ST6391 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'ST6391' PDF : 68 Pages View PDF
ST6391,92,93,95,97,99
ON-CHIP CLOCK OSCILLATOR
The internal oscillator circuit is designed to require
a minimum of external components. A crystal
quartz, a ceramic resonator, or an external signal
(provided to the OSCin pin) may be used to gener-
ate a system clock with various stability/cost trade-
offs. The typical clock frequency is 8MHz. Please
note that different frequencies will affect the operation
of those peripherals (D/As, SPI) whose reference fre-
quencies are derived from the system clock.
The different clock generator options connection
methods are shown in Figures 25 and 26. One ma-
chine cycle takes 13 oscillator pulses; 12 clock
pulses are needed to increment the PC while and
additional 13th pulse is needed to stabilize the inter-
nal latches during memory addressing. This means
that with a clock frequency of 8MHz the machine cy-
cle is 1.625µs.
The crystal oscillator start-up time is a function of
many variables: crystal parameters (especially RS),
oscillator load capacitance (CL), IC parameters, am-
bient temperature, and supply voltage.It must be ob-
served that the crystal or ceramic leads and circuit
connections must be as short as possible. Typical
values for CL1 and CL2 are in the range of 15pF to
22pF but these should be chosen based on the crys-
tal manufacturers specification. Typical input capaci-
tance for OSCin and OSCout pins is 5pF.
The oscillator output frequency is internallydivided
by 13 to produce the machine cycle and by 12 to
produce the Timer and the Watchdog clock. A byte
cycle is the smallest unit needed to execute any
operation (i.e., increment the program counter). An
instruction may need two, four, or five byte cycles
to be executed (See Table 7).
Figure 25. Clock Generator Option (1)
Figure 26. Clock Generator Option (2)
Table 7. Intructions Timing with 8MHz Clock
Instruc tion Type
Cycles
Branch if set/reset
Branch & Subroutine Branch
Bit Manipulation
Load Instruction
Arithmetic & Logic
Conditional Branch
Program Control
5 Cycles
4 Cycles
4 Cycles
4 Cycles
4 Cycles
2 Cycles
2 Cycles
Execution
Time
8.125µs
6. 50µs
6. 50µs
6. 50µs
6. 50µs
3. 25µs
3. 25µs
Figure 27. OSCin, OSCout Diagram
24/64
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