SLRC400 Datasheet, PDF (74/130 Pages) NXP Semiconductors – I·CODE Reader IC

Philips Semiconductors
I•CODE Reader IC
Product Specification Rev. 2.0 November 2001
SL RC400
9.2.2 CONTROLLING THE TIMER UNIT
The main part of the timer unit is a down-counter. As long as the down-counter value is unequal zero, it
decrements its value with each timer clock.
If TAutoRestart is enabled the timer does not decrement down to zero. Having reached the value 1 the timer
reloads with the next clock with the TimerReload value.
The timer is started by loading a value from the TimerReload Register into the counter module. This may be
triggered by one of the following events:
• Transmission of the first bit to the label (TxBegin Event) and bit TStartTxBegin is 1
• Transmission of the last bit to the label (TxEnd Event) and bit TStartTxEnd is 1
• The counter module decrements down to zero and bit TAutoRestart is 1
• Bit TStartNow is set to 1 (by the µ-Processor)
Note: Every start-event re-loads the timer from the TimerReload Register. Thus, the timer unit is re-triggered.
The timer can be configured to stop with one of the following events:
• Reception of the first valid bit from the label (RxBegin Event)and bit TStopRxBegin is set to 1
• Reception of the last bit from the label (RxEnd event) and bit TStopRxEnd is set to 1
• The counter module has decremented down to zero and bit TAutoRestart is set to 0
• Bit TStopNow is set to 1 (by the µ-Processor)
Loading a new value, e.g. zero, into the TimerReload Register does not immediately influence the counter,
since the TimerReload Register affects the counter units content only with the next start-event. Thus, the
TimerReload Register may be changed even if the timer unit is already counting. The consequence of
changing the TimerReload Register will be visible after the next start-event.
If the counter is stopped by setting bit TStopNow, no TimerIRq is signalled.
9.2.3 TIMER UNIT CLOCK AND PERIOD
The clock of the timer unit is derived from the 13.56 MHz chip clock via a programmable divider. The clock
selection is done with the TPreScaler Register, that defines the timer unit clock frequency according to the
following formula:
T_TimerClock
=
1
f_TimerClock
= ^{2T PreScaler}
13.56MHz
The possible values for the TPreScaler Register range from 0 up to 21. This results in minimum time
T_TimerClock of about 74 ns up to about 150 ms.
The time period elapsed since the last start event is calculated with
T_Timer
= ^{TReLoadValue − TimerValue}
f_TimerClock
This results in a minimum time T_Timer of about 74 ns up to about 40 s.
74
Preliminary