RAM module testing and troubleshooting

Summary

In this video, the creator tests a self-built RAM module by powering it up and verifying its functionality. They check for correct data storage and retrieval, addressing issues with wiring and pull-up resistors. The video demonstrates writing zeros and ones to memory, reading data back, and using control signals for input and output. The creator also troubleshoots a wiring mistake, discusses the importance of edge detection for RAM writing, and labels control signals for clarity. The video concludes with a functional RAM module and plans for future videos on the program counter and a binary to decimal converter.

Takeaways

• 😀 The video is focused on testing a RAM module to ensure it's functioning correctly.
• 🔌 The initial power-up of the RAM module revealed the clock was running, and the module was in program mode.
• 💡 The video demonstrates writing zeros to memory, which unexpectedly resulted in ones, indicating a potential default input issue.
• 🔍 Upon inspection, it was discovered that some wires were connected incorrectly, specifically power and ground connections were off by one pin.
• 🛠️ The video shows a hands-on approach to fixing the wiring issue, which was crucial for the proper operation of the RAM module.
• 📈 After fixing the wiring, the RAM module was tested again, and it was observed that writing different patterns to memory worked as expected.
• 🔄 The video explains the process of reading data from memory and outputting it to the bus, showcasing the functionality of the memory address register.
• 🔧 The script includes a troubleshooting session where the presenter identifies and corrects an issue with the RAM in signal and its interaction with the clock signal.
• 💻 The video concludes with the addition of labels to various control signals for convenience and better understanding of the system.
• 📚 The presenter plans to work on the program counter and the output register in upcoming videos, with the latter involving a binary to decimal converter.

Q & A

What was the initial problem the presenter encountered when testing the RAM module?

The initial problem was that the RAM module was writing all ones instead of zeros, which suggested a potential issue with the wiring or connections.

Why did the presenter suspect that the default input might be '1'?

The presenter suspected the default input might be '1' because the inputs on the 7400 series and 74 LS series chips are pulled up with pull-up resistors, which would result in a '1' if nothing is connected.

What was the wiring mistake that the presenter identified during the testing?

The wiring mistake was that the power should have been going to pin 16, but it was connected to pin 15, and the ground should have been going to pin 15 but was connected to pin 14.

How did the presenter confirm that the wiring issue was fixed?

The presenter confirmed the wiring issue was fixed by writing zeros to the memory and observing that the memory now correctly reflected the written zeros.

What is the purpose of the memory address register in the RAM module?

The memory address register is used to select a specific memory location to read from or write to within the RAM module.

Why did the presenter decide to label the control signals?

The presenter decided to label the control signals to make it easier to understand and manage the various signals during testing and future reference.

What is the function of the RAM in signal as described in the script?

The RAM in signal is used to write data from the bus into the RAM memory. It is active high and is used to transfer data during the leading edge of the clock signal.

How did the presenter create an edge detector circuit to control the RAM in signal?

The presenter created an edge detector circuit by using a 0.01 microfarad capacitor and a 1k ohm resistor, which charges during the high state of the clock signal and discharges quickly, creating a short high pulse that triggers the RAM in signal.

What is the significance of the 10 microseconds delay in the edge detector circuit?

The 10 microseconds delay ensures that the RAM in signal is only high for the leading edge of the clock pulse, allowing data to be written to the RAM during just that moment.

What is the next step the presenter plans to take after testing the RAM module?

The next steps the presenter plans to take are to work on the program counter circuit and then proceed to the output register, which will involve creating a binary to decimal converter.

Outlines

00:00

🔌 Powering Up the RAM Module

In this segment, the video creator powers up the RAM module they've been building in previous videos to test its functionality. They observe the clock running and switch to program mode to write all zeros to the memory. The initial test reveals that all memory locations are filled with ones, suggesting a default input issue. The creator inspects the connections and identifies a wiring error, where power and ground connections are off by one pin. After correcting the wiring, they retest and successfully write different patterns to the memory, confirming that the RAM module is now functioning as intended.

