82C85

FN2976 Rev.0.00

Page 5 of 21

March 1997

until the oscillator signal amplitude reaches the X1 Schmitt trig-

ger input threshold voltage and 8192 cycles of the crystal oscil-

lator output are counted by an internal counter. After this count

is complete, the stopped outputs (CLK, CLK50, PCLK, and

OSC) start cleanly with the proper phase relationships.

Typically, any input signal which meets the START input timing

requirements can be used to start the 82C85. In many cases,

this would be the INT output from an 82C59A CMOS Priority

Interrupt Controller (See Figure 1). This output, which is active

high, can be connected to both the 82C85 START pin and to

the appropriate interrupt request input on the microprocessor.

When the INT output becomes active, the oscillator/clock cir-

cuit on the 82C85 will restart. Upon completion of the appropri-

ate restart sequence, the CLK signal to the CPU will become

active. The CPU can then respond to the still pending interrupt

request.

If the 82C59A/82C85 restart combination is used in conjunc-

tion with an 82C55A STOP control, the 82C55A must be initial-

ized prior to the 82C59A after reset. The 82C59A interrupt

output is driven high at reset, causing the 82C85 to remain in

the START mode regardless of the state of the S2/STOP input.

This will avoid stopping the 82C85 due to negative transitions

on the S2/STOP input which may occur during a mode change

on the 82C55A or during the operation of any peripheral I/O

device prior to initialization.

Another method of insuring proper operation of the START

function upon reset or system initialization is to bias the

S2/STOP input low with an external pull-down resistor. The

S2/STOP input will remain low until driven high by the 82C55A

port pin or by external logic. This insures that the 82C85 STOP

command (HHH prior to LHH requirement on the status inputs)

will not be satisfied. To minimize power dissipation in this case

(using a pulldown resistor), the S2/STOP input should be nor-

mally LOW and pulsed HIGH to develop the necessary HHH-

to-LHH STOP sequence. In this manner, the output driving the

S2/STOP input will be normally LOW and will not be driving to

the opposite state of the pull-down resistor.

Fast Mode

The most common operating mode for a system is the FAST

mode. In this mode, the 82C85 operates at the maximum fre-

quency determined by the main oscillator or EFI frequency.

82C59A

82C85

START

CLK

S0

S1

S2/STOP

SLO/FST

80C86/88

INTR

CLK

PA0 PA1

82C55A

INT

VCC

FIGURE 1. CMOS PERIPHERAL CONTROL OF 82C85 STOP,

START AND SLOW/FAST OPERATIONS

TABLE 2. TYPICAL SYSTEM POWER SUPPLY CURRENT FOR STATIC CMOS OPERATING MODES

FAST

SLOW

STOP-CLOCK

STOP-OSC

CPU Frequency

5MHz

20 KHz

DC

DC

XTAL Frequency

15MHz

15MHz

15MHz

DC

ICC

82C85

24.7mA

16.9mA

14.1mA

24.4mA

80C88

23.8mA

173.0mA

106.6mA

106.6mA

82C82

1.7mA

6.5mA

1.0mA

1.0mA

82C86

1.4mA

14.0mA

1.0mA

1.0mA

82C88

3.5mA

14.3mA

3.8mA

3.8mA

82C52

151.2mA

72.0mA

1.0mA

1.0mA

82C54

943.0mA

915.0mA

3.5mA

1.0mA

82C55A

3.2mA

1.2mA

1.0mA

1.0mA

82C59A

580.0mA

520.0mA

1.0mA

1.0mA

74HCXX + other

2.9mA

10.0mA

90.0mA

90.0mA

HM-6516

820.0mA

32.0mA

1.9mA

1.9mA

HM-6616

6.3mA

52.5mA

12.0mA

12.0mA

Total

66.8mA

18.9mA

14.3mA

244.7mA

All measurements taken at room temperature, VCC = +5.0V. Power supply current levels will be dependent upon system configuration and fre-

quency of operation.