SAB80515 Datasheet, PDF(76/270 Page) Siemens Semiconductor Group – 8-Bit Single-Chip Microcontroller Family

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SAB80515 Datasheet, PDF (76/270 Pages) Siemens Semiconductor Group – 8-Bit Single-Chip Microcontroller Family

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On-Chip Peripheral Components

7.4.2 Reference Voltages

The SAB 80(C)515 has two pins to which a reference voltage range for the on-chip A/D converter

is applied (pin VAREF for the upper voltage and pin VAGND for the lower voltage). In contrast to

conventional A/D converters it is now possible to use not only these externally applied reference

voltages for the conversion but also internally generated reference voltages which are derived from

the externally applied ones. For this purpose a resistor ladder provides 16 equidistant voltage levels

between VAREF and VAGND. These steps can individually be assigned as upper and lower reference

voltage for the converter itself. These internally generated reference voltages are called VlntAREF and

V . lntAGND The internal reference voltage programming can be thought of as a programmable "D/A

converter" which provides the voltages VIntARFF and VIntAGND for the A/D converter itself.

The SFR DAPR (see figure 7-29) is provided for programming the internal reference voltages

VIntAREF and V . lntAGND For this purpose the internal reference voltages can be programmed in steps of

1/16 of the external reference voltages (VAREF â VAGND) by four bits each in register DAPR. Bits 0 to

3 specify V , lntAGND while bits 4 to 7 specify V . IntAREF A minimum of 1 V difference is required between

the internal reference voltages VlntARF and VIntAGND for proper operation of the A/D converter. This

means, for example, in the case where VAREF is 5 V and VAGND is 0 V, there must be at least four

steps difference between the internal reference voltages VIntAREF and V . IntAGND

The values of VIntAGND and VIntAREF are given by the formulas:

DAPR (.3-.0)

V V = + IntAGND

AGND

16

(VAREF â VAGND)

with DAPR (.3-.0) < CH;

DAPR (.7-.4)

V V = + IntAREF

AGND

16

(VAREF â VAGND)

with DAPR (.7-.4) > 3H;

DAPR (.3-.0) is the contents of the low-order nibble, and DAPR (.7-.4) the contents of the high-order

nibble of DAPR.

Semiconductor Group

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