Cerebot MX7cK Reference Manual
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page 11 of 34
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PIC32 Peripheral Devices
The following describes the peripheral devices
available in the PIC32MX795F512L
microcontroller and how they are accessed on
the Cerebot MX7cK board.
CPU Clock Source
The PIC32 microcontroller supports numerous
clock source options for the main processor
operating clock. The Cerebot MX7cK board is
designed to operate with either a silicon
resonator from Discera, IC2, for use with the
EC oscillator option, or an external crystal, X1,
for use with the XT oscillator option. Standard
production boards will have an 8Mhz Discera
silicon resonator loaded and the EC oscillator
option should be used. If IC2 is not loaded, an
8Mhz crystal will be loaded for X1 (on the
bottom of the board) and the XT oscillator
option should be used. Oscillator options are
selected via the configuration settings specified
using the
#pragma config
statement. Use
#pragma config POSCMOD=EC
to select the
EC option and
#pragma
config
POSCMOD=XT
to select the XT option.
Using the internal system clock phase-locked
loop (PLL), it is possible to select numerous
multiples or divisions of the 8Mhz oscillator to
produce CPU operating frequencies up to
80Mhz. The clock circuit PLL provides an input
divider, multiplier, and output divider. The
external clock frequency (8Mhz) is first divided
by the input divider value selected. This is
multiplied by the selected multiplier value and
then finally divided by the selected output
divider. The result is the system clock,
SYSCLK, frequency. The SYSCLK frequency
is used by the CPU, DMA controller, interrupt
controller and pre-fetch cache.
The operating frequency is selected using the
PIC32MX795 configuration variables. These
are set using the
#pragma config
statement. Use
#pragma
config FPLLIDIV
to set the input divider,
#pragma config
FPLLMUL
to set the multiplication factor and
#pragma config FPLLODIV
to set the
output divider. Refer to the
PIC32MX5XX/6XX/7XX Family Data Sheet
and the PIC32MX Family Reference Manual,
Section 6. Oscillators for information on how to
choose the correct values, as not all
combinations of multiplication and division
factors will work.
In addition to configuring the SYSCLK
frequency, the peripheral bus clock, PBCLK,
frequency is also configurable. The peripheral
bus clock is used for most peripheral devices,
and in particular is the clock used by the
timers, and serial controllers (UART, SPI, I
2
C).
The PBLCK frequency is a division of the
SYSCLK frequency selected using
#pragma
config FPBDIV
. The PBCLK divider can be
set to divide by 1, 2, 4, or 8.
The following example will set up the Cerebot
MX7cK for operation with a SYSCLK frequency
of 80Mhz and a PBCLK frequency of 10Mhz:
#pragma config FNOSC = PRIPLL
#pragma config POSCMOD = EC
#pragma config FPLLIDIV = DIV_2
#pragma config FPLLMUL = MUL_20
#pragma config FPLLODIV = DIV_1
#pragma config FPBDIV = DIV_8
Documentation for the PIC32 configuration
variables can be found in the PIC32MX
Configuration Settings guide. This is found
using the
“Help.Topics…”
command in the
MPLAB IDE. Also, refer to Appendix C for an
example of setting the configuration variables.
When using the Cerebot MX7cK with the
chipKIT MPIDE software, the clock source is
set by the boot loader and no action is
required.
Ethernet Interface
The Cerebot MX7cK provides the ability to
interface with 10Mbps or 100Mbps Ethernet
networks. The PIC32MX795 microcontroller
contains a 10/100 Ethernet Medium Access
Controller (MAC). External to the PIC32
microcontroller, the Cerebot MX7cK board
provides an SMSC LAN8720 Ethernet Physical