05:00

🔍 Debugging and Testing Memory Address Register

The video continues with the creator debugging the memory address register. They notice that the memory outputs random data when not all address bits are set low, indicating a potential issue with the wiring. After correcting the wiring, they proceed to test the memory address register's ability to read and write data at different memory locations. They successfully demonstrate writing to and reading from various addresses, confirming that the memory address register is working correctly. However, they encounter an unexpected behavior where the address register changes to all ones, which is a result of simultaneously writing to the memory address register and data, an action that should not normally occur.

10:02

🏷️ Labeling Control Signals for Clarity

To simplify the process and avoid confusion, the video creator decides to label all the control signals on the breadboard. They label signals such as the halt signal, memory address register in, RAM out, RAM in, instruction register out, and instruction register in, each with an explanation of their function and whether they are active high or low. This step is crucial for organizing the setup and making it easier to troubleshoot and understand the flow of data and control signals within the system.

15:04

⏲️ Implementing an Edge Detector Circuit

In the final part of the video, the creator focuses on refining the RAM write operation to occur only on the leading edge of the clock signal. They implement an edge detector circuit using a capacitor and a resistor to ensure that data is written to RAM at the precise moment the clock signal goes high. This modification ensures that the RAM write operation is synchronized with the clock's rising edge, preventing data from being written during the entire high state of the clock signal. The creator tests this setup and confirms that the RAM module now correctly writes data only during the initial 10 microseconds of the clock's high state.

Keywords

💡RAM module

RAM, or Random Access Memory, is a form of computer memory that can be read and changed in any order, typically used to store working data. In the video, the RAM module is the primary focus, as the creator is building and testing this component. The RAM module is essential for the computer's operation as it allows for the temporary storage and retrieval of data.

💡Power up

To 'power up' refers to the process of supplying power to a device or system, enabling it to operate. In the context of the video, the creator is powering up the RAM module to test its functionality and ensure it works as intended. This is a critical step in the testing phase of any electronic device.

💡Program mode

Program mode is a setting in which a device is configured to accept and store data. In the video, the creator switches to program mode to write data to the memory. This mode is essential for initializing the memory with specific data patterns to test the RAM module's ability to store and retrieve information accurately.

💡Pull-up resistors

Pull-up resistors are used in digital circuits to ensure that an input pin is at a high logic level when not actively driven by a signal. The video mentions that inputs on the 7400 and 74LS series chips are pulled up, which means they default to a high state when not connected. This is significant because it explains why the memory initially displayed ones, as the inputs were not connected and thus defaulted to the high state.

💡DIP switch

A DIP switch, or Dual In-line Package switch, is a type of electronic switch typically used for inputting binary or decimal numbers into a system. In the video, the DIP switch is used to control the input to the RAM module. The creator inspects the connections related to the DIP switch and identifies a wiring error that needed correction.

💡Memory address register

The memory address register is a component that holds the address of a memory location. In the video, the creator tests the functionality of the memory address register to ensure that data can be written to and read from specific memory locations. This is crucial for the proper operation of the RAM module as it allows for the selective access of stored data.

💡Control logic

Control logic refers to the part of a computer system that manages the flow of data and operations within the system. In the video, the creator discusses how the control logic would handle writing data to memory. Understanding control logic is vital for the proper functioning of the computer, as it dictates the sequence of operations and data flow.

💡Edge detector circuit

An edge detector circuit is a type of circuit that triggers on the rising or falling edge of a signal. In the video, the creator constructs an edge detector using a capacitor and a resistor to ensure that data is written to the RAM only on the leading edge of the clock signal. This precise timing is essential for the accurate operation of the memory module.

💡Instruction register

The instruction register is a part of a computer's central processing unit (CPU) that holds the current instruction being executed. In the video, the creator labels the instruction register and its associated signals, indicating its importance in the data processing and instruction execution within the computer.

💡Binary to decimal converter

A binary to decimal converter is a device or circuit that translates binary numbers into their decimal equivalents. The video ends with a mention of a binary to decimal converter in relation to the output register. This component is significant as it allows the computer to represent and display numbers in a format that is more easily understood by humans.

