82C85 Datasheet, PDF (8/21 Pages) Intersil Corporation – CMOS Static Clock Controller/Generator

82C85
edge after FAST command recognition. Proper CLK and
CLK 50 phase relationships are maintained and minimum
pulse width specifications are met.
FAST-to-SLOW or SLOW-to-FAST mode changes will occur
on the next rising or falling edge of PCLK. It is important to
remember that the transition time for operating frequency
changes, which are dependent upon PCLK, will vary with the
82C85 oscillator or EFI frequency.
The crystal/capacitor configuration and the formula used to
determine the capacitor values are shown in Figure 4. Crys-
tal Specifications are shown in Table 3.
CT = _C-^C----₁₁-----+^•----^C-_C----²-₂-- (Including Stray Capacitance)
(EQ. 1)
Slow Mode Control
The 82C55A programmable peripheral interface can be
used to provide control of the SLO/FST pin by connecting a
port pin of the 82C55A directly to the SLO/FST pin (See Fig-
ure 1). With the port pin configured as an output, software
control of the SLO/FST pin is provided by simply writing a
logical one (FAST mode) or logical zero (SLOW Mode) to
the corresponding port. PORT C is well-suited for this func-
tion due to it’s bit set and reset capabilities. Since PCLK con-
tinues to run at a frequency equal to the oscillator or EFI
frequency divided by 6, it can be used by other devices in
the system which need a fixed high frequency clock. For
example, PCLK could be used to clock an 82C54 program-
mable interval timer to produce a real-time clock for the sys-
tem or as a baud rate generator to maintain serial data
communications during SLOW mode operation.
Oscillator
The oscillator circuit of the 82C85 is designed primarily for
use with an external parallel resonant, fundamental mode
crystal from which the basic operating frequency is derived.
The crystal frequency should be selected at three times the
required CPU clock. X1 and X2 are the two crystal input con-
nections. The output of the oscillator is buffered and avail-
able at the OSC output (pin 18) for generation of other
system timing signals.
For the most stable operation of the oscillator (OSC) output
circuit, two capacitors (C1 = C2) are recommended. Capaci-
tors C1 and C2 are chosen such that their combined capaci-
tance matches the load capacitance as specified by the
crystal manufacturer. This insures operation within the fre-
quency tolerance specified by the crystal manufacturer.
CRYSTAL
2.4 - 25MHz
X1
C₁
X2
C₂
FIGURE 4. 82C85 CRYSTAL CONNECTION
TABLE 3. CRYSTAL SPECIFICATIONS
PARAMETER
TYPICAL CRYSTAL
SPECIFICATION
Frequency
2.4 to 25MHz
Type of Operation
Parallel Resonant, Fund. Mode
Load Capacitance
20 or 32pF
RSERIES (Max)
35X (f = 25MHz, C_L = 32pF)
66X (f = 25MHz, C_L = 20pF)
Frequency Source Selection
The F/C input is a strapping pin that selects either the crystal
oscillator or the EFI input as the source frequency for clock
generation. If the EFI input is selected as the source, the
oscillator section (OSC output) can be used independently
for another clock source. If a crystal is not used, then crystal
input X1 (pin 23) must be tied to V_CC or GND and X2 (pin
22) should be left open. If the EFI mode is not used, then EFI
(pin 20) should be tied to V_CC or GND.
EFI
OR
OSC
PCLK
SLO/FST
CLK
CLK50
FIGURE 5. SLO/FST TIMING OVERVIEW
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