SN54LVT8980, SN74LVT8980

EMBEDDED TEST-BUS CONTROLLERS

IEEE STD 1149.1 (JTAG) TAP MASTERS WITH 8-BIT GENERIC HOST INTERFACES

SCBS676C – DECEMBER 1996 – REVISED AUGUST 1997

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POST OFFICE BOX 655303

ready output

The ready output (RDY) from the host interface can be used, where the selected microprocessor/microcontroller

supports it, to insert wait or hold states back to the host. If a host-requested access cannot be performed

immediately, RDY goes inactive (low) during that given access. When the condition blocking the access clears,

RDY goes active (high) and the eTBC grants the requested access. Alternatively, where such

hardware-generated hold or wait states are not supported in the selected microprocessor/microcontroller host,

the eTBC status and/or command registers can be polled to determine its readiness to grant a given read or

write access.

Conditions that cause a host access to be blocked (and RDY to become inactive) are limited to the following:

register generates RDY inactive; this condition clears, RDY goes active, and the requested access

completes, when the TDI buffer is no longer empty.

(bit 6, TDOS), a requested write to TDO-buffer register generates RDY inactive; this condition clears, RDY

goes active, and the requested access completes, when the TDO buffer is no longer full or the TDO-buffer

reset completes, as applicable.

of the command register, a requested write to command, configurationA, configurationB, or counter

registers generates RDY inactive. This condition clears, RDY goes active, and the requested access is

complete, when the previously specified command finishes. The sole exception is the writing of a logic 1

into the software reset (bit 7, SWRST) bit of the command register, which is never blocked.

the number of writes to TDO-buffer register may not exceed, by more than 5, the number of reads to

TDI-buffer register. A write to TDO-buffer register that does exceed this limit is blocked, and generates RDY

inactive, indefinitely; the TDI-buffer register must be read before another write to TDO-buffer register.