Highlights

Powering up the RAM module for the first time to test its functionality.

Observing the clock running and switching to program mode.

Writing all zeros to memory and encountering an issue with ones being written instead.

Hypothesis that ones are the default input due to pull-up resistors on chip inputs.

Testing the hypothesis by setting specific bits and observing memory writing.

Identifying a wiring mistake with power and ground connections.

Fixing the wiring and retesting the memory writing with all zeros.

Writing different patterns to memory and verifying successful storage.

Switching to run mode and using the memory address register.

Demonstrating the ability to read data from specific memory addresses.

Testing the write operation from registers to memory.

Unexpected behavior of the memory address register writing to all ones.

Using a reset signal to return to address zero for further testing.

Labeling control signals for easier reference and troubleshooting.

Creating an edge detector circuit for the RAM in signal using a capacitor and resistor.

Calculating the time it takes for the capacitor to charge and its impact on the RAM in signal.

Testing the edge detector circuit to ensure data is written on the leading edge of the clock signal.

Final verification of RAM module functionality before moving on to the program counter.

Anticipating future work on the output register and binary to decimal converter.

Transcripts

so in the last few videos we built

pretty much all of the RAM module and in

this video I want to power it up test it

out and basically convince ourselves

that it's working right so we'll power

it up and it's on see the clock is

running and we're going to switch to

program mode and so we have all 0s here

we have all 0s here so if we push the

button in program mode it should write

all zeros to memory and ok ok however

making progress all right so it looks

like we wrote oh I think I see what's

going on oh boy so I'm just looking Colt

very closely here and what I see so

first off I guess the kind of explain

was gone through my mind here so we

wrote zeros here we did not we wrote

ones here and whenever you see ones one

thing to kind of think or maybe a

possibility is that you know one is sort

of the default input so if something's

not connected you get a one because they

all the inputs on all these 7400 series

and 74 LS series chips are pulled up too

they have like pull-up resistors on the

inputs so that makes me think that you

know maybe this is hooked up sort of

correctly I guess we could test that

actually if we maybe set that yeah so we

if we set that bit it's writing that

there if we turn that off it's not

writing it but no matter what we set

these two I'm guessing it won't change

yeah so that doesn't that does nothing

happens there so that makes me think

that we got this hooked up right we

don't have this hooked up right and the

first thing that I notice just just

inspecting it I see that I am I might be

off by by a couple are off by one here

to the power should be going to pin 16

here and I see it's going to pin 15 and

then also this this ground should be

going to pin 15 it looks like it's going

to pin 14 but then I would expect oh

yeah okay so I do have I do have this

this input from the dip switch that two

comes across and that goes to pin 15 of

course you can see

that's correct but it's just these two

wires are just off by one so I just need

to shift these over to the left so let

me pull the power off and do that so

this shift over and of course you know

if this if this chip wasn't powered

which it wasn't you know the power was

was not connected this is fallen out

whoo boy yeah so this chip wasn't

powered then you know its outputs would

be would be floating and the inputs on

these chips default to ones and so

whenever we do a write it just it just

writes all ones so that explains the

behavior that we saw so now I need to

figure out how to find a good place for

this ground what can I get it to reach

yeah think that'll work okay let's give

this try so if we power it up we get

some random stuff in memory there but

with these all zeros if we write we get

all zeros so it looks like that might

have fixed it so let's try setting a

different pattern in here and write that

okay looks like we write that and if we

switch to address one we get some other

random garbage and if we go to all zeros

here and right there then we got all

zeros if we go back to address zero we

should see yep we see that pattern

that's in address zero and roots one we

go to address two we get random stuff

but we could put in you know some other

pattern like that if we go back to

address zero we see that if we go to

address 1 we get 0 so it looks like we

are able to put whatever data we want

into memory by selecting the address

here selecting the data we want here and

pushing this button here to write that

data in that address so now if we go to

run mode then we should be now using the

memory address register here and it

looks like that's pointed to address 0

and so we're seeing what's in address 0

there now in this case we want to go to

address 1 we've got two kind

set our our our a one on the bus here so

we want to set these top three bits to

zero and then of course the bottom that

will default to one when we when we set

our read signal here so this is our read

and actually let me stop the clock for a

second here so if we say we want to well

write I guess from the from the

registers per seconds we're going to

write to the bus a 1 0 0 0 so if we set

this enabled there then on the next

clock pulse we get a 1 in our headers

register and of course we see the 0

because we have a 0 in that location of

memory if we want to go back to address

0 then we have to set all four of these

bits low so let's go ahead and set that

forth bit low and so this signal is

still inputting a right signal so on the

next clock pulse it should put a 0 into

the register here and that goes to 0 and

we see the the memory that are the the

data that's in that memory location 0

and we can try going to memory address 2

and on the next clock pulse whoops did I

oh I just have these wires mixed up let

me put these in the right order here so

I'm not confused there we go so this

should be addressed to on the next clock

pulse

yep puts it to there and then this is

the data that we had stored in address

location 2 so it looks like the memory

address register is working that's great

now if we want to read this data from

the the bus or read this data and put it

on the bus again we're going to connect

the bus over here and we can hook up

some LEDs just to to see for now what's

over here and like I did before I'm just

going to connect 3 of these LEDs just to

give us an idea so if we want to output

well right now so we have location 2 in

our address register and if we want to

take the data that's in location to put

that on the bus then that's this signal

here we take that low and we see well at

least the bottom 3 bits it looks like

we're outputting that

and I guess we can we can move this led

here and see yeah that fourth bit is is

set there and then the fifth bit should

be off sixth bit should be off seventh

bit should be on and the eighth bit

should be off okay so it looks like it's

out putting this thing on to the bus

when we tell it to and then when we tell

it not to it doesn't so then I think the

only other thing we want to test is make

sure that when we want to take data from

the bus and put it into memory that we

can do that and so in that case I'm

going to take these LEDs out of the way

and I'm going to use these wires here to

put a value on the bus so again you know

these are going to these are going to

default high so I'm just going to set a

couple of these I'm going to set this

second bit low and let's say the third

bit low may be the sixth bit low and the

seventh bit low just those four bits I'm

going to set low and then I'm going to

use this signal down here which is our

our right signal so you know we've got

this address one in the memory address

register so we see what's in memory

location one here we saw that we can

help with that too the bus now we have a

value that a different value that want

input from the bus and store in that

memory location and so if the control

logic of the computer wants to do that

then it would set this high to say I

want to write to memory and then on the

next clock pulse which if you can see

all of this and I just pulse the clock

here and we should see this new data go

into memory and there it is so that new

data is in memory oh it's something

something interesting happened here you

see the address changed to all ones and

that's because we're also writing to the

memory address register

normally that would never happen because

you wouldn't write to the memory address

register and the data at the same time

you'd want to to do one of the other so

let me back up a second

and turn off the writing to writing the

data to memory then I want to go back to

address one actually let me just double

check here address two it looks like it

wrote it there too

we'll go back actually might me would be

easier here is is we can use this reset

signal and just reset to address zero so

we're here we're at address zero and we

want to not write to the address

registers we don't want to change the

address we only want to change the

contents at that address we want to stay

on address zero and then we want to

write to that address this data until

then on the next clock pulse yes it

writes this data into memory so I'm

reasonably convinced that that this is

working now you know it's not a total

test of every connection so there might

be like one bit that's not hooked up

quite right and we'll we'll probably

find that later on when we start hooking

everything up together and running more

complex programs things will see some

subtle issues which are always fun to

troubleshoot but at least at least at a

quick quick sort of test it looks like

everything is working more or less the

way that we want one other thing I'm

going to do just as kind of a

convenience thing is we've got all these

little control signals everywhere I'm

going to I'm going to label them just to

make life a little bit easier for me so

if we go up to the clock this signal

here is the the halt signal I'm just

going to stick a little label there that

says this is this is halt which halts

the clock this is the memory address

register in signal so when this is when

this goes low we're going to input a

