17
LTC1419
1419fb
APPLICATIONS INFORMATION
WUU
U
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.
t
3
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.
t
1
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
t
5
t
CONV
t
6
t
8
t
7
DATA
(SAMPLE N)
(CONVST = )
