
REV. B
ADuC824
–48–
SERIAL PERIPHERAL INTERFACE
The ADuC824 integrates a complete hardware Serial Peripheral
Interface (SPI) interface on-chip. SPI is an industry standard syn-
chronous serial interface that allows eight bits of data to be
synchronously transmitted and received simultaneously, i.e., full
duplex. It should be noted that the SPI physical interface is shared
with the I
2C interface and therefore the user can only enable one
or the other interface at any given time (see SPE in SPICON
below). The system can be configured for Master or Slave operation
and typically consists of four pins, namely:
MISO (Master In, Slave Out Data I/O Pin), Pin#14
The MISO (master in slave out) pin is configured as an input line
in master mode and an output line in slave mode. The MISO
line on the master (data in) should be connected to the MISO
line in the slave device (data out). The data is transferred as
byte wide (8-bit) serial data, MSB first.
MOSI (Master Out, Slave In Pin), Pin#27
The MOSI (master out slave in) pin is configured as an output line
in master mode and an input line in slave mode. The MOSI
line on the master (data out) should be connected to the MOSI
line in the slave device (data in). The data is transferred as byte
wide (8-bit) serial data, MSB first.
SCLOCK (Serial Clock I/O Pin), Pin#26
The master clock (SCLOCK) is used to synchronize the data
being transmitted and received through the MOSI and MISO data
lines. A single data bit is transmitted and received in each SCLOCK
period. Therefore, a byte is transmitted/received after eight
SCLOCK periods. The SCLOCK pin is configured as an output
in master mode and as an input in slave mode. In master mode
the bit-rate, polarity and phase of the clock are controlled by
the CPOL, CPHA, SPR0 and SPR1 bits in the SPICON SFR
(see Table XIX). In slave mode the SPICON register will have
to be configured with the phase and polarity (CPHA and CPOL)
of the expected input clock. In both master and slave mode
the data is transmitted on one edge of the SCLOCK signal and
sampled on the other. It is important therefore that the CPHA
and CPOL are configured the same for the master and slave devices.
SS (Slave Select Input Pin), Pin#13
The Slave Select (
SS) input pin is only used when the ADuC824
is configured in slave mode to enable the SPI peripheral. This line
is active low. Data is only received or transmitted in slave mode
when the
SS pin is low, allowing the ADuC824 to be used in single
master, multislave SPI configurations. If CPHA = 1 then the
SS
input may be permanently pulled low. With CPHA = 0 then the
SS input must be driven low before the first bit in a byte wide
transmission or reception and return high again after the last bit
in that byte wide transmission or reception. In SPI Slave Mode,
the logic level on the external
SS pin (Pin# 13), can be read
via the SPR0 bit in the SPICON SFR.
The following SFR registers are used to control the SPI interface.
SPICON
SPI Control Register
SFR Address
F8H
Power-On Default Value
04H
Bit Addressable
Yes
I
P
S
IL
O
C
WE
P
SM
I
P
SL
O
P
CA
H
P
C1
R
P
S0
R
P
S
Table XIX. SPICON SFR Bit Designations
Bit
Name
Description
7
ISPI
SPI Interrupt Bit
Set by MicroConverter at the end of each SPI transfer.
Cleared directly by user code or indirectly by reading the SPIDAT SFR.
6
WCOL
Write Collision Error Bit
Set by MicroConverter if SPIDAT is written to while an SPI transfer is in progress.
Cleared by user code.
5
SPE
SPI Interface Enable Bit
Set by user to enable the SPI interface.
Cleared by user to enable the I
2C interface.
4
SPIM
SPI Master/Slave Mode Select Bit
Set by user to enable Master Mode operation (SCLOCK is an output).
Cleared by user to enable Slave Mode operation (SCLOCK is an input).
3
CPOL
*
Clock Polarity Select Bit
Set by user if SCLOCK idles high.
Cleared by user if SCLOCK idles low.
2
CPHA
*
Clock Phase Select Bit
Set by user if leading SCLOCK edge is to transmit data.
Cleared by user if trailing SCLOCK edge is to transmit data.
1
SPR1
SPI Bit-Rate Select Bits
0
SPR0
These bits select the SCLOCK rate (bit-rate) in Master Mode as follows:
SPR1
SPR0
Selected Bit Rate
SPR1
SPR0
Selected Bit Rate
00fCORE/2
10fCORE/8
01fCORE/4
11fCORE/16
In SPI Slave Mode, i.e., SPIM = 0, the logic level on the external
SS pin (Pin# 13), can be read via the SPR0 bit.
*Bits should contain the same values for master and slave devices.