DS1221
6 of 8
SECURITY OPTION
When activated by Dallas Semiconductor, the security option prevents unauthorized access. A sequence
of events must occur to gain access to the memories (Figure 3). First, a dummy read cycle or a 200 ns
active low reset pulse is executed to initialize the sequence. Second, a 64-bit access code must be
consecutivel y written to the DS1221 using the write enable signal ( WE ), the chip enable si gnal (CE ), and
the data input/output signal (DQ). The code is written to the DS1221 without regard to the address.
Actual RAM locations are not written, as the security option is intercepting the data path until access is
granted. Instead, a special 64-bit write only register is written. Following the 64 write cycles, the register
is compared to a 64-bit pattern uniquel y defined by the user and programmed into the DS1221 by Dallas
Semiconductor at the time of manufacture. This pattern can only be interrogated by an intelligent
controller within the DS1221 and cannot be read by the use r. If a re ad cycle occurs befo re 64 w rite cycles
are completed, the security sequence is aborted. When a correct match for 64 bits is received, the third
part of the securit y sequence begins by reading a 64-bit read only register. This register consists of 64 bits
also defined by the user and programmed into the DS1221 by Dallas Semiconductor at the time of
manufacture. For each of the 64 read cycles, 1 bit of the user-defined read only register is driven onto the
DQ line. This phase also requires that the 64 read cycles be consecutive. The data being read from the
read only register can be used by software to determine if the DS1221 will be permitted to be used with
that particular system. After the 64th read cycle has been executed the DS1221 is unlocked and all
subsequent memory cycles will be passed through and will become actual memory accesses based upon
address inputs. If VCC falls below 4.5 volts or the reset line is driven low, the entire security sequence
must be executed again in order to access memory locations.
NOTE:
Contact Dallas Semiconductor sales office for code assignments.
SECURITY OPTION
AC E LECTRICAL CHARACT ERIS TIC S (0°C to 70°C; VCC = 5V ± 10%)
PARAMETER SYMBOL MIN TYP MAX UNITS NOTES
Read Cycle Time tRC 250 ns
CE Access Time tCO 200 ns
RD Access Time tOE 100 ns
CE to Output Low Z tCOE 10 ns
RD to Output Low Z tOEE 10 ns
CE to Output High Z tOD 100 ns
RD to Output High Z tODO 100 ns
Read Recovery tRR 50 ns
Write Cycle tWC 250 ns
Write Pulse Width tWP 170 ns
Write Recovery tWR 50 ns
Data Setup tDS 100 ns
Data Hold Time tDH 0ns
CE Pulse Width tCW 170 ns
Reset Pulse Width tRST 200 ns