The Spi Protocol - FIGnition FUZE Hardware Reference Manual

In addition, the Amic serial Flash has two other connections we don't need. Pin 3 is a
write-protect pin and we connect it to 3.6v to ignore the /WP. Pin 7 is a /HOLD pin used to
suspend operations on the chip. We ignore it by connecting it to 3.6v.

5.2.2 The SPI Protocol

The Spi protocol is a widely-used minimal complexity serial hardware protocol. It has a
master/slave organisation (where the Master is usually the host computer) and uses 4 sig-
nals [Fig 5.3]:
• A Data out signal to send data from the master to a slave device.
• A Data in signal to send data from a slave device to the master.
• A Clock signal which sends one clock pulse for each bit sent and received.
• A Chip select signal which selects an SPI device.
To use SPI the clock signal must first be stopped. The computer activates chip select sig-
nal for the device it wants to talk to; then send as many bytes (or bits) of data the device
expects and finally it stops the clock signal and then deactivates the chip select [Fig 5.4].
There are alternative modes for when exactly each bit should be sent (e.g. when the clock
signal goes down or when it goes up), but the Flash chip expects mode 0 to be used as in
Fig 5.4.
Fig 5.2
Fig 5.3