SPI 4-Wire3-Wire

Real Time Embedded Systems

www.atomicrhubarb.com/embeddedLecture 1 – January 17, 2012

Topic

Section Topic

• Where in the books– Catsoulis chapter/page– Simon chapter/page– Zilog UM197 (ZNEO Z16F Series Flash Microcontroller Contest Kit User Manual)

– Zilog UM171 (ZiLOG Developer Studio II—ZNEO User Manual)

– Zilog PS220 (ZNEO Z16F Series Product Specification)

– Zilog UM188 (ZNEO CPU Core User Manual)

– Assorted datasheets

Synchronous Serial Buses

1-wire 2-wire 3-wire 4-wire

… but not in this order

Serial Buses

Communication between components Typically between integrated circuit components CPU to peripherals

Typically short distances (at least that was the plan)

Number of wires = number of SIGNAL wires Power & ground not included in the counting

Usually synchronous (data on one line and clock on another)

Lots of variety, lots of similarity.

The nice thing about standards is that there are so many of them to choose from.

- Andrew Tannenbaum (or was it Grace Hopper)

Why so many?

• 1,2,3,4 wire?

Why so many?

4-wire – SPI – Motorola 3-wire – Maxim IC

Microwire – National Semiconductor 2-wire – I2C – Phillips Semiconductor 1-wire – Dallas Semiconductor

SensorPath – National SemiconductorUNI/O - Microchip

SPI (4 wire)

Serial Peripheral Interface 4 wires (really 3 + 1 for each device) Low Cost Simple Intended for CPU to Peripheral

communication and control. Defined by Motorola.

SPI Specification

The Specification is hidden inside the HC08 microcontroller data sheets

Motorola Semiconductor is now Freescale Semiconductor.

Defines 4 signals

SPI Signals

SCLK – Serial Clock MISO – Master In, Slave Out MOSI – Master Out, Slave In SS – Slave Select

Based on shifting data between 2 shift registers.

Whats a shift register?

Shift Register

A cascade of flip flops sharing the same clock. The output of any one but the last flip-flop connected to the input of the next one in the chain. Resulting in a circuit that shifts by one position the one-dimensional "bit array" stored in it.

SPI Signals

• Swap 2 bytes, 1 bit at at time using the BRG as our serial clock.

• Multiple chips can beconnected to the master.

• Individual SS wires are necessary to select theone we want to talk to.

SPI Modes

4 timing modes Based on clock polarity and clock

phase.

Timing

It is common to see a reference to the SPI mode as CKPOL = 0, PHASE = 0

This is by far the most common SPI transfer.

SPI data

Is device specific Some need a single byte Some need multiple bytes Reads follow writes A dummy write is necessary to initiate

a read.

What?

• A dummy write is necessary to initiate a read.

To read THIS data we need to write something (anything) to shift the slave data to the master.

What if...• ... I only have• one SS pin?

?

SPI Devices

IO Expanders Clocks Temperature Sensors Non-volatile memory ... lots of things.

ESPI on the ZNEO

Enhanced Serial Peripheral Interface

ESPI• The Enhanced Serial Peripheral Interface (SPI) is a synchronous

interface allowing several SPI-type devices to be interconnected. SPI-compatible devices include EEPROMs, Analog-to-Digital Converters, and ISDN devices. Features of the SPI include:

• Full-duplex, synchronous, character-oriented communication• Four-wire interface• Data transfers rates up to a maximum of one-fourth the system

clock frequency• Error detection• Write and mode collision detection• Dedicated Baud Rate Generator

ESPI Pins

• PC2 - /SS• PC3 - SCK• PC4 - MOSI• PC5 - MISO

ESPI Block Diagram

What was that thing we could do to access multiple

SPI devices with onlyONE /SS pin?

ESPI

Several Modes Master controller of a single device Slave Device Master controller of multiple devices

ESPI Registers

ESPIDATA – Byte to be transmitted or byte received

ESPITDCR – Transmit Data Command Register ESPICTL – Configuration register ESPISTAT – Status Register ESPISTATE – State register ESPIMODE – More configuration ESPIBRH/L – Baud Rate

ESPIDATA

ESPICTL

SPISTAT

SPIMODE

SPIBRH,L

SPI recipe

SPI

1. Configure SPI pins (alternate function)2. Configure ESPISTL (phase, pol, etc)3. Configure ESPIMODE (ssio, etc)4. Set ESPI BRG5. Write the spi_read functions **6. Write the spi_write functions **7. Write device specific functions8. Configure interrupts if desired9. Write Interrupt Service Routine, set vector10. Configure SPI to generate interrupts

(set DIRQE bit of ESPICTL)10. Enable interrupts (EI)11. Enable SPI (set bit 0 of SPICTL)

** non-trivial

SPI Details

Data read/written is device specific.

DS1722

This is on the ZNEO developer board

DS1722

Get the datasheet !

Note

CE = active HIGH /SS = active LOW

Use the SSPO bit of the ESPIMODE register to control this.

DS1722 Error

3-wire

3-Wire/Microwire

The MICROWIRE protocol is essentially a subset of the SPI interface, specifically CPOL = 0 and CPHA = 0.

The Maxim 3-wire interface is found on some ICs from Maxim. The data flow to and from the device is multiplexed on one pin (DQ) while SPI needs two separate signals (MOSI, MISO).

ESPI ExampleExample_SpiDs1722

Read from the DS1722 SPI Temperature sensor