5-24

Initial Clear Circuitry

The internal logic of the 7104 is supplied by an internal

regulator between V++ and Digital Ground. The regulator

includes a low-voltage detector that will clear various

registers. This is intended to ensure that on initial power-up,

the control logic comes up in Auto-Zero, with the 2nd, 3rd,

and 4th MSB bits cleared, and the “mode” F/F cleared (i.e.,

in “direct” mode). This, however, will also clear these regis-

ters if the supply voltage “glitches” to a low enough value.

Additionally, if the supply voltage comes up too fast, this

clear pulse may be too narrow for reliable clearing. In gen-

eral, this is not a problem, but if the UART internal “MODE”

F/F should come up set, the byte and chip ENABLE lines will

become active outputs. In many systems this could lead to

bus conflicts, especially in non-handshake systems. In any

case, SEN should be high (held high for non-handshake sys-

tems) to ensure that the MODE F/F will be cleared as fast as

possible (see Figure 11 for timing). For these and other

reasons, adequate supply bypass is recommended.

Oscillator

The ICL7104-14 is provided with a versatile three terminal

oscillator to generate the internal clock. The oscillator may

be overdriven, or may be operated as an RC or crystal

oscillator.

Figure 14 shows the oscillator configured for RC operation.

The internal clock will be of the same frequency and phase

as the voltage on the CLOCK 3 pin. The resistor and

capacitor should be connected as shown. The circuit will

oscillate at a frequency given by f = 0.45/RC. A 50 - 100k

Ω

resistor is recommended for useful ranges of frequency. For

optimum 60Hz line rejection, the capacitor value should be

chosen such that 32768 (-16), 8192 (-14) clock periods is

close to an integral multiple of the 60Hz period.

As a result of pin count limitations, the ICL7104-16 has only

CLOCK 1 and CLOCK 2 available, and cannot be used as

an RC oscillator. The internal clock will correspond to the

inverse of the signal on CLOCK 2. Figure 15 shows a crystal

oscillator circuit, which can be used with both 7104 versions.

If an external clock is to be used, it should be applied to

CLOCK 1. This internal clock will correspond to the signal

applied to this pin.

FIGURE 13. HANDSHAKE TRIGGERED BY MODE

DATA VALID

INTERNAL

INTERNAL

STATUS

MODE

INTERNAL

SEN INPUT

CE/LOAD

UART

HIGH BYTE

MIDDLE

LOW BYTE

NORM

HBEN

MBEN

LBEN

ZERO-CROSSING DETECTED

ZERO-CROSSING OCCURS

CLOCK

LATCH

OUTPUT

INPUT

MODE

DATA

BYTE DATA

DATA

DATA VALID

DATA VALID

STATUS OUTPUT UNAFFECTED

BY UART MODE

LATCH PULSE INHIBITED

IN UART MODE

THREE-STATE WITH PULLUP

THREE-STATE HIGH IMPEDANCE

DON’T CARE

POSITIVE TRANSITION

CAUSES ENTRY INTO

UART MODE

DEINT PHASE III

AS OUTPUT

NOTE: Clock 3 has the same output drive as the bit outputs.

FIGURE 14. RC OSCILLATOR (ICL7104-14 ONLY)

CLOCK

25

26

R

C

2

CLOCK

3

CLOCK

1

24

FIGURE 15. CRYSTAL OSCILLATOR

CLOCK

25

26

2

CLOCK

1

V+

DEPENDS ON CRYSTAL

TYP 0-30pF

CRYSTAL

†

ICL8052/ICL7104, ICL8068/ICL7104