Clock Modes

The MAX196/MAX198 operate with either an internal or

an external clock. Control bits (D6, D7) select either

internal or external clock mode. Once the desired clock

mode is selected, changing these bits to program

power-down will not affect the clock mode. In each

mode, internal or external acquisition can be used. At

power-up, external clock mode is selected.

Internal Clock Mode

Select internal clock mode to free the µP from the

burden of running the SAR conversion clock. To select

this mode, write the control byte with D7 = 0 and D6 =

1. A 100pF capacitor between the CLK pin and ground

sets this frequency to 1.56MHz nominal. Figure 7

shows a linear relationship between the internal clock

period and the value of the external capacitor used.

External Clock Mode

Select external clock mode by writing the control byte

with D7 = 0 and D6 = 0. Figure 8 shows CLK and WR

timing relationships in internal and external acquisition

modes, with an external clock. A 100kHz to 2.0MHz

external clock with 45% to 55% duty cycle is required for

proper operation. Operating at clock frequencies lower

than 100kHz will cause a voltage droop across the hold

capacitor, and subsequently degrade performance.

Multirange, Single +5V, 12-Bit DAS

with 12-Bit Bus Interface

______________________________________________________________________________________

11

ACQMOD = "1"

CS

WR

D7–D0

INT

RD

DOUT

ACQMOD = "0"

DATA VALID

CONTROL

BYTE

CONTROL

BYTE

Figure 6. Conversion Timing Using External Acquisition Mode

2000

0

0

50

250

350

500

CLOCK PIN CAPACITANCE (pF)

100

150 200

300

1500

1000

Figure 7. Internal Clock Period vs. Clock Pin Capacitance