Datasheet

 2003-2019 Microchip Technology Inc. DS20001801J-page 55
MCP2515
8.0 OSCILLATOR
The MCP2515 is designed to operate with a crystal or
ceramic resonator connected to the OSC1 and OSC2
pins. The MCP2515 oscillator design requires the use
of a parallel cut crystal. Use of a series cut crystal may
give a frequency out of the crystal manufacturer’s
specifications. A typical oscillator circuit is shown in
Figure 8-1. The MCP2515 may also be driven by an
external clock source connected to the OSC1 pin, as
shown in Figure 8-2 and Figure 8-3.
8.1 Oscillator Start-up Timer
The MCP2515 utilizes an Oscillator Start-up Timer
(OST) that holds the MCP2515 in Reset to ensure that
the oscillator has stabilized before the internal state
machine begins to operate. The OST keeps the device
in a Reset state for 128 OSC1 clock cycles after the
occurrence of a Power-on Reset, SPI Reset, after the
assertion of the RESET pin, and after a wake-up from
Sleep mode. It should be noted that no SPI protocol
operations should be attempted until after the OST has
expired.
8.2 CLKOUT Pin
The CLKOUT pin is provided to the system designer for
use as the main system clock or as a clock input for other
devices in the system. The CLKOUT has an internal
prescaler which can divide F
OSC
by 1, 2, 4 and 8. The
CLKOUT function is enabled and the prescaler is
selected via the CANCTRL register (see Register 10-1).
The CLKOUT pin will be active upon system Reset and
default to the slowest speed (divide-by-8) so that it can
be used as the MCU clock.
When Sleep mode is requested, the MCP2515 will
drive sixteen additional clock cycles on the CLKOUT
pin before entering Sleep mode. The Idle state of the
CLKOUT pin in Sleep mode is low. When the CLKOUT
function is disabled (CLKEN (CANCTRL[2]) = 0), the
CLKOUT pin is in a high-impedance state.
The CLKOUT function is designed to ensure that
t
hCLKOUT
and t
lCLKOUT
timings are preserved when the
CLKOUT pin function is enabled, disabled or the
prescaler value is changed.
FIGURE 8-1: CRYSTAL/CERAMIC RESONATOR OPERATION
FIGURE 8-2: EXTERNAL CLOCK SOURCE
(2)
Note: The maximum frequency on CLKOUT is
specified as 25 MHz (See Table 13-5).
C
1
C
2
XTAL
OSC2
OSC1
Sleep
To Internal Logic
Note 1: A Series Resistor (R
S
) may be required for AT strip cut crystals.
2: The Feedback Resistor (R
F
) is typically in the range of 2 to 10 M.
R
S
(1)
R
F
(2)
Clock from
External System
OSC1
OSC2
Note 1: A resistor to ground may be used to reduce system noise; this may increase system current.
2: Duty cycle restrictions must be observed (see Table 13-2).
Open
(1)