Datasheet

0x7F 0x5A1
MOSI
(host)
R/W Addr Data
0x7F 0x5A1
R/W Addr Data
0x7F 0x5A1
R/W Addr Data
XXX XX XXX XX XXX
MISO
(host)
XX
0x7F 0x5A1
R/W Addr Data
0x7F1 0x5A
0x01 0x220
MOSI
(host)
R/W Addr Data
(Device 3)
0x00 0xFF1
R/W Addr Data
(Device 2)
0x00 0x100
R/W Addr Data
(Device 1)
XXX XX XXX XX XXX
MISO
(host)
0x7F 0xFF1
R/W Addr Data
(Device 3)
0x7F 0xFF1
R/W Addr Data
(Device 2)
0x7F 0xFF1
R/W Addr Data
(Device 1)
0x01 0x220 0x00 0x881 0x00 0x100
48-bit SPI Transaction #1 48-bit SPI Transaction #2
XX
LMH0394
SNLS312L –AUGUST 2010–REVISED APRIL 2013 www.ti.com
SPI Daisy-Chain Read and Write Example
The following example further clarifies LMH0394 SPI daisy-chain operation. Assume a daisy-chain of three
LMH0394 devices (Device 1, Device 2, and Device 3), with Device 1 as the first device in the chain and Device 3
as the last device in the chain, as shown by the first three devices in Figure 7. Since there are three devices in
the daisy-chain, each SPI transaction is 48-bits long.
This example shows an SPI operation combining SPI reads and writes in order to accomplish the following three
tasks:
1. Write 0x22 to register 0x01 of Device 1 in order to set the output swing to 400 mV
P-P
.
2. Read the contents of register 0x00 of Device 2.
3. Write 0x10 to register 0x00 of Device 3 in order to force the sleep mode.
Figure 10 shows the two 48-bit SPI transactions required to complete these tasks (the bits are shifted in left to
right).
Figure 10. SPI Daisy-Chain Read and Write Example
The following occurs at the end of the first transaction:
1. Write 0x22 to register 0x01 of Device 1.
2. Latch the data from register 0x00 of Device 2.
3. Write 0x10 to register 0x00 of Device 3.
In the second transaction, three dummy reads (each consisting of 16 “1”s) are shifted in, and the read data from
Device 2 (with value 0x88) appears on MISO in the 25th through 32nd clock cycles.
SPI Daisy-Chain Length Detection
A useful operation for the host may be to detect the length of the daisy-chain. This is a simple matter of shifting
in a series of dummy reads with a known data value (such as 0x5A). For an SPI daisy-chain of N LMH0394
devices, the known data value will appear on the host's MISO pin after N+1 writes. Assuming a daisy-chain of
three LMH0394 devices, the result of this operation is shown in Figure 11.
Figure 11. SPI Daisy-Chain Length Detection
OUTPUT DRIVER ADJUSTMENTS AND DE-EMPHASIS SETTING
The output driver swing (amplitude), offset voltage (common mode voltage), and de-emphasis level are
adjustable via SPI register 01h.
The output swing is adjustable via bits [7:6] of SPI register 01h. The default value for these register bits is “10”
for a peak to peak differential output voltage of 700 mV
P-P
. The output swing can be set for 400 mV
P-P
,
600 mV
P‑P
, 700 mV
P-P
, or 800 mV
P-P
.
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