F1978 datasheet(12/26 Pages) IDT

12 / 26 page

F1978 Datasheet

Rev O August 28, 2017

Programming

The F1978 can be programmed using either the parallel or the serial interface, which is selectable via VMODE (pin 13). Serial Mode is selected

by floating VMODE or pulling it to a logic HIGH, and Parallel Mode is selected by setting VMODE to a logic LOW.

For a comparison of the F1975 and F1978 products, see the Application Note AN945

– Comparison of F1975 and F1978 Digital Step Attenuator

Serial Programming Methods.

Serial Control Mode

The F1978 Serial Mode is selected by floating VMODE (pin 13) or pulling it to a logic HIGH. The serial interface is a 6-bit shift register and shifts

in the most significant bit (MSB) (D5 bit) first.

The F1978 can be programmed through the serial interface using one of two methods, Conventional Serial Mode or Direct Serial Mode, as

described in the following sections.

Table 7.

6 Bit SPI DATA Word Sequence

Bit

Definition

Attenuation 16dB Control Bit

Attenuator 8dB Control Bit

Attenuator 4dB Control Bit

Attenuator 2dB Control Bit

Attenuator 1dB Control Bit

Attenuator 0.5dB Control Bit

Table 8.

Truth Table for Serial Control Word Bits

(MSB)

(LSB)

Attenuation (dB)

0.5

31.5

F1978 Datasheet(PDF) 12 Page - Integrated Device Technology