WM9705 datasheet(46/55 Pages) WOLFSON

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WM9705

Production Data

PD Rev 4.4 August 2006

CONTINUOUS METHOD

The continuous method of conversion allows for an autonomous free running operation of the

digitiser, converting pen X,Y results and either pressure or AUX channels continuously at a pre-

set rate. This removes from the controller the overhead associated with instructing which

conversion to do next etc. Continuous conversion is enabled by setting the CTC bit in register

76h. The rate of operation of the conversion sequence is set by writing bits CR0/1 into register

76h. These set the sequence rate as follows:

CR1

CR0

CONVERSION RATE

93.75Hz or 512 AC link frames

187.5Hz or 256 AC link frames

375Hz or 128 AC link frames

750Hz or 64 AC link frames

Table 26 Conversion Rate

Note that this rate is the MAXIMUM that may be obtained. In practice the rate will typically be

slower, if the number of conversions to be performed, and the delay applied by the DEL[3-0] bits

to each conversion, add up to a time greater than the nominal period for conversion set above.

Likewise, if conversions are halted by pen going up, or MASK being applied, then this conversion

rate will not be met. If MSK[1,0] is set to 11, then the conversion rate becomes synchronised to

the rate of application of edges to the MASK input. The CR[1,0] value is ignored. Conversions are

performed after the edge on the MASK pin, delayed by the value set in the DEL [3-0] register.

Normal operation will be to perform X,Y screen drives, and conversions, followed by a pressure or

AUX conversion if requested by setting the appropriate address in the ADR[2-0] bits of register

76h. So long as the ADR[2-0] value remains set to auxiliary or pressure inputs, X,Y and the extra

auxiliary conversions will be made in every cycle.

Pen-down detection is performed after completion of each set of conversions and the output sent

to the PENDET pin, and output in bit 15 of the data read-back word. If the PDEN bit is left at the

default value ‘0’, if pen-down is not detected, then the continuous conversion process including

driving the screen, will continue anyway. If the PDEN bit is set to ‘1’, and a pen-down is NOT

detected, then the conversion process will come to a stop, until the pen is once more returned to

the screen. DEL[3-0] delays are applied as for polled operation to all conversions.

Once a conversion is complete, the result is written into the results register 7Ah where it is

available for readback. Additionally the results may be output into a chosen SLOT by enabling slot

readback bit SLEN in register 76h and selecting the desired slot by setting SLT[2-0].

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