Cirrus Logic CDB5374 - Manual (Page 28)

CDB5374
28 DS862DB1
2.3.6 RS-485 Telemetry
By default, CDB5374 communicates with the PC evaluation software through the microcontroller USB
port. Additional hardware is designed onto CDB5374 to use the microcontroller I
2
C
®
port as a low-level
local telemetry, but it is provided for custom programming convenience only and is not directly supported
by the CDB5374 PC evaluation software or microcontroller firmware.
Telemetry signals enter CDB5374 through RS-485 transceivers, which are differential current mode trans-
ceivers that can reliably drive long distance communication. Data passes through the RS-485 transceiv-
ers to the microcontroller I
2
C interface and the clock and synchronization inputs.
2.3.6.1 CLK, SYNC
Clock and synchronization telemetry signals into CDB5374 are received through RS-485 twisted pairs.
These signals are required to be distributed through the external system with minimal jitter and timing
skew, and so are normally driven through high-speed bus connections.
Synchronization of the measurement channel is critical to ensure simultaneous analog sampling across
a network. Several options are available for connecting a SYNC signal through the RS-485 telemetry to
the digital filter.
A direct connection is made when the SYNC_IO signal is received over the dedicated RS-485 twisted pair
and sent directly to the digital filter SYNC pin through jumper J56. The incoming SYNC_IO signal must be
synchronized to the network at the transmitter since no local timing adjustment is available.
A microcontroller hardware connection is made when the SYNC_IO signal is received over the dedicated
RS-485 twisted pair and detected by a microcontroller interrupt. The microcontroller can then use an in-
ternal counter to re-time the SYNC_MC signal output to the digital filter SYNC input as required.
Specification Value
RS-485 Transceiver - Linear Tech
LTC1480IS8
Surface Mount Package Type SOIC-8, 5mm x 6mm
Supply Voltage, Quiescent Current 3.3V, 600
µA
Maximum Data Rate 2.5 Mbps
Transmitter Delay, Receiver Delay 25 - 80 ns, 30 - 200 ns
Transmitter Current, Full Termination (60
Ω)
25 mA
Transmitter Current, Half Termination (120
Ω)
13 mA
Specification Value
Synchronous Inputs, 2 wires each
CLK±, SYNC±
Specification Value
Distributed SYNC Signal Synchronization
± 240 ns
Distributed Clock Synchronization ± 240 ns
Analog Sampling Synchronization Accuracy ± 480 ns