C8051F50x-F51x
19.3. Clock Multiplier
The Clock Multiplier generates an output clock which is 4 times the input clock frequency scaled by a pro-
grammable factor of 1, 2/3, 2/4 (or 1/2), 2/5, 2/6 (or 1/3), or 2/7. The Clock Multiplier’s input can be
selected from the external oscillator, or the internal or external oscillators divided by 2. This produces three
possible base outputs which can be scaled by a programmable factor: Internal Oscillator x 2, External
Oscillator x 2, or External Oscillator x 4. See Section 19.1 on page 165 for details on system clock selec-
tion.
The Clock Multiplier is configured via the CLKMUL register (SFR Definition 19.5). The procedure for con-
figuring and enabling the Clock Multiplier is as follows:
1. Reset the Multiplier by writing 0x00 to register CLKMUL.
2. Select the Multiplier input source via the MULSEL bits.
3. Select the Multiplier output scaling factor via the MULDIV bits
4. Enable the Multiplier with the MULEN bit (CLKMUL | = 0x80).
5. Delay for >5 µs.
6. Initialize the Multiplier with the MULINIT bit (CLKMUL | = 0xC0).
7. Poll for MULRDY => 1.
Important Note
: When using an external oscillator as the input to the Clock Multiplier, the external source
must be enabled and stable before the Multiplier is initialized. See “19.4. External Oscillator Drive Circuit”
The Clock Multiplier allows faster operation of the CIP-51 core and is intended to generate an output fre-
quency between 25 and 50 MHz. The clock multiplier can also be used with slow input clocks. However, if
the clock is below the minimum Clock Multiplier input frequency (FCM
min
), the generated clock will consist
of four fast pulses followed by a long delay until the next input clock rising edge. The average frequency of
the output is equal to 4x the input, but the instantaneous frequency may be faster. See Figure 19.2 below
for more information.
if F
CM
in
>= F
CM
min
F
CM
in
F
CM
out
if F
CM
in
< F
CM
min
F
CM
in
F
CM
out
Figure 19.2. Example Clock Multiplier Output
170
Rev. 1.1
SILABS [ SILICON LABORATORIES ]
