Digital frequency meter using DMA Terminal Count stop method
Abstract-
This paper presents a new wide-range speed
measurement method, using the direct memory access
(DMA) terminal count register(TCR). The DMA
method is based on both pulse counting in the constant
sampling time at terminal count stop pin of a DMA
controller. The hardware configuration and algorithms
for a microcontroller implementation are also
presented. The proposed method is suitable in systems
using microcontrollers with DMA controller and
timers. Limitations and sources of errors are discussed
in details. The DMA Terminal count register method
is suitable for real-time speed control systems.
I. Introduction:
The speed measurement can be
achieved using the following methods
1. Time measurement-determines time interval
between pulses[1]
2. Pulse counting-counts input pulses within sampling
time[2]
3. Combined method[3]
4. Constant Elapsed Time method(CET)[4]
5. DMA Transfer method.[4][5]
When the hardware configuration of the DMA
Transfer method executes a long processor instruction,
it is possible that DMA acknowledge signal (DACK)
is not received before the next rising edge of input
pulses. Therefore, this next pulse will not be detected.
In order not to lose any input pulse the counter of
unperformed DMA requests is used. This can also be
solved using a simple h/w configuration and less
power consumption method which can be achieved
using terminal count register and TC stop pin of DMA
controller. The TC stop pin of DMA Controller
changes its state after fixed number of DMA cycles.
The numbers of pulses generated from oscillator are
counted within these DMA cycles. This is
proportional to the speed to be measured. A DMA
controller 8257 is selected in my work because of its
simple architecture, easy to understand its
management and controlling operations. The 8257 can
gain control on the system bus using the CPU’ hold
function. The DMA count for each channel and
outputs a control signal to notify the peripherals that
the programmed number of DMA cycles
complete[6].The DMA channel that is to be
programmed should always be “masked” before
loading any settings. This is because the hardware
might unexpectedly assert the DRQ for that channel,
and the DMA might respond, even though not all of
the parameters have been loaded or updated.
II. Terminal count DMA method
In this method the average number of pulses in the
buffer is counted which indicates the speed of the disk
or the number of pulses arrived at DRQ.The rotational
speed can be calculated from the quotient ΔΦ/Δt. ΔΦ
is the increment of the rotational angle during the time
interval Δt, where
IV. Hardware Configuration:
The rising edge of the pulses from the encoder is
recognized by DRQ pin of the DMA controller. As the
DRQ responds to the pulses the terminal count register
of the DMA controller which is set by user program is
decremented. Initially the TC pin is low. When
terminal count register become zero the TC signal
become high. The TC is enabled by Mode set register.
When the
timer recognizes the TC signal it resets the pulse count
in the buffer. Before the TC become high the pulses
from the free running timer is counted by a buffer.
The buffer will count the number of the pulses until
the terminal count register value is zero. When it is
zero the TC will enable the Output enable pin of the
buffer which allow to write the data from buffer into
memory. When the encoder pulse width is large
terminal count register will take more time to reach
zero. So the buffer will store maximum count pulses
from the free running timer. If the encoder pulse width
is small in case of high speed the number of pulses
stored in the buffer will be minimum. So the number
of pulses in the buffer is counted which indicates the
speed of the disk or the number of pulses arrived at
DRQ.
V. Software Routines
Step1: mask the channel which is used to access the
peripheral
Step2: Enable TC stop bit and Auto reload bit of
mode set register.
Step3: After the TC pin enables the buffer unmask the
channel.
Step4: Repeat from step1 to step 4 as per the user
requirement to take the average values.
The code is developed in assembly level language
using MASM software. The code can also be compile
and execute in personal computer the debug level by
giving internal DMA port addresses[6].
VI. Experimental setup
The 16 bit microprocessor is used to control the DMA
controller. The microprocessor is operated at 5MHz
clock frequency. The DMA controller is operated at
8MHz. The reference clock for the external freerunning
timer and the interval timer inside the
microcontroller, fREF is 2 MHz. The terminal count
DMA method is verified using 20MHz Frequency
generator, a tachometer is interfaced to generate the
pulses for DRQ of DMA. Two buffers are used in
between DMA and Timer to make delay. The
hardware delay is created to enable buffer output data
pins before enabling the timer. The number of auto
reload mode is decided by user program. It can also be
set using timer reset signal by connecting more logic
circuit.
VII CONCLUSIONS
The TC DMA method is very easy to interface. The
cost effect is less when compared to other methods. It
need less hard ware for measurement of speed. The
TC DMA method reduces the power consumption. It
avoids the time constraints in accessing the capture
registers. The TC DMA allows to measure large range
of frequencies. The measurement error has a zero
average value and almost fixed value over a wide
frequency range. Further the execution time can be
improved using ARM processor.