Preliminary Technical Data

AD7938/AD7939

Writing Data to the AD7938/AD7939

With W/B tied logic high, a single write operation transfers the

full data-word on DB0 to DB11 to the control register on the

AD7938/AD7939. The DB8/HBEN pin assumes its DB8

function. Data written to the AD7938/AD7939 should be

provided on the DB0 to DB11 inputs with DB0 being the LSB of

the data-word. With W/B tied logic low, the AD7938/AD7939

requires two write operations to transfer a full 12-bit word.

DB8/HBEN assumes its HBEN function. Data written to the

AD7938/AD7939 should be provided on the DB0 to DB7

inputs. HBEN determines whether the byte written is high byte

or low byte data. The low byte of the data-word should be

written first with DB0 being the LSB of the full data-word. For

the high byte write, HBEN should be high and the data on the

DB0 input should be data bit 8 of the 12-bit word. In both word

and byte mode, a single write operation to the shadow register is

always sufficient since it is only 8-bits wide.

Figure 40

Figure 40. AD7938/AD7939 Parallel Interface—Write Cycle Timing for Word Mode Operation (W/B = 1)

shows the write cycle timing diagram of the

AD7938/AD7939. When operated in word mode, the HBEN

input does not exist and only the one write operation is required

to write the word of data to the device. Data should be provided

on DB0 to DB11. When operated in byte mode, the two write

cycles shown in F

are required to write the full data-

word to the AD7938/AD7939. In F

, the first write

transfers the lower 8 bits of the data-word from DB0 to DB7,

and the second write transfers the upper 4 bits of the data-word.

When writing to the AD7938/AD7939, the top 4 bits in the high

byte must be 0s.

igure 41

igure 41

Figure 41. AD7938/AD7939 Parallel Interface—Write Cycle Timing for Byte Mode Operation (W/B = 0)

The data is latched into the device on the rising edge of WR.

signals are gated internally. CS and WR may be tied together as

(assuming CS and RD have not already been tied together).

DATA

DB0 TO DB11

WR

CS

LOW BYTE

HIGH BYTE

DB0 TO DB11

HBEN/DB8

WR

CS

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