When the port latch contains a 0, all pFETS in Figure 4.
are off while the nFET is turned on. When the port latch
makes a 0-to-1 transition, the nFET turns off. The strong
pFET, T1, turns on for two oscillator periods, pulling the
output high very rapidly. As the output line is drawn high,
pFET T3 turns on through the inverter to supply the IOH
source current. This inverter and T form a latch which
holds the 1 and is supported by T2.
When Port 2 is used as an address port, for access to
external program of data memory, any address bit that
contains a 1 will have his strong pullup turned on for the
entire duration of the external memory access.
When an I/O pin son Ports 1, 2, or 3 is used as an input,
the user should be aware that the external circuit must
sink current during the logical 1-to-0 transition. The
maximum sink current is specified as ITL under the D.C.
Specifications. When the input goes below
approximately 2 V, T3 turns off to save ICC current. Note,
when returning to a logical 1, T2 is the only internal
pullup that is on. This will result in a slow rise time if the
user’s circuit does not force the input line high.
XTAL1 and XTAL2 are the input and output respectively,
of an inverting amplifier which is configured for use as an
on-chip oscillator, as shown in Figure 5. Either a quartz
crystal or ceramic resonator may be used.
Figure 5. Crystal Oscillator.
To drive the device from an external clock source,
XTAL1 should be driven while XTAL2 is left
unconnected as shown in Figure 6. There are no
requirements on the duty cycle of the external clock
signal, since the input to the internal clocking circuitry is
through a divide-by-two flip-flop, but minimum and
maximum high and low times specified on the Data Sheet
must be observed.
Figure 6. External Drive Configuration.
TSC80C51 with Secret ROM
TEMIC offers TSC80C31/80C51 with the encrypted
secret ROM option to secure the ROM code contained in
the TSC80C31/80C51 microcontrollers.
The clear reading of the program contained in the ROM
is made impossible due to an encryption through several
random keys implemented during the manufacturing
The keys used to do such encryption are selected
randomwise and are definitely different from one
microcontroller to another.
This encryption is activated during the following phases :
– Everytime a byte is addressed during a verify of the
ROM content, a byte of the encryption array is
– MOVC instructions executed from external program
memory are disabled when fetching code bytes from
– EA is sampled and latched on reset, thus all state
modification are disabled.
For further information please refer to the application
note (ANM053) available upon request.
Rev. E (14 Jan.97)