4
FN8194.3
October 12, 2006
The X9418 will respond with an acknowledge after
recognition of a start condition and its slave address
and once again after successful receipt of the
command byte. If the command is followed by a data
byte the X9418 will respond with a final acknowledge.
Array Description
The X9418 is comprised of two resistor arrays. Each
array contains 63 discrete resistive segments that are
connected in series. The physical ends of each array
are equivalent to the fixed terminals of a mechanical
potentiometer (V
H
/R
H
and V
L
/R
L
inputs).
At both ends of each array and between each resistor
segment is a CMOS switch connected to the wiper
(V
W
/R
W
) output. Within each individual array only one
switch may be turned on at a time. These switches are
controlled by the Wiper Counter Register (WCR). The
six bits of the WCR are decoded to select, and enable,
one of sixty-four switches.
The WCR may be written directly, or it can be changed
by transferring the contents of one of four associated
Data Registers into the WCR. These Data Registers
and the WCR can be read and written by the host
system.
Device Addressing
Following a start condition the master must output the
address of the slave it is accessing. The most
significant four bits of the slave address are the device
type identifier (refer to Figure 1 below). For the X9418
this is fixed as 0101[B].
Figure 1. Slave Address
The next four bits of the slave address are the device
address. The physical device address is defined by
the state of the A
0
- A
3
inputs. The X9418 compares
the serial data stream with the address input state; a
successful compare of all four address bits is required
for the X9418 to respond with an acknowledge. The
A
0
- A
3
inputs can be actively driven by CMOS input
signals or tied to V
CC
or V
SS
.
Acknowledge Polling
The disabling of the inputs, during the internal
nonvolatile write operation, can be used to take
advantage of the typical 5ms EEPROM write cycle
time. Once the stop condition is issued to indicate the
end of the nonvolatile write command the X9418
initiates the internal write cycle. ACK polling can be
initiated immediately. This involves issuing the start
condition followed by the device slave address. If the
X9418 is still busy with the write operation no ACK will
be returned. If the X9418 has completed the write
operation an ACK will be returned, and the master can
then proceed with the next operation.
Flow 1. ACK Polling Sequence
Instruction Structure
The next byte sent to the X9418 contains the instruction
and register pointer information. The four most
significant bits are the instruction. The next four bits
point to one of the two pots and when applicable they
point to one of four associated registers. The format is
shown Figure 2.
100
A3 A2 A1 A0
Device Type
Identifier
Device Address
1
Nonvolatile Write
Command Completed
Enter ACK Polling
Issue
START
Issue Slave
Address
ACK
Returned?
Further
Operation?
Issue
Instruction
Issue STOP
NO
YES
YES
Proceed
Issue STOP
NO
Proceed
X9418
