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2-16

FUNCTIONAL DESCRIPTION

2.3 Memory assignment

7220 Group User’s Manual

2.3.1 Internal RAM

The static RAM is assigned.

The internal RAM is used as a stack area for subroutine calls and interrupts as well as for storing data.

Both zero page and page 1 are used as a stack area. At reset, the page 1 is specified automatically.

Ordinary, the stack pointer is set to the highest address in the internal RAM of the page 1 during initialization

immediately after power on.

This stack pointer moves to lower addresses as the nesting depth increases; therefore, make sure the

subroutine nesting and interrupt levels to prevent the stored data destroying necessary data in the RAM.

When the stack page is specified “1,” if the value of stack pointer exceeds address 010016, the value of

stack page never change to “0” automatically. In this case, set the stack page value to “0” and set the stack

pointer value to the highest address by software.

2.3.2 I/O ports (addresses 00C016 to 00CD16)

Addresses 00C016 to 00CD16 are assigned to the ports, port direction registers and the port P3 output mode

control register. There are 5 ports: P0, P1, P2, P3 and P5. Ports P0, P1 and P2 are the 8-bit programmable

I/O ports. Port P3 consists of 5 bits. The low-order 3 bits (P30–P32) are the programmable I/O ports, and

the high-order 2 bits (P33 and P34) are the input ports.

For I/O ports P0, P1, P2 and P30–P32, input or output can be specified in bit units by setting the relevant

values to each port direction register.

To specify port bits as output pins, write “1” to the corresponding bit of the port direction register.

Conversely, write “0” to the corresponding bit to specify as an input pin.

For example, to use the even numbered bits of port P2 as output ports and the odd numbered bits as input

ports, write “5516 (010101012)” to address 00C516 (the port P2 direction register) at initialization.

Although Port P5 is an output port, it can be specified as the CRT output pins (R, G, B, OUT1) or as

general-purpose port (P52–P55) by setting each bit in the port P5 direction register.

When setting “0,” it is used for the CRT output pins (R, G, B, OUT1), and when setting “1,” it is used as

general-purpose output ports (P52–P55).

Note: Each port direction register default is “input” (port P5 is “CRT output”) immediately after reset

release.

Fig. 2.3.6 I/O setting example of port

00C416

00C516

Port P2 direction register

Bit

Port P2

Bit

Port P2

Port P2 direction register

Write “5516” to port P2 direction register

01010101

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