Z8F640x/Z8F480x/Z8F320x/Z8F240x/Z8F160x
Z8 Encore!®
101
mitter and receiver sections, a Baud Rate (clock) Generator and a control unit. The trans-
mitter and receiver sections use the same clock.
During an SPI transfer, data is sent and received simultaneously by both the Master and
the Slave SPI devices. Separate signals are required for data and the serial clock. When an
SPI transfer occurs, a multi-bit (typically 8-bit) character is shifted out one data pin and an
multi-bit character is simultaneously shifted in on a second data pin. An 8-bit shift register
in the Master and another 8-bit shift register in the Slave are connected as a circular buffer.
The SPI shift register is single-buffered in the transmit and receive directions. New data to
be transmitted cannot be written into the shift register until the previous transmission is
complete and receive data (if valid) has been read.
SPI Signals
The four basic SPI signals are:
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•
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MISO (Master-In, Slave-Out)
MOSI (Master-Out, Slave-In)
SCK (SPI Serial Clock)
SS (Slave Select)
The following paragraphs discuss these SPI signals. Each signal is described in both Mas-
ter and Slave modes.
Master-In, Slave-Out
The Master-In, Slave-Out (MISO) pin is configured as an input in a Master device and as
an output in a Slave device. It is one of the two lines that transfer serial data, with the most
significant bit sent first. The MISO pin of a Slave device is placed in a high-impedance
state if the Slave is not selected. When the SPI is not enabled, this signal is in a high-
impedance state.
Master-Out, Slave-In
The Master-Out, Slave-In (MOSI) pin is configured as an output in a Master device and as
an input in a Slave device. It is one of the two lines that transfer serial data, with the most
significant bit sent first. When the SPI is not enabled, this signal is in a high-impedance
state.
Serial Clock
The Serial Clock (SCK) is used to synchronize data movement both in and out of the
device through its MOSI and MISO pins. In Master mode, the SPI’s Baud Rate Generator
creates the serial clock. The Master drives the serial clock out its own SCK pin to the
Slave’s SCK pin. When the SPI is configured as a Slave, the SCK pin is an input and the
clock signal from the Master synchronizes the data transfer between the Master and Slave
devices. Slave devices ignore the SCK signal, unless the SS pin is asserted.
PS017610-0404
Serial Peripheral Interface
ZILOG [ ZILOG, INC. ]