value into the memory address register

so I'm going to call that M I for memory

address register in and I've got the bar

there because it's a active low so

normally want that high this is the RAM

out signal so yeah when this is when

this goes low as an active low when this

goes low it outputs the

of RAM to the bus so mark that is ro for

RAM out and that's active low so we'll

keep that high normally and then of

course the input from input from the bus

to RAM we've got the RAM in signal down

here so I'll mark that as RI for RAM in

and that's active high so I don't have

the bar there so normally that will be

low unless we're inputting from RAM and

then while we're here since we've got

the instruction register down here I'll

label those as well this is the

instruction register out so this will

output whatever is on the in the

instruction register to the bus so

that's the i/o and that's an active low

so normally that'll be high unless we're

outputting the contents of the

instruction register to the bus and

finally the the instruction register in

so this will input data from the bus and

store it in the instruction register and

that's also active low so that's III and

then normal that'll be high and actually

one last thing that I want to do which

is when we're for the RAM in when we're

writing data from the bus into Ram which

you'll notice if we set this high so

we're Ram in when the clock signal goes

high you see that this is all zeros and

that's because when the clock signal is

high we're writing for the entire

duration of the of the clock signal high

really what we want to do is write just

on the leading edge of the clock so

right now we're landing the clock and

the RAM in signal oops you can't quite

see that we're Nanding the clock and the

RAM in signal so this this white pin one

here is the clock and this is the RAM in

signal but really what we want to do is

we want to look at the ramming signal

and just the leading edge of the clock

and just at that moment where the clock

goes high not the entire time that it's

high we want to actually that's the

that's the moment that we want to read

data from the bus and

put it into into RAM here so we can do a

really simple edge detector circuit by

pulling this pin low and then instead of

inputting the clock directly into that

pin if we if we go through a capacitor

then what will happen is this will only

be high for the time it takes the

capacitor to charge through this

resistor right because the clock will go

high so we get five volts over here and

then of course we've got zero volts

we've got ground here and so this will

charge through the capacitor and while

it's charging this pin will be high but

as soon as the capacitor charges current

will stop flowing through the capacitor

and this will go low again so it will

only be high for the for sort of that

moment that it takes for the capacitor

charge so this is a this is a point zero

one micro farad capacitor and this is a

1k resistor so a 0.01 micro farad

capacitor and a 1k ohm resistor and so

if we want to know the time it takes for

this to charge we just multiply these

together but first we've got to get it

into into standard units so 0.01 micro

farad's in standard units which we

farad's would be zero point zero zero

zero zero zero zero zero one farad's

times and then 1 kilo ohm is one

thousand ohms and if we multiply these

two together basically we're just moving

the decimal point over three here so we

get zero point zero zero zero zero one

and then the answer will be in seconds

and that'll be the time that it takes

for the capacitor to charge and so in

other words at point zero zero zero one

seconds is going to be ten ten

microseconds so basically the the right

signal here that ends up going to the 74

LS 189 s are our actual ram chips in

this in this scenario it was only going

to be ten microseconds so it's just

going to be four for an instant it's

going to write

whatever is on the bus in that instant

when the clock goes high which is which

is exactly the moment that we want to

register that so we can kind of test

this out now right now we have this

value in the bus if we just change one

of these bits to something else we've

got our RAM in notes can't quite see

I've got a ram in signal high here so on

the next clock cycle it should write

that value in here we'll just change

this this this bit here to high and so

on that clock signal you see it goes

high it doesn't matter how long that

clock signal is it's it's just the first

10 microseconds of the clock that it's

using to clock in that new value and so

if we change this value on the bus new

clock doesn't matter how long that clock

signal is it's the first 10 microseconds

that clocks in that value so now we have

a little bit of an edge detector here as

well so so that pretty much should do it

this this here is our Ram module I think

we we've pretty much got everything

working at least the first initial tests

that we can do here so I think in the

next videos will either work on the

program counter which is a fairly simple

circuit or I think I'll do that first

and then and then I'll get into the the

actual output register which is which is

kind of interesting because we're going

to do a binary to decimal converter