A/D Control/Status Register
M68HC11E Family Data Sheet, Rev. 5.1
Freescale Semiconductor
63
When this control bit is clear, the four requested conversions are performed once to fill the four result
registers. When this control bit is set, conversions are performed continuously with the result registers
updated as data becomes available.
MULT — Multiple Channel/Single Channel Control Bit
When this bit is clear, the A/D converter system is configured to perform four consecutive conversions
on the single channel specified by the four channel select bits CD:CA (bits [3:0] of the ADCTL register).
When this bit is set, the A/D system is configured to perform a conversion on each of four channels
where each result register corresponds to one channel.
NOTE
When the multiple-channel continuous scan mode is used, extra care is
needed in the design of circuitry driving the A/D inputs. The charge on the
capacitive DAC array before the sample time is related to the voltage on the
previously converted channel. A charge share situation exists between the
internal DAC capacitance and the external circuit capacitance. Although
the amount of charge involved is small, the rate at which it is repeated is
every 64
s for an E clock of 2 MHz. The RC charging rate of the external
circuit must be balanced against this charge sharing effect to avoid errors
in accuracy. Refer to M68HC11 Reference Manual, Freescale document
order number M68HC11RM/AD, for further information.
CD:CA — Channel Selects D:A Bits
Refer to
Table 3-2. When a multiple channel mode is selected (MULT = 1), the two least significant
channel select bits (CB and CA) have no meaning and the CD and CC bits specify which group of four
channels is to be converted.
Table 3-2. A/D Converter Channel Selection
Channel Select
Control Bits
Channel Signal
Result in ADRx
if MULT = 1
CD:CC:CB:CA
0000
AN0
ADR1
0001
AN1
ADR2
0010
AN2
ADR3
0011
AN3
ADR4
0100
AN4
ADR1
0101
AN5
ADR2
0110
AN6
ADR3
0111
AN7
ADR4
10XX
Reserved
—
1100
VRH
(1)
1. Used for factory testing
ADR1
1101
VRL
(1)
ADR2
1110
(VRH)/2
(1)
ADR3
1111
Reserved(1)
ADR4