Datasheet

 2018 Microchip Technology Inc. Data Sheet Complete DS40002060A-page 108
ATtiny4/5/9/10
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 example, 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 single 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.
15.3.10 Direction Change
In order to ensure correct timing of the half-duplex operation, a simple guard time mechanism has been added to the phys-
ical 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 communication. 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.
15.4 Access Layer of Tiny Programming Interface
The TPI access layer is responsible for handling the communication with the external programmer. 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.
The TPI access layer controls the character transfer direction on the TPI physical layer. It also handles the recovery from
the error state after exception.
The Control and Status Space (CSS) of the Tiny Programming Interface is allocated for control and status registers in the
TPI access Layer. The CSS consist of registers directly involved in the operation of the TPI itself. These register are acces-
sible using the SLDCS and SSTCS instructions.
The access layer can also access the data space, either directly or indirectly using the Pointer Register (PR) as the
address pointer. The data space is accessible using the SLD, SST, SIN and SOUT instructions. The address pointer can
be stored in the Pointer Register using the SLDPR instruction.
15.4.1 Message format
Each message comprises an instruction followed by one or more byte operands. The instruction is always sent by the
external programmer. Depending on the instruction all the following operands are sent either by the external programmer
or by the TPI.
The messages can be categorized in two types based on the instruction, as follows: