AVR-CDC-SPI.pdf

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Virtual COM Port over Low-Speed USB

AVR-CDC

Home About CDC-232 CDC-IO CDC-SPI Driver Download FAQ Reference

About

CDC-SPI

CDC-232

Usage

The SPI (Serial Peripheral Interface Bus) is a standard for a synchronous serial communication between devices.

Devices communicate in master-slave mode where the master controls the data transfer. After generating eight shift-

clocks by the master, byte data are exchanged with the slave. SPI bus is adapted for many devices such as

microcontrollers, EEPROM, sensors..

Schematics

CDC-IO

Usage

Schematics

Sample Program

CDC-SPI

Usage

Schematics

SPI Data Exchange

Sample Program

The CDC-SPI is a damn COM-SPI bridge over the low-speed CDC protocol. The AVR receives byte data stream from

the host PC via USB and sends them to MOSI line. At the same time, it returns data stream received from MISO.

Driver

CDC Protocol

Usage

Windows

Mac OS

The installation is the same as CDC-232. The baudrate is arbitrary. Send binary data from terminal software or from

your application on PC through the virtual COM port.

Linux

FAQ

Device Recognition

The CDC-SPI has two slave-selectors (ATtiny45 version has one selector). /SS0 is controlled by SendBreak signal

from the host, and /SS1 by DTR. It may control four slave devices if these lines are decoded.

Communication Problem

Others

Download

CDC-SPI Implementation on ATtiny45

Reference

Schematic s

The registers on SPI lines are for short protection. Use higher value(3-10Kohms) when connecting to the slave with

different Vcc.

The Reset pin is used as the slave selector in ATtiny45 version. It disables further SPI downloading.

CDC-SPI for ATtiny45-20

The clock speed is selectable in ATtiny461 and ATtiny2313 version.

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CDC-SPI for ATtiny2313-20

There are four SPI modes (clock polarity and phase) supported in ATmega version. See the datasheet in detail.

CDC-SPI for ATmega8/48/88-20

Sample Program

It's sometimes necessary to test the actual SPI protocol before embedding a device into system. This is a simple test

program to check the SPI sequence. It just switches the /SS lines and send/receive data through virtual COM port.

This is an example to access to EEPROM 93C46 (AT93C46D).

Testing EEPROM (93C46)

C:\cdcspi.2010-02-06\spitest\release>spitest 13

CDC-SPI Tester by Osamu Tamura

com13 < ss0 0

com13 < ss0 1

com13 < 2 60 ; EWEN

com13 > FF FF

com13 < ss0 0

com13 < ss0 1

com13 < 3 1 0 ; READ $01

com13 > FF FF FF ; = FF

com13 < ss0 0

com13 < ss0 1

com13 < 2 81 73 ; WRITE $01 <= 73

com13 > FF FF FF

com13 < ss0 0

com13 < ss0 1

com13 < 3 1 0 ; READ $01

com13 > FF FF 73 ; = 73

com13 < ss0 0

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com13 < ss0 1

com13 < 3 81 ; ERASE $01

com13 > FF FF

com13 < ss0 0

com13 < ss0 1

com13 < 3 1 0 ; READ $01

com13 > FF FF FF ; = FF

com13 < ss0 0

com13 < ss0 1

com13 < 2 0

; EWDS

com13 > FF FF

com13 < ss0 0

com13 < q ; quit

One of the most familiar SPI implementations is the AVR's serial downloader. With this SPI bridge, you can easily

program the AVR chips. This is an example of the AVR writer. It enables writing to the SPI programmable

ATtiny/ATmega devices (AT90*** are not supported).

Downloading to ATmega8

C:\cdcspi.2010-02-06\spiprog\release>spiprog -c13 -f8FFF spimega8.hex

CDC-SPI AVR Programmer Ver. 0.5 by Osamu Tamura

Signature: 1E-93-07 (Flash 8KB, Page 32words)

LockBits: FF

FuseBits: FF-8F-FF

2226 bytes loaded.

Device erased.

Writing ................................... done.

Verifying ................................... done.

Writing FuseBits [8F-FF] ... done.

2240 bytes programmed successfully.

Since the CDC-SPI was designed as a general purpose COM-SPI bridge, it takes 8 bytes transfer to program one byte

flash data. If you are just interested in AVR downloading, consider other projects (e.g. USBasp).

All the source code, hex files, schematics and sample programs are in download page.

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