SAB80C515A-5 Datasheet, PDF (22/108 Pages) Siemens Semiconductor Group – 8-Bit CMOS Single-Chip Microcontroller Family

Memory Organization
A hardware protection is done by an unsymmetric latch at XMAP0-bit. A unintentional disabling of
XRAM could be dangerous since indeterminate values could be read from external bus. To avoid
this the XMAP-bit is forced to ’1’ only by reset. Additional during reset an internal capacitor is loaded.
So the reset state is a disabled XRAM. Because of the load time of the capacitor XMAP0-bit once
written to ’0’ (that is, discharging capacitor) cannot be set to ’1’ again by software. On the other hand
any distortion (software hang up, noise,...) is not able to load this capacitor, too. That is, the stable
status is XRAM enabled. The only way to disable XRAM after it was enabled is a reset.
The clear instruction for the XMAP0-bit should be integrated in the program initialization routine
before XRAM is used. In extremely noisy systems the user may have redundant clear instructions.
The control bit XMAP1 is relevant only if the XRAM is accessed. In this case the external RD and
WR signals at P3.6 and P3.7 are not activated during the access, if XMAP1 is cleared. For debug
purposes it might be useful to have these signals and the Ports 0, 2 available. This is performed if
XMAP1 is set.
3.4.3 Behaviour of Port0 and Port2
The behaviour of Port 0 and P2 during a MOVX access depends on the control bits in register
SYSCON and on the state of pin EA. The table 3-3 lists the various operating conditions. It shows
the following characteristics:
a) Use of P0 and P2 pins during the MOVX access.
Bus: The pins work as external address/data bus. If (internal) XRAM is accessed, the data
written to the XRAM can be seen on the bus in debug mode.
I/0: The pins work as Input/Output lines under control of their latch.
b) Activation of the RD and WR pin during the access.
c) Use of internal or external XDATA memory.
The shaded areas describe the standard operation as each 80C51 device without on-chip XRAM
behaves.
Semiconductor Group
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