pca8885 NXP Semiconductors, pca8885 Datasheet - Page 7

• Manufacturer Part Number: pca8885
• Description: Capacitive 8-channel Proximity Switch With Auto-calibration And Very Low Power Consumption
• Manufacturer: NXP Semiconductors

NXP Semiconductors
PCA8885
Objective data sheet
When the voltage of an RC combination falls below the level V
comparator output changes. The logic following the comparators determines which
comparator switched first. If the reference comparator switched first, then a pulse is given
on CUP. If the sensor comparator switched first, then a pulse is given on CDN.
illustrates the functional principle of the PCA8885.
The pulses control the charge on the external capacitors C
Every time a pulse is given on CUP, the capacitor C
source (I
small increment. Likewise when a pulse occurs on CDN, capacitor C
through a current sink (I
to fall by a small decrement. The voltage on C
that causes the capacitance attached to the input pin INx to be discharged more quickly.
This arrangement constitutes a closed loop control system, that constantly tries to
equalize the discharge time (t
equilibrium state, the discharge times are nearly equal and the pulses alternate between
CUP and CDN.
The counter following this logic counts the pulses CUP or CDN respectively. The counter
is reset every time the pulse sequence changes from CUP to CDN or the other way round.
The outputs OUT0 to OUT7 are only activated when enough consecutive CUP or CDN
pulses occur. Low-level interference or slow changes in the input capacitance do not
cause the output to switch.
Various measures, such as asymmetrical charge and discharge steps, are taken to
ensure that the output switches off correctly. A special start-up circuit ensures that the
device reaches equilibrium quickly when the supply is attached.
The sampling rate (f
oscillator frequency can be adjusted within a specified range by programming the
CLKREG register (see
The status of the output signals OUT0 to OUT7 is stored in the SENS register (see
Table 8 on page
source
) from V
All information provided in this document is subject to legal disclaimers.
14). An interrupt is generated on changes of the sensor states.
s
) is derived from the internally generated oscillator frequency. The
Rev. 1 — 24 November 2011
DD(INTREGD)
Table 9 on page
sink
Capacitive 8-channel proximity switch with auto-calibration
) towards ground for a fixed time, causing the voltage on C
dch
) with the reference discharge time (t
for a fixed time causing the voltage on C
14).
CPC
controls an additional current sink (I
CPC
is charged through a current
CPC
ref
on pins CPC0 to CPC7.
, the appropriate
CPC
dch(ref)
PCA8885
© NXP B.V. 2011. All rights reserved.
is discharged
CPC
). In the
to rise by a
Figure 4
7 of 43
CPC
CPC
)