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AD7938/AD7939

Preliminary Technical Data

Reading Data from the AD7938/AD7939

With the W/B pin tied logic high, the AD7938/AD7939

interface operates in word mode. In this case, a single read

operation from the device accesses the conversion data-word on

Pins DB0 to DB11. The DB8/HBEN pin assumes its DB8

function. With the W/B pin tied to logic low, the

AD7938/AD7939 interface operates in byte mode. In this case,

the DB8/HBEN pin assumes its HBEN function. Conversion

data from the AD7938/ AD7939 must be accessed in two read

operations with 8 bits of data provided on DB0 to DB7 for each

of the read operations. The HBEN pin determines whether the

read operation accesses the high byte or the low byte of the12-

or 10-bit word. For a low byte read, DB0 to DB7 provide the

eight LSBs of the 12-bit word. For 10-bit operation, the two

LSBs of the low byte are 0s and are followed by 6 bits of

conversion data. For a high byte read, DB0 to DB4 provide the

four MSBs of the 12-/10-bit word. DB5 to DB7 of the high byte

provide the Channel ID. F

shows the read cycle timing

diagram for a 12-/10-bit transfer. When operated in word mode,

the HBEN input does not exist, and only the first read operation

is required to access data from the device. When operated in

byte mode, the two read cycles shown in F

are required

to access the full data-word from the device.

igure 38

igure 39

Figure 39. AD7938/AD7939 Parallel Interface—Read Cycle Timing for Byte Mode Operation (W/B = 0)

The CS and RD signals are gated internally and level triggered

active low. In either word mode or byte mode, CS and RD may

be tied together as the timing specifications for t10 and t11 are

0 ns minimum. This would mean the bus would be constantly

driven by the AD7938/AD7939.

The data is placed onto the data-bus a time t13 after both CS and

RD go low. The RD rising edge can be used to latch data out of

the device. After a time, t14, the data lines will become three-

stated.

Alternatively, CS and RD can be tied permanently low and the

conversion data is valid and placed onto the data-bus a time, t9,

before the falling edge of BUSY.

t11

t10

t13

t15

t16

t14

t12

t17

LOW BYTE

HIGH BYTE

DB0 TO DB7

HBEN/DB8

Rev. PrN | Page 26 of 32

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