AD7699

Rev. 0 | Page 24 of 28

GENERAL TIMING WITH A BUSY INDICATOR

Figure 38 details the timing for all three modes: reading/writing

during conversion, after conversion, and spanning conversion.

Note that the gating item for both CFG and data readback is at

the end of conversion (EOC). As detailed previously, the data

If the full CFG word is not written to prior to EOC, it is

discarded and the current configuration remains.

At the EOC, if CNV is low, the busy indicator is enabled. In

addition, to generate the busy indicator properly, the host must

assert a minimum of 17 SCK falling edges to return SDO to

high impedance because the last bit of data on SDO remains

active. Unlike the case detailed in the General Timing Without

a Busy Indicator section, if the conversion result is not read out

fully prior to EOC, the last bit clocked out remains. If this bit is

low, the busy signal indicator cannot be generated because the

digital output requires a high impedance, or a bit remaining high,

to low transition for the interrupt input of the host. A good

example of this occurs when an SPI host sends 16 SCKs because

these are usually limited to 8-bit or 16-bit bursts, thus the LSB

remains. Because the transition noise of the AD7699 is 4 LSBs

peak to peak (or greater), the LSB is low 50% of the time. For

this interface, the SPI host needs to burst 24 SCKs, or a QSPI

interface can be used and programmed for 17 SCKs.

The SCK can idle high or low depending on the CPOL and

CPHA settings if SPI is used. A simple solution is to use CPOL

= CPHA = 1 (not shown) with SCK idling high.

POWER

UP

END OF CONVERSION (EOC)

START OF CONVERSION

EOC

EOC

ACQUISITION

(n –1)

ACQUISITION

(n)

ACQUISITION

(n + 1)

ACQUISITION

(n + 2)

CONVERSION (n – 1)

CONVERSION (n)

CONVERSION (n + 1)

CONVERSION (n – 2)

PHASE

CNV

CNV

CNV

DIN

SDO

DIN

SDO

DIN

SDO

SCK

SCK

SCK

NOTES

1. CNV MUST BE HIGH PRIOR TO THE END OF CONVERSION (EOC) TO AVOID THE BUSY INDICATOR.

A TOTAL OF 17 SCK FALLING EDGES IS REQUIRED TO RETURN SDO TO HIGH-Z. IF CFG READBACK

IS ENABLED, A TOTAL OF 31 SCK FALLING EDGES IS REQUIRED TO RETURN SDO TO HIGH-Z.

DATA (n – 2)

DATA

(n – 2)

DATA

(n – 1)

DATA

(n – 1)

DATA (n)

DATA (n)

DATA (n + 1)

CFG (n)

CFG (n + 2)

CFG (n + 1)

CFG (n + 3)

XXX

DATA

(n – 2)

DATA

(n – 1)

DATA (n)

1

17/31

1

17/31

1

17/31

1

1

17/31

1

17/31

1

17/31

1

1

17/31

1

17/31

1

17/31

1

17/31

XXX

CFG (n)

CFG (n + 1)

CFG (n + 2)

DATA

(n – 2)

DATA

(n – 1)

DATA (n)

DATA (n + 1)

CFG (n)

CFG (n + 1)

CFG (n + 2)

CFG (n + 3)

Figure 38. General Interface Timing for the AD7699 With a Busy Indicator