ACD82124 Datasheet, PDF (12/48 Pages) List of Unclassifed Manufacturers – 24 Ports 10/100 Fast Ethernet Switch Controller

Prior to any transaction, the ACD82124 will output
thirty-two bits of ‘1’ as a preamble signal. After the
preamble, a ‘01’ signal is used to indicate the start of
the frame.
For a write operation, the device will send a ‘01’ to
signal a write operation. Following the ‘01’ write signal
will be the 5 bit ID address of the PHY device and the
5 bit register address. A ‘10’ turn around signal is then
followed. After the turn around, the 16 bit of data will
be written into the register. After the completion of the
write transaction, the line will be left in a high imped-
ance state.
For a read operation, the ACD82124 will output a ‘10’
to indicate read operation after the start of frame indi-
cator. Following the ‘10’ read signal will be the 5-bit ID
address of the PHY device and the 5-bit register ad-
dress. Then, the ACD82124 will cease driving the MDIO
line, and wait for one BT. During this time, the MDIO
should be in a high impedance state. The ACD82124
will then synchronize with the next bit of ‘0’ driven by
the PHY device, and continue on to read 16 bits of
data from the PHY device.
The system designer should set the ID of the PHY
devices as ‘1’ for port-0, ‘2’ for port-1, … and ‘24’ for
port-23. The detail timing requirement on PHY man-
agement signals are described in the chapter of “Tim-
ing Description.”
CPU Interface
The ACD82124 includes a CPU interface to enable an
external CPU to access the internal registers of the
ACD82124. The protocol used in the CPU is the asyn-
chronous serial signal (UART). The baud rate can be
from 1200 bps to 76800 bps. The ACD82124 auto-
matically detects the baud rate for each command,
and returns the result at the same baud rate. The sig-
nals in CPU interface are described in Table-6.5.
Table-6.5: CPU Interface Signals
Name
Type
Description
CPUDI
I
CPU data input
CPUDO
O
CPU data output
CPUIRQ
O
CPU interrupt request
A command sent by CPU comes through the CPUDI
line. The command consists of 9 octets. Command
frames transmitted on CPUDI have the following for-
mat (Table-6.6):
Table-6.6: CPU Command Format
Operation Command Register Index Data Checksum
Write 0010XX11 8-bit 8-bit 24-bit 8-bit
Read 0010XX01 8-bit 8-bit 24-bit 8-bit
The byte order of data in all fields follows the big-endian
convention, i.e. most significant octet first. The bit or-
der is least significant order first. The Command octet
specifies the type of the operation. Bit 2 and bit 3 of
the command octet is used to specify the device ID of
the chip. They are set by bit 16 and bit 17 of the Reg-
ister 25 at power on strobing. The address octet speci-
fies the type of the register. The index octet specifies
the ID of the register in a register array. For write
operation, the Data field is a 4-octet value to specify
what to write into the register. For read operation, the
Data field is a 4-octet 0 as padded data. The checksum
value is an 8-bit value of exclusive-OR of all octets in
the frame, starting from the Command octet.
The ACD82124 will respond to each valid command
received by sending a response frame through the
CPUDO line. The response frames have the following
format (Table-6.7):
Table-6.7: Response Format
Response Command Result
Write
00100011 8-bit
Read
00100001 8-bit
Data
24-bit
24-bit
Checksum
8-bit
8-bit
The command octet specifies the type of the response.
The result octet specifies the result of the execution.
The Result field in a response frame is defined as:
• 00 for no error
• 01 for Checksum
• 10 for address incorrect
• 11 for MDIO waiting time-out
For response to a read operation, the Data field is a 3-
octet value to indicate the content of the register. For
response to a write operation, the Data field is 24 bits
of 0. The checksum value is an 8-bit value of exclu-
sive-OR of all octets in the response frame, starting
from the Command octet.
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