AD9142ABCPZ datasheet(31/73 Pages) AD

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AD9142A

Data Sheet

RESETTING THE FIFO

Upon power-on of the device, the read and write pointers start

to roll around the FIFO from an arbitrary slot; consequently, the

FIFO depth is unknown. To avoid a concurrent read and write

to the same FIFO address and to assure a fixed pipeline delay

from power-on to power-on, it is important to reset the FIFO

pointers to a known state each time the device powers on or

wakes up. This state is specified in the requested FIFO level

(FIFO depth and FIFO level are used interchangeably in this

data sheet), which consists of two sections: the integer FIFO

level and the fractional FIFO level.

The integer FIFO level represents the difference of the states

between the read and write points in the unit of an input data

period (1/fDATA). The fractional FIFO level represents the

difference of the FIFO pointers that is smaller than the input

data period. The resolution of the fractional FIFO level is the

input data period divided by the interpolation ratio and, thus, it

is equal to one DACCLK cycle.

The exact FIFO level, that is, the FIFO latency, can be calculated

FIFO Latency = Integer Level + Fractional Level

Because the FIFO has eight data slots, there are eight possible

FIFO integer levels. The maximum supported interpolation rate

in the AD9142A is 8× interpolation. Therefore, there are eight

possible FIFO fractional levels. Two 3-bit registers in Register 0x23

are assigned to represent the two FIFO levels, as follows:

•

Bits[6:4] represent the FIFO integer level

•

Bits[2:0] represent the FIFO fractional level.

For example, if the interpolation rate is 4× and the total FIFO

depth is 4.5 input data periods, set the FIFO_LEVEL_CONFIG

(Register 0x23) to 0x42 (4 here means four data cycles and 2

means two DAC cycles, which is half of a data cycle). Note that

there are only four possible fractional levels in the case of 4×

interpolation. Table 17 shows additional examples of configuring

the FIFO level in various interpolation rate modes.

Table 17. Examples of FIFO Level Configuration

Interpolation

Rate

Example FIFO

Level (1/fDATA)

Integer Level

(Reg. 0x23[6:4])

Fractional Level

(Reg. 0x23[2:0])

2×

3 + 1/2

4×

4 + 1/4

8×

4 + 3/8

By default, the FIFO level is 4.0. It can be programmed to any

allowed value from 0.0 to 7.x. The maximum allowed number

for x is the interpolation rate minus 1. For example, in 8×

interpolation, the maximum value allowed for x is 7.

The following two ways are used to reset the FIFO and initialize

the FIFO level:

•

Serial port (SPI) initiated FIFO reset.

•

Frame initiated FIFO reset.

SERIAL PORT INITIATED FIFO RESET

A SPI initiated FIFO reset is the most common method to reset

the FIFO. To initialize the FIFO level through the serial port,

toggle FIFO_SPI_RESET_REQUEST (Register 0x25[0]) from

0 to 1 and back to 0. When the write to this register is complete,

the FIFO level is initialized to the requested FIFO level and the

readback of FIFO_SPI_RESET_ACK (Register 0x25[1]) is set

to 1. The FIFO level readback, in the same format as the FIFO

level request, should be within ±1 DACCLK cycle of the

requested level. For example, if the requested value is 0x40 in

4× interpolation, the readback value should be one of the

following: 0x33, 0x40, or 0x41. The range of ±1 DACCLK cycle

indicates the default DAC latency uncertainty from power-on

to power-on without turning on synchronization.

The recommended procedure for a serial port FIFO reset is as

follows:

1. Configure the DAC in the desired interpolation mode

(Register 0x28[1:0]).

2. Ensure that the DACCLK and DCI are running and stable

at the clock inputs.

3. Program Register 0x23 to the customized value, if the

desired value is not 0x40.

4. Request the FIFO level reset by setting Register 0x25[0] to 1.

5. Verify that the device acknowledges the request by setting

6. Remove the request by setting Register 0x25[0] to 0.

7. Verify that the device drops the acknowledge signal by

setting Register 0x25[1] to 0.

8. Read back Register 0x24 multiple times to verify that the

actual FIFO level is set to the requested level and that the

readback values are stable. By design, the readback is

within ±1 DACCLK around the requested level.

FRAME INITIATED FIFO RESET

The frame input has two functions. One function is to indicate

the beginning of a byte stream in the byte interface mode, as

discussed in the Data Interface section. The other function is

to initialize the FIFO level by asserting the frame signal high

for at least the time interval required to load complete data to

the I and Q DACs. This corresponds to one DCI period in word

interface mode and two DCI periods in byte interface mode.

Note that this requirement of the frame pulse length is longer

than that of the frame signal when it serves only to assemble the

byte stream. The device accepts either a continuous frame or a

one shot frame signal.

In the continuous reset mode, the FIFO responds to every valid

frame pulse and resets itself. In the one shot reset mode, the

FIFO responds only to the first valid frame pulse after the

FRAME_RESET_MODE bits (Register 0x22[1:0]) are set.

Therefore, even with a continuous frame input, the FIFO resets

one time only; this prevents the FIFO from toggling between

the two states from periodic resets. The one shot frame reset

mode is the default and the recommended mode.

Rev. A | Page 30 of 72

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