XRT8001IP datasheet(8/48 Pages) EXAR

8 / 48 page

XRT8001

Rev. 1.01

Bits 2 Through 5: The Four (4) Bit Address Values

(Labeled A0, A1, A2 and A3)

The next four rising edges of the SCLK signal will clock

in the 4-bit address value for this particular Read (or

Write) operation. The address selects the Command

be reading data from, or writing data to. The user must

supply the address bits to the SDI input pin in ascend-

ing order with the LSB (least significant bit) first.

Bits 6 and 7:

The next two bits, A4 and A5, must be set to “0”, as

shown in Figure 19.

Bit 8 - A6

The value of “A6” is a “don’t care”. Once these first

eight bits have been written into the Microprocessor

Serial Interface, the subsequent action depends upon

whether the current operation is a “Read” or “Write”

operation.

Read Operation

Once the last address bit (A3) has been clocked into

the SDI input, the “Read” operation will proceed through

an idle period, lasting three SCLK periods. On the

falling edge of SCLK Cycle #8 (see Figure 19) the serial

data output signal (SDO) becomes active. At this point

the user can begin reading the data contents of the

addressed Command Register (at Address [A3, A2,

A1, A0]) via the SDO output pin. The Microprocessor

Serial Interface will output this 5-bit data word (D0

through D4) in ascending order (with the LSB first), on

the falling edges of the SCLK pin. As a consequence,

the data (on the SDO output pin) will be sufficiently

stable for reading (by the Microprocessor), on the very

next rising edge of the SCLK pin.

1.0 Operating the Microprocessor Serial

Interface

The XRT8001 Serial Interface is a simple four-wire

interface that is compatible with many of the

microcontrollers available in the market. This interface

consists of the following signals:

CSB

Chip Select (Active Low)

SCLK

Serial Clock

SDI

Serial Data Input

SDO

Serial Data Output

Using the Microprocessor Serial Interface

The following instructions, for using the Micropro-

cessor Serial Interface, are best understood by

referring to the diagram in Figure 19.

In order to use the Microprocessor Serial Interface

the user must first provide a clock signal to the

SCLK input pin. Afterwards, the user will initiate a

“Read” or “Write” operation by asserting the “active-

low” Chip Select input pin (CSB). It is important to

assert the CSB pin (e.g., toggle it “low”) at least

50ns prior to the very first rising edge of the clock

signal.

Once the CSB input pin has been asserted, the type of

operation and the target register address must now be

specified by the user. The user provides this informa-

tion to the Microprocessor Serial Interface by writing

eight serial bits of data into the SDI input. Note: each

of these bits will be “clocked” into the SDI input on the

rising edge of SCLK. These eight bits are identified and

described below.

Bit 1 - “R/W” (Read/Write) Bit

This bit will be clocked into the SDI input on the first

rising edge of SCLK (after CSB has been asserted).

This bit indicates whether the current operation is a

“Read” or “Write” operation. A “1” in this bit specifies

a “Read” operation; whereas, a “0” in this bit specifies

a “Write” operation.

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