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ST9 USER GUIDE

4.5 SERIAL PERIPHERAL INTERFACE

4.5.1 Description

The SPI is a synchronous input-output port that you can configure in various modes, including

S-Bus. It can have many more uses, of which two are considered here: interfacing with serial-

access EEPROMs and interfacing with a liquid-crystal display.

The main block of the SPI is an 8-bit shift register which can be read or written in parallel

through the internal data bus of the ST9, and that can shift the data in or out on two separate

pins, named SDI and SDO, respectively. The serial transfer is initiated with a write to the SPI

Data Register (SPIDR). Data is input and output simultaneously with each most significant bit

being output on SDO while the level at SDI becomes the least significant bit. Each time a bit

is transferred, a pulse is output at the SCK pin. When eight bits are transferred, eight pulses

have been sent on SCK, and the process stops. If the proper bits are set in the SPI Control

– Transfers are started by writing a byte into the SPI data register

– Data is input and output at the same time

– Data is input and output with the most-significant bit being sent first

– Eight clock pulses are output on the SCK pin synchronously with bit shifting

– An interrupt request can be issued on end of transmission

The SPI is configured with the SPICR register. Four bits of this register are of special interest

in the applications detailed here: these are the CPOL-CPHA pair, and the SPR1-SPR0 pair.

The CPOL-CPHA pair defines the polarity and phase of the clock pulses. This allows them to

adapt to the external device. CPOL selects between rising edge or falling edge as the active

transition, and CPHA selects whether the first active edge is the first edge of the pulse train or

the second one.

The SPR1-SPR0 pair selects one of four different transfer speeds.

4.5.2 Static Liquid-Crystal Display Interface Example

This example uses a static liquid-crystal display that shows a simple number. This can be the

voltage in a digital voltmeter, or any 3 1/2 digit value. A very simple demonstration program is

supplied in the Companion Software. Some of the routines are reused in the bar code reader

described later.

A static liquid-crystal display is composed of a glass back-panel, and a set of front-panel elec-

trodes printed on a glass front-panel. The space between both panels is filled with a liquid-

crystal solution. The electrodes are transparent, and the panels may be fitted with a combina-

tion of polarisers, so that the whole display, unpowered, is either transparent or opaque ac-

cording to the selected polariser combination. When a difference of potential is applied be-

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