Rev. 1.1 12/03 Copyright © 2003 by Silicon Laboratories AN128-DS11
AN128
SOFTWARE SPI EXAMPLES FOR THE C8051F30X FAMILY
Relevant Devices
This application note applies to the following devices:
C8051F300, C8051F301, C8051F302, and
C8051F303.
Introduction
This application note is a collection of routines that
can be used to implement a master-mode SPI
device in software. Eight different examples of a
SPI master-mode transfer are provided. The exam-
ples contain two functions for each of the SPI clock
phase and polarity options: an example written in
‘C’, and for increased speed, an assembly routine
that can be called from ‘C’. An example of how to
call the routines from a ‘C’ program and an
EEPROM interface example are also provided. SPI
is a trademark of Motorola Inc.
The SPI functions described in this document are
written to minimize the software overhead required
for the SPI transaction. They are intended for use in
a system where the C8051F30x device is the only
SPI master on the bus.
Hardware Interface
These examples implement the SPI interface using
three GPIO pins for transactions:
MOSI (Master Out / Slave In): This pin is used for
serial data output from the C8051F30x, and should
be configured as a digital push-pull output.
MISO (Master In / Slave Out): This pin is used for
serial data input from the slave device, and should
be configured as an open-drain digital pin.
SCK (Serial Clock): This pin is used as the serial
data clock output from the C8051F30x device, and
should be configured as a digital push-pull output.
Additionally, if a slave select signal is required for
the slave device, a fourth GPIO pin is needed, and
must be declared as a digital push-pull output for
this purpose. All of the dedicated GPIO pins should
be skipped by the crossbar. Figure 1 shows the con-
nections between the SPI master (C8051F30x) and
SPI slave devices.
Function Descriptions
There are eight examples of a software SPI master
contained in this applications note. Each of the four
different SPI modes (Mode 0, Mode 1, Mode 2 and
Mode 3) are given as examples in both ‘C’ and
assembly. Table 1 lists the source files for each
implementation. All of the routines can be called
from ‘C’ using the same function prototype.
Because of this, only one of the sample implemen-
tations should be included when building a project.
The functions can be renamed if multiple SPI
modes are needed in the same system.
MISO
SCK
C8051F30x
MOSI
P0.3
SPI Slave
MOSI
MISO
SCK
NSS
Figure 1. Hardware Configuration
