PBM 990 80
External LPO clock requirements
If an external LPO square wave clock is available it can be applied directly on the LPOXIN pin, with LPOSELECT and
LPOXOUT pins left open. The clock frequency shall be either 3.2kHz, 32.000 kHz or 32.768 kHz. After power-up, the applied
clock is always divided down internally to 3.2 kHz by a programmable divider.
Input frequency
3.200 kHz
32.000 kHz
32.768 kHz
Parameter
Input frequency accuracy
Input jitter
Input frequency duty cycle
Input frequency accuracy
Input jitter
Input frequency duty cycle
Input frequency accuracy
Total litter: Input jitter + 30 µs inherent jitter
Input frequency duty cycle
Symbol
f
ACC1
t
J1
f
DUTY1
f
ACC2
t
J2
f
DUTY2
f
ACC3
t
JTOT
f
DUTY3
Max
+250
10
45
55
-250
+250
10
30
70
See Note See Note
See Note
30
70
Min
-250
Unit
ppm
µs
%
ppm
µs
%
ppm
µs
%
Note on LPOXIN pin:
When using the 32.768 kHz option it should be noted that the clock will be divided internally by 10.24 to achieve the required
3.2 kHz LPO frequency. This division factor implies that there will be an inherent jitter of max 30 µs on the resulting 3.2 kHz
clock. This 30 µs jitter will affect the minimum low power stop mode time (input clock stopped), that can be achieved for a
specific frequency accuracy f
ACC3
.
In order to fulfil the max real time error corresponding to the Bluetooth LPO clock requirement of ± 250 ppm and max 10 µs
jitter the minimum stop mode time specification must be weighed against the cost to achieve a certain frequency accuracy
f
ACC3.
The minimum low power mode stop time that can be used to fulfil the maximum real time error corresponding to the
Bluetooth LPO clock requirement is calculated as follows:
t
STOPMIN
[seconds] = (t
JTOT
[µs] – 10 µs) / (250 ppm – f
ACC3
[ppm])
Example:
A 32.768 kHz clock with max f
ACC3
= ±180 ppm and input jitter of 1 µs will require a minimum low power mode stop time of
(31-10)/(250-180) = 0.3 s to fulfil the maximum real time error corresponding to the Bluetooth LPO clock requirement.
LPO crystal and oscillator requirements
The internal LPO clock oscillator can be used when no external LPO clock is available. It works with an external crystal at
either 32.000 or 32.768 kHz. The crystal is connected between LPOXIN and LPOXOUT pins. Each pin shall have a capacitor
connected to V
SSCORE
. The capacitor value is determined by the selected crystal.
The oscillator is enabled by tying the LPOSELECT pin to logic high. Note that there is an internal pull-down resistor on the
LPOSELECT pin, which means that there will be a current flow from V
DDIO4
down to the tie-off circuit for LPOSELECT when the
oscillator is enabled.
After power-up the software needs to set up the appropriate internal divider ratio depending on the frequency of the external
crystal in order to generate the expected 3.2 kHz internal LPO clock. The crystal and the external capacitors shall be selected
such that the combined crystal + oscillator requirements stated in the requirements table below are fulfilled.
Parameter
Operating ambient temperature range
32.000 kHz Crystal oscillator accuracy, including total
frequency tolerance and stability from nominal frequency
over the complete temperature range.
32.768 kHz Crystal oscillator accuracy, including total
frequency tolerance and stability from nominal frequency
over the complete temperature range.
Note on LPO crystal oscillator:
Symbol
T
AMB
f
ACC1
f
ACC2
Min
-40
-250
See Note
Typ
+25
Max
+85
+250
See Note
Unit
o
C
ppm
ppm
When using the 32.768 kHz crystal option it should be noted that the oscillator frequency will be divided internally by 10.24 to
achieve the required 3.2 kHz LPO frequency. This division factor implies that there will be an inherent jitter of max 30 µs on the
resulting 3.2 kHz clock. This 30 µs jitter will affect the minimum low power stop mode time (input clock stopped), that can be
achieved for a specific frequency accuracy f
ACC2
.
In order to fulfil the max real time error corresponding to the Bluetooth LPO clock requirement of ± 250 ppm and max 10 µs
jitter the minimum stop mode time specification must be weighed against the cost to achieve a certain frequency accuracy
f
ACC2.
The minimum low power mode stop time that can be used to fulfil the maximum real time error corresponding to the
Bluetooth LPO clock requirement is calculated as follows:
t
STOPMIN
[seconds] = (30 µs – 10 µs) / (250 ppm – f
ACC2
[ppm])
6
ERICSSON [ ERICSSON ]
