ADFS5758 datasheet(39/75 Pages) AD

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Data Sheet

ADFS5758

Rev. 0 | Page 39 of 75

Bits[D31:D30] = 0b10 are used for synchronization purposes

during readback.

If autostatus readback mode is selected, the contents of the status

This ability allows the user to continuously monitor the status

powers up with this feature disabled. When this feature is enabled,

the normal two-stage readback feature is not available. Only the

status register is available on SDO. To read back any other

following the two-stage readback sequence. The automatic

status readback can be reenabled after the register is read back.

The shared SYNC autostatus readback is a special version of the

autostatus readback mode used to avoid SDO bus contention

when multiple devices are sharing the same SYNC line.

Echo mode behaves similarly to autostatus readback mode,

except that every second readback consists of an echo of the

previous command written to the ADFS5758 (see Figure 81).

See the Reading from Registers section for further details on the

readback modes.

COMMAND

STATUS

CONTENTS

COMMAND

Figure 81. SDO Contents, Echo Mode

WINDOWED WATCHDOG TIMER (WDT)

This watchdog timer feature is useful to ensure that communica-

tion has not been lost between the system controller and the

ADFS5758 and that the SPI datapath lines are functioning as

expected.

When enabled, the WDT alerts the system if the ADFS5758 does

not receive a specific SPI frame in the user-programmable timeout

period. A valid watchdog kick is a specific SPI frame received

within the pass window of the WDT. When the specific SPI

frame is received, the watchdog resets the timer controlling the

timeout alert. The SPI frame used to reset the WDT is

configurable as one of three choices:

•

A valid SPI write to any register.

•

A specific key code write to the key register (default).

•

Two specific consecutive key code writes to the key register.

As shown in Figure 82, the watchdog timer uses a center

threshold and a window width, where the width is a fraction of

the center threshold. For example, if the center threshold is set

to 100 ms and the window width set to 1/2, the valid region is

100 ms ± 25 ms. A WDT kick before 75 ms or after 125 ms is

registered as a fault. The windowing feature of the WDT can be

disabled by setting the window width to 1/1. In this scenario,

the user must simply kick the WDT any time before the center

threshold, eliminating the possibility of an early reset and thus

simplifying the operation of the WDT. For highest safety, the

window feature requires that the user write the correct key (or

two keys, if enabled) within the timeout window. If the signal

arrives before or after this timing window, the watchdog times

out and a dedicated WDT_STATUS bit in the status register alerts

the user that the WDT has timed out. The DIGITAL_DIAG_

RESULTS register can be read to clarify whether the WDT

timeout was due to a late or early kick. Note that, when a WDT

timeout occurs, all writes to the DAC_INPUT register as well as

hardware or software LDAC events are ignored until the active

WDT fault flag within the DIGITAL_DIAG_RESULTS register

is cleared. Once this flag has been cleared the WDT can be

restarted by performing a subsequent WDT kick command.

On power-up, the WDT is disabled by default. The default settings

of the center threshold and window width are 1 sec and 1/1,

respectively. The default method to kick the WDT is to write

one specific key and, upon timeout, the default action is to set

the relevant flag bits and the FAULT pin. See Table 41 for

specific register bit details to support the configurability of the

WDT operation.

USER DIGITAL OFFSET AND GAIN CONTROL

The ADFS5758 has a USER_GAIN and a USER_OFFSET register

that allow trimming of the gain and offset errors from the

entire signal chain. The 16-bit USER_GAIN register allows the

user to adjust the gain of the DAC channel in steps of 1 LSB. The

USER_GAIN register coding is straight binary, as shown in

Table 14. The default code in the USER_GAIN register is 0xFFFF,

which results in no gain factor applied to the programmed

output. In theory, the gain can be tuned across the full range of

the output. In practice, the maximum recommended gain trim

is approximately 50% of the programmed range to maintain

accuracy.

Table 14. Gain Register Adjustment

Gain Adjustment Factor

D15

D14 to D1

65,535/65,536

…

2/65,536

1/65,536

CENTER THRESHOLD

MUST RESET HERE

FAULT: A RESET HERE

IS TOO EARLY

CENTER THRESHOLD –

WINDOW WIDTH/2

CENTER THRESHOLD +

WINDOW WIDTH/2

FAULT: RESET IS

TOO LATE

Figure 82. Windowed Watchdog Timer

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