AN637
Si890
X
M
A S TER
C
O N T R O L L E R
R
ECOMMENDATIONS
1. Introduction
The Si890x isolated 10-bit monitoring ADCs integrate a 10-bit SAR ADC subsystem, supervisory state machine
and isolated UART (Si8900), I
2
C/SMbus port (Si8901), or SPI Port (Si8902) into a single 16 SOW package. Based
on Silicon Labs' proprietary CMOS isolation technology, the Si890x series offers isolation ratings of 2.5 or 5.0 kV
and finds use across a wide range of low-frequency analog data acquisition applications including isolated ac line
monitors, sensor interface circuits, power-delivery systems, and solar inverters. Si890x devices operate in slave
mode and must be managed by an external master controller (typically an MCU). This application note provides
useful recommendations for configuring the user's master controller for operation with the Si890x including serial
port resynchronization and recovery from loss of VDD.
2. Si890x Overview
VDDA
AIN0
AIN1
AIN2
VREF
ADC Subsystem
MUX
PGA
VREF
D3[CNFG_0]
VDDB
Tx
Data
Rx
Data
To All
Blocks
State Machine/
User Registers
10‐Bit
ADC
Serial
Port
Serial I/O
To User’s
Master
VREF
ISOLATION
GNDB
/RST
GNDB
Si890x
Figure 1. Si890x Generic Block Diagram
Figure 1 shows a generic block diagram representative of any member of the Si890x family. The Si890x family
requires input and output VDD supplies that are isolated from each other. The input-side supply (VDDA) provides
bias to the ADC subsystem, state machine, serial port, and the input side of the digital isolator. The VDDB supply
powers the isolator output circuits and typically powers the user's master as well. The user's master controller
sends a serial command byte to the Si890x that specifies the ADC operating mode, MUX channel address, and
voltage reference source. Upon receipt of the command byte, the Si890x executes the specified ADC
conversion(s) and makes conversion data available for the master to read at the isolated serial port interface. An
understanding of the integrated CMOS digital isolator operation is necessary to understanding the overall Si890x
operation. For detailed information on Si890x operation, see the Si890x data sheet available for download at
www.silabs.com/isolation.
Rev. 0.1 10/11
Copyright © 2011 by Silicon Laboratories
AN637
AN637
2.1. Integrated Galvanic Isolation
The CMOS isolation technology offers uniform, stable performance over operating voltage and temperature with
none of the fundamental weaknesses of optocouplers. A block diagram of a CMOS digital isolator channel is
shown in Figure 2.
Bondwire
INPUT
PIN
CMOS Digital Isolator
ISOLATOR DIE
RF
ISOLATION
BARRIER
RF
ISOLATION
BARRIER
Bondwire
OUTPUT
PIN
ISOLATOR DIE
RECEIVER
TRANSMITTER
RECEIVER
TRANSMITTER
Bondwire
Input Pin
Transmitter Output
Receiver Input
Output Pin
Figure 2. CMOS Isolator Block Diagram
The operation of the digital isolator is analogous to that of an optocoupler, except a high-frequency carrier is used
instead of light. An isolator channel consists of two identical semiconductor dies in a standard IC package where
each die contains both transmit and receive circuits. When they are wire-bonded together, the two dies form a high-
frequency transmitter and receiver separated by a differential capacitive isolation barrier. Input/output data is
transferred using simple on/off keying (OOK) modulation; when an input pin (VIN) is high, the transmitter generates
a carrier that propagates across the isolation barrier to the receiver. The receiver asserts logic 1 on the output pin
(VOUT) when sufficient in-band carrier energy is detected. When VIN is low, the transmitter is off, and the receiver
drives VOUT low because no in-band RF energy is detected. Refer to Table 1 to determine outputs when one or
both VDD supplies are not present. For more information, see the Si8660/1/2/3 data sheet for the CMOS digital
isolator.
RF ENERGY
RF ENERGY
2
Rev. 0.1
AN637
Table 1. CMOS Digital Isolator Operation
Input
1,2
H
L
X
X
V
I
VDDI
State
1,3,4
P
P
UP
P
VDDO
State
1,3,4
P
P
P
UP
V
O
Output
1,2
H
L
L
H
Undetermined
Normal operation.
Upon transition of VDDI from unpowered to powered, V
O
returns to the same state as V
I
in less than 1 µs.
Upon transition of VDDO from unpowered to powered, V
O
returns to the same state as V
I
within 1 µs.
Comments
Notes:
1.
VDDI and VDDO are the input and output power supplies. V
I
and V
O
are the respective input and output terminals.
2.
X = not applicable; H = Logic High; L = Logic Low; Hi-Z = High Impedance.
3.
“Powered” state (P) is defined as 2.5 V < VDD < 5.5 V.
4.
“Unpowered” state (UP) is defined as VDD = 0 V.
