MC68HC05B6_13 Datasheet, PDF (99/302 Pages) Freescale Semiconductor, Inc – Microcontrollers

8
ANALOG TO DIGITAL CONVERTER
The analog to digital converter system consists of a single 8-bit successive approximation
converter and a sixteen channel multiplexer. Eight of the channels are connected to the
PD0/AN0 – PD7/AN7 pins of the MC68HC05B6 and the other eight channels are dedicated to
internal reference points for test functions. The channel input pins do not have any internal output
driver circuitry connected to them because such circuitry would load the analog input signals due
to output buffer leakage current. There is one 8-bit result data register (address $08) and one 8-bit
status/control register (address $09).
The A/D converter is ratiometric and two dedicated pins, VRH and VRL, are used to supply the
reference voltage levels for all analog inputs. 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_RH converts to $FF (full scale) with no overflow indication and an input voltage equal to V_RL
8
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 the A/D status/control register (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 results. When the A/D converter is not being used it can be disconnected,
by clearing the ADON bit in the ADSTAT register, in order to save power (see Section 8.2.3).
For further information on A/D converter operation please refer to the M68HC11 Reference
Manual — M68HC11RM/AD.
8.1
A/D converter operation
The A/D converter consists of an analog multiplexer, an 8-bit digital to analog converter capacitor
array, a comparator and a successive approximation register (SAR) (see Figure 8-1).
There are eleven options that can be selected by the multiplexer; AN0–AN7, VRH, (VRH+VRL)/2
or VRL. Selection is done via the CHx bits in the ADSTAT register (see Section 8.2.3). AN0–AN7
are the only input points for A/D conversion operations; the others are reference points that can
be used for test purposes.
MC68HC05B6
Rev. 4.1
ANALOG TO DIGITAL CONVERTER
Freescale
8-1