HT80C51 User Manual
Memory Organization – MOVC protection (option HT80C51_CPU_MOVCP)
Handshake Solutions
© Philips Electronics N.V. 2005
Page 17 of 132
In the Power Down mode of operation, VDD can be reduced to a level that is still sufficient for logic
and RAM to keep their contents. Care must be taken, however, to ensure that VDD is not reduced
before the Power Down mode is invoked, and that VDD is restored to its normal operating level, be-
fore the Power Down mode is terminated. The reset that terminates Power Down also should switch
on the core clock again. The reset should not be activated before VDD is restored to its normal operat-
ing level, and must be held active long enough to allow an oscillator to restart and stabilize.
2.5.7.
External RAM Page XRAMP (option HT80C51_CPU_XRAMP)
The MOVX-instruction comes in two flavors: MOVX @DPTR and MOVX @Ri. For the second version
(MOVX @Ri) the contents of one index register Ri specify the lower half of the 16bit address for the
access to the external data memory. The upper half is not specified by the instruction, but the SFR
XRAMP supplies it. In other words, the external data memory is divided into pages of 256bytes, with
XRAMP selecting the page and @Ri addressing within this page.
XRAMP
External RAM Page
addr = C9H
reset value = 00H
bits
7
6
5
4
3
2
1
0
XRAMP
2.6.
MOVC protection (option HT80C51_CPU_MOVCP)
This optional feature protects a memory region in program memory from being read out by a program
outside this region. Thus any MOVC instruction, that is executed outside the protected region and tries
to access the protected region, will return the value 00H instead of the real memory content. The pro-
tection is only one-way, so a protected program can read the complete program memory area.
The protected region is defined to start at address 0000H. The upper limit of the protected region is
defined by static inputs ht80c51_movcp_uaddr_i, so the protected code memory region is from
0000H <= addr < ht80c51_movcp_uaddr_i. The value of this upper limit is under control of the
customer, but must not change during execution. Usually it is hard wired to a constant value.
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