XC886/888CLM
Analog-to-Digital Converter
User’s Manual
16-9
V1.3, 2010-02
ADC, V 1.0
16.4.1
Request Source Arbiter
The arbiter can operate in two modes that are selectable by bit ARBM:
•
Permanent arbitration:
In this mode, the arbiter will continuously poll the request sources even when there
is no pending conversion request.
•
Arbitration started by pending conversion request:
In this mode, the arbiter will start polling the request sources only if there is at least
one conversion pending request.
Once started, the arbiter polls the two request sources (source x at slot x, x = 0 - 1) to
find the analog channel with the highest priority that must be converted. For each
arbitration slot, the arbiter polls the request pending signal (REQPND) and the channel
number valid signal (REQCHNRV) of one request source. The sum of all arbitration slots
is called an arbitration round. An arbitration slot must be enabled (ASENx = 1) before it
can take part in the arbitration.
Each request source has a source priority that can be programmed via bit PRIOx.
Starting with request source 0 (arbitration slot 0), the arbiter checks if a request source
has a pending request (REQPND = 1) for a conversion. If more than one request source
is found with the same programmed priority level and a pending conversion request, the
channel specified by the request source that was found first is selected. The
REQCHNRV signal is also checked by the arbiter and a conversion can only be started
if REQCHNRV = 1 (and REQPND = 1). If both request sources are programmed with the
same priority, the channel number specified by request source 0 will be converted first
since it is connected to arbitration slot 0.
The period
t
ARB
of a complete arbitration round is fixed at:
t
ARB
= 4 *
t
ADCD
(16.3)
Refer to
for register description of priority and arbitration control.
*
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