SAH-C515 SIEMENS [Siemens Semiconductor Group], SAH-C515 Datasheet - Page 34

• Manufacturer Part Number: SAH-C515
• Description: 8-Bit CMOS Microcontroller
• Manufacturer: SIEMENS [Siemens Semiconductor Group]

4.6
The C515-1R allows to protect the contents of the internal ROM against unauthorized read out. The
type of ROM protection (protected or unprotected) is fixed with the ROM mask. Therefore, the
customer of a C515-1R version has to define whether ROM protection has to be selected or not.
The C515-1R devices, which operate from internal ROM, are always checked for correct ROM
contents during production test. Therefore, unprotected as well as protected ROMs must provide a
procedure to verify the ROM contents. In ROM verification mode 1, which is used to verify
unprotected ROMs, a ROM address is applied externally to the C515-1R and the ROM data byte is
output at port 0. ROM verification mode 2, which is used to verify ROM protected devices, operates
different : ROM addresses are generated internally and the expected data bytes must be applied
externally to the device (by the manufacturer or by the customer) and are compared internally with
the data bytes from the ROM. After 16 byte verify operations the state of the P3.5 pin shows whether
the last 16 bytes have been verified correctly.
This mechanism provides a very high security of ROM protection. Only the owner of the ROM code
and the manufacturer who know the contents of the ROM can read out and verify it with less effort.
The behaviour of the move code instruction, when the code is executed from the external ROM, is
in such a way that accessing a code byte from a protected on-chip ROM address is not possible. In
this case the state of the EA pin is always latched with the rising edge of RESET and the byte
accessed will be invalid.
4.6.1 Unprotected ROM Mode
If the ROM is unprotected, the ROM verification mode 1 as shown in figure 4-3 is used to read out
the contents of the ROM. The AC timing characteristics of the ROM verification mode is shown in
the AC specifications (chapter 10).
Figure 4-3
ROM Verification Mode 1
ROM verification mode 1 is selected if the inputs PSEN, ALE, EA, and RESET are put to the
specified logic level. Then the 14-bit address of the internal ROM byte to be read is applied to the
port 1 and port 2 lines. After a delay time, port 0 outputs the content of the addressed ROM cell. In
ROM verification mode 1, the C515 must be provided with a system clock at the XTAL pins and
pullup resistors on the port 0 lines.
Semiconductor Group
ROM Protection for the C515
P1.0
P2.0 - P2.4
Port 0
Inputs: PSEN =
-
P1.7
ALE, EA = V
RESET = V
V
SS
IL2
IH
/
V
IH2
Address
4-6
Data
1 Out
1
Address
Data
External Bus Interface
2 Out
2
MCT03221
C515