Rainbow Electronics ATtiny10 - Manual (Page 101)

101
8127B–AVR–08/09
ATtiny4/5/9/10
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 accessible 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.
14.4.1 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 oper-
ands 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:
• Write messages. A write message is a request to write data. The write message is sent
entirely by the external programmer. This message type is used with the SSTCS, SST,
STPR, SOUT and SKEY instructions.
• Read messages. A read message is a request to read data. The TPI reacts to the request by
sending the byte operands. This message type is used with the SLDCS, SLD and SIN
instructions.
All the instructions except the SKEY instruction require the instruction to be followed by one byte
operand. The SKEY instruction requires 8 byte operands. For more information, see the TPI
instruction set on
page 102 .
14.4.2 Exception Handling and Synchronisation
Several situations are considered exceptions from normal operation of the TPI. When the TPI
physical layer is in receive mode, these exceptions are:
• The TPI physical layer detects a parity error.
• The TPI physical layer detects a frame error.
• The TPI physical layer recognizes a BREAK character.
When the TPI physical layer is in transmit mode, the possible exceptions are:
• The TPI physical layer detects a data collision.
All these exceptions are signalized to the TPI access layer. The access layer responds to an
exception by aborting any on-going operation and enters the error state. The access layer will
stay in the error state until a BREAK character has been received, after which it is taken back to
its default state. As a consequence, the external programmer can always synchronize the proto-
col by simply transmitting two successive BREAK characters.