17

LTC1419

In slow memory and ROM modes (Figures 19 and 20) CS

is tied low and CONVST and RD are tied together. The MPU

starts the conversion and reads the output with the RD

signal. Conversions are started by the MPU or DSP (no

external sample clock).

In slow memory mode the processor applies a logic low to

RD (= CONVST), starting the conversion. BUSY goes low,

forcing the processor into a WAIT state. The previous

conversion result appears on the data outputs. When the

conversion is complete, the new conversion results ap-

pear on the data outputs; BUSY goes high, releasing the

processor and the processor takes RD (= CONVST) back

high and reads the new conversion data.

In ROM mode, the processor takes RD (= CONVST) low,

starting a conversion and reading the previous conversion

result. After the conversion is complete, the processor can

read the new result and initiate another conversion.

SHDN

CONVST

1419 F14b

Figure 14b. SHDN to CONVST Wake-Up Timing

Timing and Control

Conversion start and data read operations are controlled

by three digital inputs: CONVST, CS and RD. A logic “0”

applied to the CONVST pin will start a conversion after the

ADC has been selected (i.e., CS is low). Once initiated, it

cannot be restarted until the conversion is complete.

Converter status is indicated by the BUSY output. BUSY

is low during a conversion.

Figures 16 through 20 show several different modes of

operation. In modes 1a and 1b (Figures 16 and 17) CS

and RD are both tied low. The falling edge of CONVST

starts the conversion. The data outputs are always enabled

and data can be latched with the BUSY rising edge. Mode

1a shows operation with a narrow logic low CONVST

pulse. Mode 1b shows a narrow logic high CONVST pulse.

In mode 2 (Figure 18) CS is tied low. The falling edge of the

CONVST signal again starts the conversion. Data outputs

are in three-state until read by the MPU with the RD signal.

Mode 2 can be used for operation with a shared MPU

databus.

CS

RD

1419 F15

Figure 15. CS to CONVST Set-Up Timing

Figure 16. Mode 1a. CONVST Starts a Conversion. Data Outputs Always Enabled

DATA N

DB13 TO DB0

DATA (N + 1)

DB13 TO DB0

DATA (N – 1)

DB13 TO DB0

CONVST

CS = RD = 0

BUSY

1419 F16

DATA

(SAMPLE N)

(CONVST =

)