. When
the CLK input transitions low to high, bit 17 is loaded into
the first position of the I/O shift register and bit 16 be-
comes present on COUT and DQ. After 17 bits (or 17
times the number of devices for a daisy chain), the data
has shifted completely around and back to its original
position. When RST is transitioned back low to end data
transfer, the value (the same as before the read oc-
curred) is loaded into the wiper 0 and wiper 1 registers
and the stack select bit.
When power is applied to the DS1667, the device al-
ways has the wiper settings at half position and the
stack select bit is at zero.