C504 Datasheet, PDF (27/49 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

C504
The A/D converter uses two clock signals for operation : the conversion clock f_ADC (= 1/ t_ADC) and
the input clock f_IN (= 1/ t_IN). Both clock signals are derived from the C504 system clock f_OSC which
is applied at the XTAL pins. The duration of an A/D conversion is a multiple of the period of the f_IN
clock signal. The table in figure 10 shows the prescaler ratios and the resulting A/D conversion
times which must be selected for typical system clock rates.
MCU System Clock f_IN
Rate (f_OSC)
[MHz]
3.5 MHz
1.75
12 MHz
6
16 MHz
8
24 MHz
12
32 MHz
16
40 MHz
20
Ratio
÷4
÷4
÷4
÷8
÷8
÷ 16
Prescaler
f_ADC
ADCL1 ADCL0 ^[MHz]
0
0
.438
0
0
1.5
0
0
2
0
1
1.5
0
1
2
1
0
1.25
A/D Conversion
Time [µs]
48 x t_IN = 27.4
48 x t_IN = 8
48 x t_IN = 6
96 x t_IN = 8
96 x t_IN = 6
192 x t_IN = 9.6
Figure 10
A/D Converter Clock Selection
The analog inputs are located at port 1 and port 3 (4 lines on each port). The corresponding port 1
and port 3 pins have a port structure, which allows to use it either as digital I/Os or analog inputs.
The analog input function of these mixed digital/analog port lines is selected via the registers
P1ANA and P3ANA.
Semiconductor Group
27