AS3900 datasheet(52/59 Pages) AMSCO

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AS3900

Data Sheet - Detail ed Description

8.10.2 Communication Error Counter (cesmax)

The communication error counter can be enabled, in order to automatically turn off the communication to a lost Client. This feature allows the

automatic disabling of the Timer at Master and/or Client side.

If the actual communication error count of one Client cesX in register SREG 0x14 to 0x17 reaches the maximum allowed communication errors

set in cesmax in REG 0x02<2:0>, the according Timer is turned off.

At the Master, the dedicated Client Timer goes inactive that can be read out via the register activeTimers in REG 0x27. This register needs to be

polled. It is only possible to set one limit for the communication error counter cesmax for all available Clients at the Master. The only possibility to

set individual limits for the different Clients is, to set an initial value at the actual communication error count cesX in the SHADOW register. An

error would increase the cesX field starting from the initial value, which allows the disabling of the dedicated Timer at fewer counts.

At the Client, the Timer goes inactive that is signaled when the activeTimer0 is reset to ‘0’. REG 0x27<0> is set to 0.

The communication error counter can be disabled by setting cesmax to 0x00 in register REG 0x02<2:0> at the Clients and at the Master. If some

Clients are lost, the Master does not stop its transmission at the dedicated time slots.

The following table summarizes the changing of the cesX (cesX stands for ces0, ces1 … ces7):

Table 40. Actual Communication Error Count cesX

Master: cesX in SREG 0x14 to 0x17

Client: ces0 in SREG 0x14<2:0>

8.10.3 Cycle Time and Random Pointer

The Cycle time defines the starting point of the dedicated time slot. The cycle time can be set individually at the Client in REG 0x0C<7:5>. This

individual cycle time is included in every packet send to the Master. This timing information is stored into the timing table of the Master in REG

0x0C to 0x13.

The cycle time can be fixed or varied by the pseudo random pointer. If the pseudo random pointer in REG 0x0C<4:0> is set to 0 the cycle time is

constant. A different value leads to a random variation of maximal ±30% of the cycle time. This feature reduces the likelihood of a collision. If the

random pointer is used, each Client should use a different one. Otherwise the shift of the cycle time would be the same, because the algorithm

behind is the identically for each Client (pseudo random). The random pointer is included in the timing information of each packet and is stored

automatically in the Masters timing table in register REG 0x0C to 0x13<4:0>.

The length of the time slot is defined by the data rate multiplied by the frame size. The frame mainly depends on the payload that can be set by

dlen in register REG 0x37<4:0>. The data rate is defined in REG 0x06<1:0>.

The following example shows the timing separation with 8 Clients. Therefore a cycle time of 125ms for each Client was set, resulting in a

maximum time slot of 15.6ms. The actual allocated time slot is defined by:

Timeslot

dlen 11ByteHeader 3ByteACK

()

bitrate

-----------------------------------------------------------------------------------------------

(EQ 1)

This equals for the maximum payload of 24 bytes and a bit rate of 105kbps to an allocated time slot of 2.9ms. As seen in the figure all packets

are static. Using the random pointer results in spontaneously changing the time slot, as denoted for Client 1 (yellow packets).

ACK

cesX is cleared, set to zero

ces0 is cleared, set to zero

ACP

cesX is cleared, set to zero

ces0 is cleared, set to zero

NAK

cesX is increased

ces0 is increased

NAR

cesX is not changed

ces0 is not changed

No reply

cesX is increased

ces0 is increased

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