epc120
9. Timing
Overview
To operate the individual elements at the 2-wire bus, some steps per element are necessary. The following drawing shows the concept:
Rx & Tx
wakeup the epc120 element
(from standby to operation)
open the receive window and
send a light pulse out through
the LED pin after PDELAY
close the receive window and
put the device to standby
read the result of the last light
reception
Figure 14: Basic sequence to operate one light beam. Note that the process in the
receiver and in the transmitter are running concurrently.
The individual epc120 elements at the 2-wire bus are normally in a sleep mode in order to keep the overall power consumption as low as
possible. Thus, an epc120 element has to be activated before it can be used. This wakeup procedure needs a certain time until all internal
operating levels have been stabilized. This time is called settling time which can be set with the parameter TSET. Then, the receive window
can be opened and the internal LED driver send a pulse out through the LED pin, which the chip can receive, if no obstacle is in the light
beam. After that, the receive window must be turned off which also puts the receiver to standby. Finally, the receive results which are stored in
the ecp120 element can be read.
In fact, there are several steps to operate one light beam only. This needs quite a long time if everything is done in a strictly sequential way. In
order to improve the performance of the whole system, certain steps can be done in parallel. The following chapters describe the timing
processes in more detail.
Timing
The microprocessor in the bus controller controls epc120 with SCAN commands. Every SCAN command includes an address which selects
the requested epc120 element.
PD PIN operation: A first SCAN command switches the selected epc120 element from standby into operation mode. The process from
standby to operation requires a certain time which is called settling time (see Figure 15). The settling time minimum is 60μs. The second
SCAN command opens the the reception window, there also the pulse at the LED PIN is sent, where a third SCAN command closes the
reception window and puts the epc120 element back to standby. The fourth SCAN command fetches the received results.
LED PIN operation: A first SCAN command switches the selected epc120 element from standby into operation mode. The process from
standby to operation requires a certain time which is called settling time (see Figure 15). The second SCAN command starts the light pulse
window. After the time PDELAY, one light pulse of the length TPULSE is generated. A third SCAN command puts the element back to standby.
If the TSTMP and the period of the SCAN commands of the microprocessor are equal the pulse will be emitted exactly in the middle of the
reception window.
The whole operation is optimized for shortest possible scan periods. Figure 15 shows the timing for a settling time of one scan period
(TSET=0) and the addresses given in the shortest possible sequence.
© 2011 ESPROS Photonics Corporation
Characteristics subject to change without notice
16
Datasheet epc12x - V2.1
www.espros.ch
EPC [ ESPROS PHOTONICS CORP ]
