83C055 Philips Semiconductors, 83C055 Datasheet - Page 11

• Manufacturer Part Number: 83C055
• Description: Microcontrollers for TV and video MTV
• Manufacturer: Philips Semiconductors

Philips Semiconductors
11 14-BIT PWM DAC (TDAC)
11.1
The 14-bit counter was already mentioned in Section 10.
The nature of the counter is such that it can achieve a
stable output value through its MSB, and the value can
propagate through logic like that shown in Fig.5. The logic
output can be stable within:
The 14-bit (TDAC) counter is a ripple counter (cost and
die-size reasons).
The 14-bit PWM DAC is controlled by two special function
registers TDACL and TDACH.
11.2
When software wishes to change the 14-bit value
(TD0 to TD13), it should first write to TDACL and then
write to TDACH. Alternatively, if the required precision of
the duty cycle is satisfied by 6 bits or less, software can
simply write to TDACH (TD8 to TD13).
11.2.1
Figure 5 shows that this block includes an ‘extra’ 14-bit
latch between TDACL - TDACH and the comparator and
other logic. The programmed value is clocked into the
operative latch when the 7 low-order bits of the counter roll
over to zero, provided that the software is not in the midst
of loading a new 14-bit value, i.e. it is not between writing
TDACL and writing TDACH.
In a similar fashion to the lower-precision PWMs, this
facility has an output flip-flop that is set when the lower
7 bits of the counter overflow/wrap. The more significant
7 bits of the operative latch’s programmed value are
compared for equality against the less significant 7 bits of
the counter, and the output FF is cleared when they match.
Thus this output has a fixed period of 128 PWM clock
cycles, and the duty cycle is determined by the
programmed value.
1996 Mar 22
one period of the PWM clock (e.g. 250 ns) if
edge-triggered logic is used to capture the logic output,
or
one phase of the PWM clock (e.g. 125 ns) if a phase of
the PWM clock is used to capture the logic output.
Microcontrollers for TV and video (MTV)
14-bit counter
14-bit DAC operation
L
OW PRECISION OPERATION
11
11.2.2
For the higher-precision aspect of this feature, the 7 MSBs
of the counter are used in a logic block with the 7 LSBs of
the programmed value.
The 7
ANDed with the 7
of the value is ANDed with the counter’s 6
being 10, and so on through the LSB of the programmed
value being ANDed with the counter’s 7 MSBs being
100000. Then these 7 ANDed terms are ORed. If the
result is true (logic 1) at the time the 7 LSBs of the counter
match the MSBs of the programmed value, the output is
forced high for 1 (additional) PWM clock cycle.
The result is that, if the value-64 bit of the 14-bit value is
programmed to a logic 1, every other cycle of 128 PWM
counter clocks has its duty cycle stretched by one counter
clock; if the value-32 bit is programmed to logic 1, every
4
is programmed to logic 1, one cycle out of each 128 is
stretched.
11.2.3
Assuming the external integrator can handle all this, the
net effect is a PWM DAC that has the period of a 7-bit
design (which makes the integrator easier and more
feasible to design) with the accuracy of a 14-bit one.
An obvious prerequisite for such precision is that the load
on the voltage must be very light, like a single op-amp or
comparator.
11.2.3.1
The TDAC feature differs from the corresponding features
of predecessor parts in several ways:
1. The 14-bit value is functionally composed of major and
2. The 14-bit value is programmed as a contiguous
3. As discussed for the 6-bit DACs, both of the preceding
th
cycle is stretched, and so on through, if the value-1 bit
minor portions of 7 bits each.
multi-register value that can be manipulated
straight-forwardly via arithmetic instructions.
parts had a feature whereby the PWM output could be
inverted, redundantly with complementing the 14-bit
value. This feature has been eliminated.
th
LSB (binary value 64) of the programmed value is
H
14-
IGH PRECISION OPERATION
Note
BIT
DAC
th
MSB (128) of the counter, the 6
OUTPUT
83C145; 83C845
83C055; 87C055
Product specification
th
and 7
th
th
MSBs
LSB