Z8 Encore! XP® 4K Series
Product Specification
118
Programmable Trigger Point Alarm
The ADC contains two programmable trigger values, defined in the ADC High Threshold
(ADCTHH) Register (Table 76 on page 128) and the ADC Low Threshold (ADCTLH)
Register (Table 77 on page 128). Each of these values is 8 bits and is NOT a two’s com-
plement number. The alarm is intended for single-ended operation and so the alarm values
reflect positive numbers only. Both thresholds have independent control and status bits.
When the ADC is enabled and the ADC value exceeds the high threshold, an ADC inter-
rupt is asserted and the high threshold status bit is set. When enabled and the ADC value is
less than the low threshold, an ADC interrupt is asserted and the low threshold status bit is
set.
Because the alarm value is positive it is compared to the most significant 8 data bits of the
ADC value, excluding the sign bit. The ADC alarm bits are compared to
{ADCD_H[6:0],ADCD_L[7]}. Alternatively, the alarm value is compared to the ADC
value shifted left by one bit. Negative ADC values never trigger the high alarm and always
trigger the low alarm. Because the ADC output is software compensated for offset, nega-
tive (pre-compensated) values can occur in SINGLE-ENDED mode.
The alarm is primarily intended for use in CONTINUOUS mode so that the CPU can
determine threshold crossings without servicing interrupts for all ADC samples. If used in
SINGLE-SHOT mode, the ADC will only interrupt the CPU if the single sample triggers
an alarm.
The alarm status bits are updated on each conversion, regardless of the alarm enable bit
values. The alarm enable bits only determine whether or not an interrupt is generated.
Interrupts
The ADC is able to interrupt the CPU under three conditions:
•
•
When a conversion has been completed
When the 8 Most Significant Bits of a sample exceed the programmable high threshold
ADCTHH[7:0]
•
When the 8 Most Significant Bits of a sample is less than the programmable low threshold
ADCTLH[7:0]
The conversion interrupt occurs when the ADC is enabled and both alarms are disabled.
When either or both alarms are enabled, the conversion interrupt is disabled and only the
alarm interrupts may occur.
When the ADC is disabled, none of the three sources can cause an interrupt to be asserted;
however, an interrupt pending when the ADC is disabled is not cleared.
The three interrupt events share a common CPU interrupt. The interrupt service routine
must query the ADC Control/Status (ADCCTL1) Register to determine the cause of an
PS022815-0206
Analog-to-Digital Converter
ZILOG [ ZILOG, INC. ]
