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CHAPTER 4 INTERFACES
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4.1.1 Bus Operation Control Pins
(1) OE_B pin
OE_B is an output enable signal input pin for AD31 through AD0 and PAR3 through PAR0 of the
μ
PD98405. While a low level is input to the OE_B pin, the
μ
PD98405 executes output from AD31 through
AD0 and PAR3 through PAR0. When a high level is input as this signal, the
μ
PD98405 sets AD31 through
AD0 and PAR3 through PAR0 to high impedance. This operation is performed independently of the
internal state of the
μ
PD98405. For example, if a high level is input to the OE_B pin while the
μ
PD98405 is
operating as the master and performing DMA write, the
μ
PD98405 immediately performs output to the
high-impedance bus. Internally, however, the
μ
PD98405 continues the DMA cycle. Consequently, data
being transferred is lost.
This signal is used to forcibly abort the bus operation of the
μ
PD98405 from an external source. Fix the
OE_B signal to low level if there is no special case where output to the bus must be forcibly set to high
impedance.
(2) ERR_B pin
The ERR_B pin enables the device controlling the system bus to input a signal to stop bus operation by the
μ
PD98405 if an error is detected on the bus. If a low level is input to this pin during DMA transfer, the
μ
PD98405 first stops DMA transfer then immediately deactivates the ATTN_B signal. In addition, the
μ
PD98405 sets the SBE (system bus error) bit of the GSR register to 1, then issues an interrupt to the host
provided the interrupt is not masked. A system bus error prevents the
μ
PD98405 from continuing DMA
transfer (master operation), thus disabling transmission/reception. Because the slaves can still be
accessed, however, a software or hardware reset must be executed to recover from the error.
4.1.2 Parity Check Function
The
μ
PD98405 has parity signal I/O pins PAR3 through PAR0 on the bus interface and, therefore, is capable
of outputting/checking a parity signal.
The user can enable or disable the parity function, select byte or word parity, and odd or even parity by
setting the GMR register accordingly.