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CONNECTION WITH EXTERNAL DEVICES
7902 Group User’s Manual
3-5
3.1 Signals required for accessing external devices
The pins required for accessing external devices are explained below:
(1) In order to switch the processor mode from the single-chip mode to memory expansion or microprocessor
mode after reset with Vss applied to pin MD0, specify the function of the related pins by using the
following bits. (Reset with MD0 = Vss makes these pins to be programmable I/O port pins).
Pin RDY: RDY input select bit (bit 2 at address 5F16)
Pin ALE : ALE output select bit (bit 3 at address 5F16)
Pin
φ1 : clock φ1 output select bit (bit 7 at address 5E16)
pins HOLD, HLDA
: HOLD input, HLDA output select bit (bit 5 at address 5F16)
Pins CS0–CS3: CS0 output select bit (bit 7 at address 8016)
CS1 output select bit (bit 7 at address 8216)
CS2 output select bit (bit 7 at address 8416)
CS3 output select bit (bit 7 at address 8616)
(2) Switches between addresses and ports
The address/port switch bits switch between addresses A8–A23 and ports P0, P11 (See Figure 3.1.3.),
so that address output pins not needed can be used as programmable I/O port pins, according to the
access space required. (Refer to section “2.3 Access space.”)
(3) Pullups of P44 (CS0)–P47 (CS3)
While pin RESET = “L” level and after reset, pins P44 (CS0)–P47 (CS3) are pulled up. Therefore,
external pullup resistors are not needed for these pins.
Setting the pins P44–P47 pullup select bit to “1” removes the pullups. (See Figure 3.1.3.)
Either of the following settings also removes the pullups regardless of the contents of the pins P44–
P47 pullup select bit. (The bit’s contents remain unchanged).
Setting the port direction register corresponding to P44–P47 to “1” (output mode).
Setting the CS0–CS3 output select bits (bit 7 at addresses 8016, 8216, 8416, and 8616) corresponding
to P44 (CS0)–P47 (CS3) to “1” (CS0–CS3 outputs enabled).
Note that P44–P47 in the flash memory parallel I/O mode (MD1 and MD0 = Vcc), and CS0 (P44) in the
microprocessor mode (MD1 = Vss and MD0 = Vcc) will not be pulled up regardless of the pins P44–
P47 pullup select bit.
For details of the flash memory parallel I/O mode, refer to section “20.4 Flash memory parallel
I/O mode.”