COP888GD Datasheet, PDF (22/42 Pages) National Semiconductor (TI) – 8-Bit CMOS ROM Based Microcontrollers with 16k

A/D Converter (Continued)
The A/D converter takes 17 A/D clock cycles to complete a
conversion. Thus the minimum A/D conversion time for the
device is 10.2 µs when a prescaler of 6 has been selected.
The 17 A/D clock cycles needed for conversion consist of 1
cycle at the beginning for reset, 7 cycles for sampling, 8
cycles for converting, and 1 cycle for loading the result into
the A/D result register (ADRSLT). This A/D result register is a
read-only register. The user cannot write into ADRSLT.
The ADBSY flag provides an A/D clock inhibit function, which
saves power by powering down the A/D when it is not in use.
Note: The A/D converter is also powered down when the device is in either
the HALT or IDLE modes. If the A/D is running when the device enters
the HALT or IDLE modes, the A/D powers down and then restarts the
conversion with a corrupted sampled voltage (and thus an invalid re-
sult) when the device comes out of the HALT or IDLE modes.
Analog Input and Source Resistance Considerations
Figure 13 shows the A/D pin model in single ended mode.
The differential mode has a similar A/D pin model. The leads
to the analog inputs should be kept as short as possible.
Both noise and digital clock coupling to an A/D input can
cause conversion errors. The clock lead should be kept
away from the analog input line to reduce coupling. The A/D
channel input pins do not have any internal output driver cir-
cuitry connected to them because this circuitry would load
the analog input signals due to output buffer leakage current.
Source impedances greater than 3 kΩ on the analog input
lines will adversely affect the internal RC charging time dur-
ing input sampling. As shown in Figure 13, the analog switch
to the DAC array is closed only during the 7 A/D cycle
sample time. Large source impedances on the analog inputs
may result in the DAC array not being charged to the correct
voltage levels, causing scale errors.
If large source resistance is necessary, the recommended
solution is to slow down the A/D clock speed in proportion to
the source resistance. The A/D converter may be operated
at the maximum speed for R_S less than 3 kΩ. For R_S greater
than 3 kΩ, A/D clock speed needs to be reduced. For ex-
ample, with R_S = 6 kΩ, the A/D converter may be operated
at half the maximum speed. A/D converter clock speed may
be slowed down by either increasing the A/D prescaler
divide-by or decreasing the CKI clock frquency. The A/D
minimum clock speed is 100 kHz.
*The analog switch is closed only during the sample time.
FIGURE 13. A/D Pin Model (Single Ended Mode)
DS100076-18
Interrupts
INTRODUCTION
Each device supports eleven vectored interrupts. Interrupt
sources include Timer 0, Timer 1, Timer 2, Timer 3, Port L
Wakeup, Software Trap, MICROWIRE/PLUS, and External
Input.
All interrupts force a branch to location 00FF Hex in program
memory. The VIS instruction may be used to vector to the
appropriate service routine from location 00FF Hex.
The Software trap has the highest priority while the default
VIS has the lowest priority.
Each of the 11 maskable inputs has a fixed arbitration rank-
ing and vector.
Figure 14 shows the Interrupt Block Diagram.
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