SuperH (SH) 64-bit RISC Series SH-5 System Architecture, Volume ...

PRELIMINARY DATA
218 JTAG interface
Polling out a message
After the five status bits have been shifted out, the tool can determine that an
output message exists and then continues shifting a further 8, 16, 24 or 32 times
depending on which VOMx bits = ‘1’. The tool now has assembled the first bytes of
an output message.
The tool continues this process of shifting out just the IBF and VOMx bits, testing
the VOMx bits and then shifting data bits depending on which VOMx bits = ‘1’. The
detection of any VOMx bit = ‘0’ indicates the end of the message. Once the end of the
message has been reached, the tool does not need to shift any more data bits out.
Tool sending a message to SH-5
Input messages, longer than 4-bytes, are sent as one or more 4-byte segments
possibly followed by a segment containing fewer bytes. For each 4-byte segment of a
message, the tool shifts 37 bits into TDI with the VIMx status bits in the last
positions (the positions closest to TDI). For the last segment of a message containing
fewer than 4 bytes, the tool shifts 13, 21 or 29 bits into TDI with appropriate VIMx
bits indicating the number of valid bytes.
During message transfers, the first VIMx bit = '0' indicates the end of the input
message. When the length of an input message is a multiple of 4 bytes, the end of
the message is indicated by a 1-byte segment (13 bits) with all VIMx bits = ‘0’.
Flow control
The SH-5 JTAG tap controller has an input message buffer large enough to hold the
largest input message plus an output message buffer large enough to hold the
largest output message. The DBUS protocol allows one outstanding response in
each direction, so it is possible that the tool may want to send a response message
immediately following a new request. Flow control is needed to eliminate the
possibility of ever having a message in the input buffer overwritten before its has
been moved into the DM. This flow control is done by sending including an input
buffer status bit in the DR register, adjacent to the VOM3 bit. The tool can poll out
just five bits (if it can handle variable-length shifting), one of which determines
whether the input buffer can accept a new message and the other four determine
whether there is an output message pending.
D R A FT
There is no need for a tool input buffer status bit in the other direction. The tool will
only poll SH-5 when its input buffer has space for a new message from SH-5.
SuperH, Inc.
SH-5 System Architecture, Volume 3: Debug 05-SA-10003 v1.0