ATtiny5 datasheet(100/168 Pages) ATMEL

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14.3.9

Collision Detection Exception

The TPI physical layer uses one bi-directional data line for both data reception and transmission.

A possible drive contention may occur, if the external programmer and the TPI physical layer

drive the TPIDATA line simultaneously. In order to reduce the effect of the drive contention, a

collision detection mechanism is supported. The collision detection is based on the way the TPI

physical layer drives the TPIDATA line.

The TPIDATA line is driven by a tri-state, push-pull driver with internal pull-up. The output driver

is always enabled when a logical zero is sent. When sending successive logical ones, the output

is only driven actively during the first clock cycle. After this, the output driver is automatically tri-

stated and the TPIDATA line is kept high by the internal pull-up. The output is re-enabled, when

the next logical zero is sent.

The collision detection is enabled in transmit mode, when the output driver has been disabled.

The data line should now be kept high by the internal pull-up and it is monitored to see, if it is

driven low by the external programmer. If the output is read low, a collision has been detected.

There are some potential pit-falls related to the way collision detection is performed. For exam-

ple, collisions cannot be detected when the TPI physical layer transmits a bit-stream of

successive logical zeros, or bit-stream of alternating logical ones and zeros. This is because the

output driver is active all the time, preventing polling of the TPIDATA line. However, within a sin-

gle frame the two stop bits should always be transmitted as logical ones, enabling collision

detection at least once per frame (as long as the frame format is not violated regarding the stop

bits).

The TPI physical layer will cease transmission when it detects a collision on the TPIDATA line.

The collision is signalized to the TPI access layer, which immediately changes the physical layer

to receive mode and goes to the error state. The TPI access layer can be recovered from the

error state only by sending a BREAK character.

14.3.10

Direction Change

In order to ensure correct timing of the half-duplex operation, a simple guard time mechanism

has been added to the physical layer. When the TPI physical layer changes from receive to

transmit mode, a configurable number of additional IDLE bits are inserted before the start bit is

transmitted. The minimum transition time between receive and transmit mode is two IDLE bits.

The total IDLE time is the specified guard time plus two IDLE bits.

The guard time is configured by dedicated bits in the TPIPCR register. The default guard time

value after the physical layer is initialized is 128 bits.

The external programmer looses control of the TPIDATA line when the TPI target changes from

receive mode to transmit. The guard time feature relaxes this critical phase of the communica-

tion. When the external programmer changes from receive mode to transmit, a minimum of one

IDLE bit should be inserted before the start bit is transmitted.

14.4 Access Layer of Tiny Programming Interface

The TPI access layer is responsible for handling the communication with the external program-

mer. The communication is based on message format, where each message comprises an

instruction followed by one or more byte-sized operands. The instruction is always sent by the

external programmer but operands are sent either by the external programmer or by the TPI

access layer, depending on the type of instruction issued.

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