SX8801IRXXMLTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SX8801IRXXMLTRT' PDF : 135 Pages View PDF
XE8801A – SX8801R
7.6 Clock source selection
There are three possible clock sources available for the CPU clock. The RC clock is always selected after power-
up or after Sleep mode. The CPU clock selection is done with the bit CpuSel in RegSysClock (0= RC clock, 1= 32
kHz from Xtal if EnableXtal =1, ExtClk = 0 and EnExtClk = 0 else external clock).
Switching from one clock source to another is glitch free.
The next table summarizes the different clock configurations of the circuit:
Mode
name
Clock Sources
Clock targets
Cpuck Note 1
High
Prescaler
CpuSel=0 CpuSel=1
Clock
input
Low
Prescaler
Clock
input
Sleep
Xtal
RC
0
0
0
Off
0
0
1
Off
0
1
0
RC
RC + Xtal
0
1
1
RC
Off
Off
Off
Xtal
Off
Xtal
RC
RC
High presc.
Xtal
RC
Xtal
External
1
0
X
Off
External
Off
Off
RC
+
External
1
1
X
RC
External
RC
High presc.
Table 7-12: Table of clocking modes.
Note 1: The CPU clock can be divided by using the freq instruction (see coolrisc instruction set)
Switching from one clock source to another and stopping the unused clock source must be performed using 3
MOVE instructions to RegSysClock. First select the new clock source, secondly change the CpuSel bit and finally
stop the unused one.
7.7 RegSysMisc Description
The RCOnPA0 bit in RegSysMisc can be used to enable the RC oscillator on an event external to the circuit. If
RCOnPA0 is 1, the RC oscillator is enabled (EnableRC bit is set to 1) as soon as the value on port A pin PA[0] is
equal to 1. The Port A pin can be debounced (see Port A documentation).
Bit DebFast in the RegSysMisc register allows to chose the debouncer clock between 256Hz and 8kHz (DebFast
= 0 and DebFast = 1 respectively). The Debouncer clock is used to debounce PA inputs (see Port A
documentation).
Bit OutputCkXtal allows to show the Xtal clock on PB[3]. The EnableXtal bit must be set to 1 else PB[3] equals 0
(see port B documentation to set up the Port B).
Bit OutputCpuCk allows to show the CpuClock on PB[2] (see Port B documentation).
© Semtech 2005
www.semtech.com
7-8
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