Rev. 0.1
3
AN637
3. Si8900 Auto Baud Master Controller Algorithm
Figure 3 shows the master/Si8900 adaptive automatic baud rate detection (“auto baud”). The Si8900 baud rate
detection algorithm uses the master's UART baud rate to determine the upper and lower baud rate boundaries and
centers the operating baud rate between these two limits. Adaptive baud rate detection begins with the Si8900
setting its baud rate to its maximum value of 520 kbps and waiting for the master to send a stream of bytes (each
having a hex value of 0xAA) to the Si8900. The Si8900 compares each received byte to 0xAA and decreases the
baud rate when the received byte is other than 0xAA. This process continues until 0xAA is successfully received by
the Si8900, at which time the upper baud rate limit value is stored in Si8900 memory.
Master
Start
Si8900 (Slave)
Maximum
BR Setting
Serial Hex Byte (0xAA) Transmission from Master
Reset
Clear Registers,
Start Over
Baud Rate
Invalid
Upper BR
Limit Setting
No Data
Upper BR
Detect
Transmit
=0xAA
?0xAA
Re‐
Transmit
Process
Complete
Validate
BR
Baud Rate
Valid
Center BR
Lower BR
Limit Setting
=0xAA
Decrease
Baud Rate
? 0x55,
0x55
Receive
=0xAA
Exit
Process
=0x55,
0x55
Store
Final Setting
Minimum
BR Setting
Lower BR
Detect
No Data
Terminate
Process
No Data
?0xAA
Exit
Process
Increase
Baud Rate
Figure 3. Si8900 Auto Baud Process Functional Flow Diagram
The Si8900 then discovers its lower baud rate limit by setting the baud rate to its minimum value of 58 kbps and
again comparing each incoming byte to 0xAA, increasing the baud rate until 0xAA is successfully received and
stored in Si8900 memory. The Si8900 then optimizes the baud rate by centering the operating baud rate half-way
between the upper and lower baud rate limits. The Si8900 then terminates the adaptive baud rate sequence by
transmitting two sequential 0x55 bytes to the master. Master controller C-code firmware examples for Figure 3 are
available in the application note, “AN635: Si8900 Automatic Baud Rate Detection” available for download at
www.silabs.com/isolation.
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Rev. 0.1
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3.1. Si8900 Communication Recovery after VDD Failure
A VDD power failure after the adaptive baud rate process has been completed will halt communication between the
Si8900 and the master. Referring to Table 2, loss of VDDB deprives the isolator output driver of bias voltage,
causing the Tx and Rx pins to enter an “undetermined” state. That is, Tx and Rx may fall to zero or be pulled to
arbitrary voltage by local parasitic coupling. (Tx and Rx can be made to be deterministic by attaching resistors from
Tx and Rx to either ground or VDD.) Because VDDA is uninterrupted, the Si8900 memory still contains the
previously calculated, valid baud rate setting. Consequently, the master need only resume communication with the
Si8900 after VDDB is restored. Note that the auto baud algorithm should not be invoked because the 0xAA byte
will be rejected by the Si8900 as an invalid command byte as input-side power was not interrupted. Loss of VDDA
disables the ADC subsystem, state machine, and memory, serial port and input-side isolator bias voltage. In this
case, the previously calculated baud rate setting is lost; so, the master must invoke the auto baud synchronization
cycle after VDDA recovers.
Table 2. Si8900 Power Fail Behavior
VDDA
Supply
Powered
Powered
Unpowered
Unpowered
VDDB
Supply
Powered
Unpowered
Powered
Unpowered
ADC Status
Enabled
Enabled
Disabled
Disabled
State Machine/
Memory
Enabled
Enabled
Disabled
Disabled
Si8900 Rx
Enabled
Undetermined
Low
Undetermined
Si8900 Tx
Enabled
Undetermined
High
Undetermined
Invoke Auto
Baud?
Yes
No
Yes
N/A
Note that all Si890x devices return every command byte they receive to the master for verification. For more
information, see “AN635: Si8900 Automatic Baud Rate Detection”, which contains master controller code
examples for verifying returned Si8900 commands against the master's original transmission. If the master
receives a returned command that differs from its original transmission, the Si8900 baud rate value is likely
incorrect. This can be confirmed by having the master send the same command byte to the Si8900 three to five
consecutive times and invoke auto baud for re-synchronization if the results are consistently incorrect. Note that if
the Si8900 loses power, the master's command transmissions will not be returned to the Si8900, thus providing the
master a way to distinguish between an incorrect baud rate and an unpowered Si8900 slave.
3.2. Si8901 and Si8902 Master Controller Recommendations
Unlike the Si8900 UART (asynchronous) communication port, the Si8901 I
2
C port is a synchronous port that is
clocked by the master controller. This port uses an ACK/NAK (acknowledge/not acknowledge) scheme to indicate
a successful or unsuccessful data transfer between the Si8901 and the master, respectively. Upon receipt of a
NAK, the master should re-transmit the CNFG_0 command byte to the Si8901 to re-establish communication and
should receive a copy of this byte back from the Si8901. Should the Si8901 repeatedly fail to return a copy of the
master's Si8901 CNFG_0 command byte, the master can either continue trying to establish communication or
service other unrelated tasks and return later to attempt communication. Loss of Si8901 input or output-side VDD
results in the I
2
C bus lines being pulled high by the bus pull-up resistors (see Table 3). For more information about
the operation of the Si8901 I
2
C port, see the Si890x data sheet available at
www.silabs.com/isolation.
Rev. 0.1
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