ST92E163-EPB/US STMicroelectronics, ST92E163-EPB/US Datasheet - Page 78

• Manufacturer Part Number: ST92E163-EPB/US
• Description: KIT DEMO MASS STORAGE
• Manufacturer: STMicroelectronics
• Type: Microcontroller Programmerr

Specifications of ST92E163-EPB/US

• Contents: Programmer, Cable, Power Supply, Software, Manual and more
• For Use With/related Products: ST9 MCUs
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

ST92163R4 - RESET AND CLOCK CONTROL UNIT (RCCU)
CLOCK MANAGEMENT (Cont’d)
5.3.1 PLL Clock Multiplier Programming
The CLOCK1 signal generated by the oscillator
drives a programmable divide-by-two circuit. If the
DIV2 control bit in MODER is set (Reset Condi-
tion), CLOCK2, is equal to CLOCK1 divided by
two; if DIV2 is reset, CLOCK2 is identical to
CLOCK1. A CLOCK1 signal with a semiperiod
(high or low) shorter than 40ns is forbidden if the
divider by two is disabled.
When the PLL is active, it multiplies CLOCK2 by 6,
8, 10 or 14, depending on the status of the MX0 -1
bits in PLLCONF. The multiplied clock is then di-
vided by a factor in the range 1 to 7, determined by
the status of the DX0-2 bits; when these bits are
programmed to 111, the PLL is switched off.
Following a RESET phase, programming bits
DX0-2 to a value different from 111 will turn the
PLL on. After allowing a stabilisation period for the
PLL,
CLK_FLAG Register selects the multiplier clock
This peripheral contains a lock-in logic that verifies
if the PLL is locked to the CLOCK2 frequency. The
bit LOCK in CLK_FLAG register becomes 1 when
this event occurs.
The maximum frequency allowed for INTCLK is
25MHz for 5V operation, and 12MHz for 3V opera-
tion. Care is required, when programming the PLL
multiplier and divider factors, not to exceed the
maximum permissible operating frequency for
INTCLK, according to supply voltage.
The ST9 being a static machine, there is no lower
limit for INTCLK. However, below 1MHz, A/D con-
verter precision (if present) decreases.
5.3.2 CPU Clock Prescaling
The system clock, INTCLK, which may be the out-
put of the PLL clock multiplier, CLOCK2, CLOCK2/
16 or CK_AF, drives a programmable prescaler
which generates the basic time base, CPUCLK,
for the instruction executer of the ST9 CPU core.
This allows the user to slow down program execu-
tion during non processor intensive routines, thus
reducing power dissipation.
The internal peripherals are not affected by the
CPUCLK prescaler and continue to operate at the
full INTCLK frequency. This is particularly useful
78/230
setting
the
CSU_CKSEL
bit
in
the
when little processing is being done and the pe-
ripherals are doing most of the work.
The prescaler divides the input clock by the value
programmed in the control bits PRS2,1,0 in the
MODER register. If the prescaler value is zero, no
prescaling takes place, thus CPUCLK has the
same period and phase as INTCLK. If the value is
different from 0, the prescaling is equal to the val-
ue plus one, ranging thus from two (PRS2,1,0 = 1)
to eight (PRS2,1,0 = 7).
The clock generated is shown in
will be noted that the prescaling of the clock does
not preserve the 50% duty cycle, since the high
level is stretched to replace the missing cycles.
This is analogous to the introduction of wait cycles
for access to external memory. When External
Memory Wait or Bus Request events occur, CPU-
CLK is stretched at the high level for the whole pe-
riod required by the function.
Figure 37. CPU Clock Prescaling
5.3.3 Peripheral Clock
The system clock, INTCLK, which may be the out-
put of the PLL clock multiplier, CLOCK2, CLOCK2/
16 or CK_AF, is also routed to all ST9 on-chip pe-
ripherals and acts as the central timebase for all
timing functions.
CPUCLK
INTCLK
PRS VALUE
000
001
010
011
100
101
110
111
Figure
37, and it
VA00260