305

Clock Generator

The clock generator consists of two synchronous divide-by-

three counters with special clear inputs that inhibit the count-

ing. One counter generates a 33% duty cycle waveform

(CLK) and the other generates a 50% duty cycle waveform

(CLK50). These two counters are negative-edge synchro-

nized, with the low-going transitions of both waveforms

occurring on the same oscillator transition. The CLK and

CLK50 output frequencies are one-third of the base input

frequency when SLO/FST is high and are equal to the base

input frequency divided by 768 when SLO/FST is low.

The CLK output is a 33% duty cycle clock signal designed to

drive the 80C86 and 80C88 microprocessors directly.

CLK50 has a 50% duty cycle output synchronous with CLK,

designed to drive co-processors and peripherals requiring a

50% duty cycle clock. When SLO/FST is high, CLK and

CLK50 have output frequencies which are 1/3 that of EFI/

OSC. When SLO/FST is low, CLK and CLK50 have output

frequencies which are OSC (EFI) divided by 768.

PCLK is a peripheral clock signal with an output frequency

equal to the oscillator or EFI frequency divided by 6. PCLK

has a 50% duty cycle. PCLK is unaffected by SLO/FST.

When the 82C85 is placed in the STOP mode, PCLK will

remain in it’s current state (logic high or logic low) until a

RESET or START command restarts the 82C85 clock cir-

cuitry. PCLK is negative-edge synchronized with CLK and

CLK50.

Clock Synchronization

The clock synchronization (CSYNC) input allows the output

clocks to be synchronized with an external event (such as

another 82C85 or 82C84A clock signal). CSYNC going

active causes all clocks (CLK, CLK50 and PCLK) to stop in

the HIGH state.

It is necessary to synchronize the CSYNC input to the EFI

clock external to the 82C85. This is accomplished with two

flip-flops when synchronizing two 82C85s and with three flip-

flops when synchronizing an 82C85 to an 82C84A (See Fig-

ure 6). Multiple external flip-flops are necessary to minimize

the occurrence of metastable (or indeterminate) states.

Ready Synchronization

Two READY inputs (RDY1, RDY2) are provided to accom-

modate two system busses. Each READY input is qualified

by (AEN1 and AEN2, respectively). The AEN signals vali-

date their respective RDY signals.

Synchronization is required for all asynchronous active-

going edges of either RDY input to guarantee that the RDY

set up and hold times are met. Inactive-going edges of RDY

in normally ready systems do not require synchronization but

must satisfy RDY setup and hold as a matter of proper sys-

tem design.

The ASYNC input defines two modes of READY synchroni-

zation operation. When ASYNC is LOW, two stages of syn-

chronization are provided for active READY input signals.

Positive-going asynchronous READY inputs will first be syn-

chronized to flip-flop one at the rising edge of CLK (requiring

a setup time TR1VCH) and then synchronized to flip-flop two

at the next falling edge of CLK, after which time the READY

output will go HIGH.

Negative-going asynchronous READY inputs will be syn-

chronized directly to flip-flop two at the falling edge of CLK,

after which time the READY output will go inactive. This

mode of operation is intended for use by asynchronous (nor-

mally not ready) devices in the system which cannot be

guaranteed by design to meet the required RDY setup timing

(TR1VCL) on each bus cycle.

When ASYNC is high or left open, the first READY flip-flop is

bypassed in the READY synchronization logic. READY

inputs are synchronized by flip-flop two on the falling edge of

CLK before they are presented to the processor. This mode

is available for synchronous devices that can be guaranteed

to meet the required RDY setup time. ASYNC can be

changed on every bus cycle to select the appropriate mode

of synchronization for each device in the system.

EFI

82C85

CSYNC

CSYNC

82C84A

CSYNC WITH 82C85(s)

CLOCK

SYNC

EFI

DQ

>

↑

>

↑

>

↑

D

Q

Q

D

(TO OTHER

82C85s)

FIGURE 6. 82C85 AND 82C84A CSYNC SYNCHRONIZATION

METHODS

82C85