ADXL180
Rev. 0 | Page 15 of 56
COMMUNICATIONS TIMING AND BUS TOPOLOGIES
ASYNCHRONOUS COMMUNICATION
LOOP CURRENT
I
I
MOD
IDLE
t
M
**
t
M
**
TIME
ADC SAMPLE
t
ADC
t
P
*
4-
03
0
07
54
t
P
*
*
t
P
=
t
DD
DURING PHASE 2, MODE 0
**
t
M
=
t
CLK
TIMES THE NUMBER OF BITS TRANSMITTED
DATA FRAME
DATA FRAME
Figure 10. Asynchronous Mode Data Transmission Timing
The ADXL180 data transmissions in their default mode run
asynchronous to the control module. In this mode, the ADXL180
timing is entirely based on the internal clock of the device. After
the initialization phases are complete, the ADXL180 begins to
transmit sensor data every 228 μs. The device transmits sensor
data until the supply voltage falls below the required minimum
operating level. If an internal error is detected, the device trans-
mits the appropriate error code until the supply voltage falls
below the required minimum operating level.
Asynchronous Single Device Point-to-Point Topology
A single device is wired in the point-to-point configuration
as shown in Figure 11. This configuration must be used in
asynchronous mode. Do not use two asynchronous devices on
one bus because communications errors are very likely to occur.
CENTER
MODULE
V
BP
DEVICE 1
NC
NC
V
BN
V
BN
V
BC
07
54
4
-03
1
Figure 11. Asynchronous Point-To-Point Topology
SYNCHRONOUS COMMUNICATION
The ADXL180 data transmission can be synchronized to the
control module. This synchronization is accomplished by the
control module generating a synchronization pulse to the
ADXL180. The synchronization pulse is a voltage pulse that
is superimposed on the supply voltage by the center module.
Figure 12 shows the synchronization pulse timing. Upon detecting
a synchronization pulse, the ADXL180 transmits its data.
Configuring the ADXL180 for Synchronous Operation
Table 7. Sync Enable (SYEN) Options
SYEN Definition
0
Synchronization pulse disabled. The device transmits
data every 228 μs based on the internal clock of the
device. Data is transmitted according to an internal state
machine sequence when powered on (default).
1
Synchronization pulse enabled. The device requires a
synchronization pulse to sample and transmit data. Data
transmission is in accordance with the internal state
machine of the device.
The user-defined SYEN bit determines whether the device is
used in synchronous operation or remains asynchronous.
SYEN, as shown in Table 7, must be set to SYEN = 1 to enable
synchronous operation.
Synchronization Pulse Detection
The ADXL180 uses a digital integration method to validate the
synchronization pulse. The ADXL180 detects the supply voltage
(V
BP
) rising above the level of V
SPT
. The state of the level detection
circuit controls the count direction of an up-down counter. The
counter is clocked every 1 μs. The counter is incremented if the
ADXL180 detects a level exceeding V
SPT
. The counter is decre-
mented if the ADXL180 detects a level below V
SPND
. Operation
is not defined between these thresholds. If the synchronization
pulse is fully below V
SPND
, the pulse is rejected and not detected.
The counter saturates at zero. The synchronization pulse is con-
sidered valid on the next clock after the counter is incremented
to seven counts. The counter is gated off (blanked) after a valid
synchronization pulse is detected. Once the sync pulse has been
recognized as valid, a command is issued to start the acceleration
data analog-to-digital conversion. The ADC does not run conti-
nuously in synchronous mode, ensuring that only the acceleration
data present at the time of the sync pulse is output from the device.
