3-88

TELCOM SEMICONDUCTOR, INC.

15-BIT, FAST-INTEGRATING CMOS

ANALOG-TO-DIGITAL CONVERTER

TC850

Figure 7. Interface to Typical

ADDRESS

DECODE

DB0

DB1

DB2

DB3

DB4

DB5

DB6

DB7

CS

A2

L/H

OVR/POL

RD

WR

BUSY

CS

+5V

DB0

DB1

DB2

DB3

DB4

DB5

DB6

DB7

A15

A0

A1

RD

WR

INTERRUPT

ADDRESS

DATA BUS

CONT/DEMAND

µP

TC850

X00

X01

X10

HIGH BYTE AND POLARITY

LOW BYTE

HIGH BYTE AND OVERRANGE

DB0

DB1

DB2

DB3

DB4

DB5

DB6

DB7

BUSY

RD

PB0

CS

NC

CS

WR

+5V

PA0

PA1

PA2

PA3

PA4

PA5

PA6

PA7

INTERRUPT

CONT/DEMAND

µC OR µP

I/O PORT

TC850

Figure 8. Interface to Typical

Since the PA0–PA7 inputs are dedicated to reading

A/D data, the A/D CS/CE inputs can be enabled continu-

ously. In continuous mode, data must be read in 3 bytes, as

shown in Table I. The required RD pulses are provided by a

µC/µP output pin.

The circuit of Figure 8 can also operate in the demand

mode, with the start-up conversion strobe generated by a

µC/µP output pin. In this case, the L/H and CONT/DEMAND

inputs can be controlled by I/O pins and the RD input

connected to digital ground.

Demand Mode Interface Timing

When CONT/DEMAND input is LOW, the TC850 per-

forms a conversion each time CE and CS are active and WR

is strobed LOW.

The demand mode conversion timing is shown in Figure

9. BUSY goes LOW and data is valid 1155 clock pulses after

WR goes LOW. After BUSY goes low, 125 additional clock

cycles are required before the next conversion cycle will

begin.

Once conversion is started, WR is ignored for 1100

internal clock cycles. After 1100 clock cycles, another WR

pulse is recognized and initiates a new conversion when the

present conversion is complete. A negative edge on WR is

required to begin conversion. If WR is held LOW, conver-

sions will not occur continuously.

The A/D conversion data is valid on the falling edge of

BUSY, and remains valid until one-half internal clock cycle

before BUSY goes HIGH on the succeeding conversion.

BUSY can be monitored with an I/O pin to determine end of

conversion, or to generate a

µP interrupt.

In demand mode, the three data bytes can be read in

any desired order. The TC850 is simply regarded as three

bytes of memory and accessed accordingly. The bus output

timing is shown in Figure 10.

Continuous Mode Interface Timing

When the CONT/DEMAND input is HIGH, the TC850

performs conversions continuously. Data will be valid on the

falling edge of BUSY, and all three bytes must be read within

443-1/2 internal clock cycles of BUSY going LOW. The

timing diagram is shown in Figure 11.

In continuous mode, OVR/POL and L/H byte-select

inputs are ignored. The TC850 automatically cycles through

three data bytes, as shown in Table I. Bus output timing in

the continuous mode is shown in Figure 12.