MC68HC05L28 Datasheet, PDF (97/156 Pages) Motorola, Inc – Flexible general-purpose microcomputer

9
A/D CONVERTER
The analog to digital converter system consists of a single 8-bit successive approximation
converter and a 16-channel multiplexer. There are only two A/D channels available on the
MC68HC05L28. These are connected to the ADx pins of the MC68HC05L28 and the other
channels are dedicated to internal reference points for test functions. The ADx pins do not have
any internal output driver circuitry connected to them because this circuitry would load the analog
input signal due to output buffer leakage current.
There is one 8-bit result data register, ADDATA and one 8-bit status/control register, ADSTAT.
The A/D converter is ratiometric and two dedicated pins, VREFH and VREFL, are used to supply the
reference voltage levels of each analog input. These pins are used in preference to the system power
supply lines because any voltage drops in the bonding wires of the heavily loaded supply pins could
degrade the accuracy of the A/D conversion. An input voltage equal to or greater than V_REFH converts
to $FF (full scale) with no overflow indication and an input voltage equal to V_REFL converts to $00.
The A/D converter can operate from either the bus clock or an internal RC type oscillator. The
internal RC type oscillator is activated by the ADRC bit in ADSTAT and can be used to give a
sufficiently high clock rate to the A/D converter when the bus speed is too low to provide accurate
9
results (see Section 9.2.1.2). When the A/D converter is not being used it can be disconnected,
using the ADON bit in the ADSTAT register, in order to save power (see Section 9.2.1.3).
9.1 A/D converter operation
The A/D converter consists of an analog multiplexer, an 8-bit digital to analog capacitor array, a
comparator and a successive approximation register (SAR) (see Figure 9-1).
There are four options that can be selected by the multiplexer; the ADx input pin, VRH,
(VRH+VRL)/2 or VRL. Selection is made via the CHx bits in the ADSTAT register (see
Section 9.2.1.4). ADx are the only input points for A/D conversion operations; the others are
reference points which can be used for test purposes.
The A/D reference input (ADx) is applied to a precision internal digital to analog converter. Control logic
drives this D/A converter and the analog output is successively compared with the analog input (ADx)
sampled at the beginning of the conversion. The conversion is monotonic with no missing codes.
MC68HC05L28
A/D CONVERTER
TPG
MOTOROLA
9-1