Specifications

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Analogue to Digital Converter (ADC)
In many microcontroller based systems,
some way of measuring analogue
electrical values is necessary. With this in
mind,all members of the H8S and
H8/300H families are equipped with a
10-bit A/D converter,in most cases with
an 8 (up to 16) channel analog
multiplexer to allow numerous signals to
be processed easily. The converter works
using a successive approximation
algorithm and conversions take 138 states
(or 8.6µs if ø - 16MHz, respectively 6.
7µs at 20MHz in case of H8S). A sample
and hold circuit is used to ensure that
once a conversion begins, a change of
the input value will not be reflected in a
different conversion result. A conversion
can be initiated by software or by means
of an external trigger input,and in some
devices by the timer unit. At the end of
the conversion an interrupt may be
requested.
As well as being able to perform single
conversion (single mode,where only one
channel is converted), the A/D
converter can also be used to scan up to
four channels (scan mode). To support
this mode of operation,four A/D result
registers (ADDRA to D) are provided.
Once the scan mode is selected,each
channel specified is converted
sequentially with the conversion value
being stored in the appropriate result
register. This mode of operation allows
the user software to sample the current
analogue value of an input by simply
reading the appropriate data register.
High speed ADC on
H8S/2655
On H8S/2655 Hitachi has implemented
a new high speed ADC,capable of
sustaining a conversion rate of 1 million
samples per second. This is achieved by
using two sample and hold (S&H)
circuits which are used interleaved,i. e.
while S&H A is sampling,S&H B is
being converted.
But not only is this new ADC faster ,but
it has also new features to make it more
flexible.
These new features include:
• 8 A/D data registers to store the
results of each channel individually
• Enhanced conversion modes, created
by combining ‘select’ (one channel) or
‘group’(more than one) with ‘single’
(one shot conversion) or ‘scan’
(continuous conversion).
• Select single mode:one shot
conversion of one channel
• Select scan mode:continuous
conversion of one channel
• Group single mode: one shot
conversion of a number of
channels
• Group scan mode: continuous
conversion of a number of
channels
• Buffer operation allows to store results
in FIFO-fashion in the ADDR
registers. Double or quadruple
buffering can be selected.
• Simultaneous sampling operation
allows the two S&H to sample
simultaneously. An important feature
in some applications,e.g. where at
least 2 out of the 3 phases of a 3-
phase mains have to be observed.
• Conversion can be triggered by
software,an external pin or the on-
chip timers
Digital to Analogue
Converter (DAC)
The H8/304x derivatives within
H8/300H and most H8S derivatives
feature a two channel, 8-bit resolution
Digital-to-Analog-Converter (DAC).
The conversion time is 10µs with a 20pF
load and the output voltage range is 0V
to Vref. The outputs can be maintained
during software standby.
Both channels can be enabled or disabled
independently. Once a channel is
enabled,the corresponding pin becomes
an analog output pin and the data in the
D/A data register (DADR) is output at
the pin after the D/A conversion time
has elapsed. The pin remains in that state
until DADR is written to again or until
the D/A channel or the complete DAC
is disabled.
The voltage at the D/A output pin can
be calculated as
DADR contents
x Vref.
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