Motorola M6800 - Manual (Page 8)

MPU-8
MPU Signal Descriptions
1. READ/WRITE (R/W):
This output line is used to signal all devices external to
the MPU that the MPU is in a read state (R/W = High) or a
write state (R/W = Low). The normal standby state of this
line when no external devices are being accessed is a high
state. This line is three-state. When three-state goes
high, this line enters the high impedance mode.
2. VALID MEMORY ADDRESS(VMA):
This output line, (when in the high state) tells all
devices external to the MPU that there is a valid address
in the address bus. For RAM's and ROM's, this line should
be ANDed with 12 clock and used as one of the enables. For
PIA's, this line should be ANDed with one of the PIA
address lines. This signal is not three-state.
3. DATA BUS ENABLE(DBE):
This signal will enable the data bus drives when in the
high state. This input is normally the phase 2 (12) clock.
During the high state, it will permit data to be output
during a write cycle. During an MPU read cycle, the data
bus drives will be disabled internally.
4. INTERRUPT REQUEST(IRQ):
This input from the PIA's requests that an interrupt
sequence be generated within the machine. The processor
will wait until it completes the current instruction that
is being executed before it recognizes the request. At that
time, if the interrupt mask bit in the Condition Code
Register is not set (interrupt masked), the machine will
begin an interrupt sequence. The Index Register, Program