How to connect Maple Serial1 pins (RX and TX) to a 9 pin COM port?

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How to connect Maple Serial1 pins (RX and TX) to a 9 pin COM port?

(16 posts) (3 voices)

Started 5 years ago by StephenFromNYC
Latest reply from gbulmer

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StephenFromNYC
Moderator

Monday October 11, 2010

Hello,

I want to use the Maple to collect ADC data at a fast rate. DMA may be the more elegant way to go (especially with the Maple Native, assuming the native will have High Speed USB), but my programming skills are not ready for that project. Still doing some background reading on USB endpoints.

Using SerialUSB.println() for high rates of data transfer is still causing me trouble, so I am curious about switching to Serial1.println().

My question is this, if I am using Maple Pins 7 and 8 for Serial1 transmit and receive, do I need a special adapter or gizmo to connect the Maple pins to the corresponding 9 pin serial COM port (pins 2 and 3 are receive and transmit on the 9 pin COM port) on my computer. Yes, I have a COM port on my system!

I am concerned about potential voltage problems. I do not want to fry my Maple.

If possible, I am also interested in having the CTS and RTS lines (and probably the CK line) connected between the Maple and my COM port for possible use.

If a voltage regulator of some type is required please let me know. Any specific model numbers for the adapter would also be appreciated.

Thanks!

Stephen from NYC

Posted 5 years ago #
leaflabsandy
Member

You need a 3.3 VDC TTL to USB converter. Also, you could use the old TTL to RS232 converters but the USB with mini connector is better.

There are really two good ones. One is from Sparkfun and the other from Modern Device. (Modern Device is closer to you and I used this one for flash downloading the bootloader)

Sparkfun link: http://www.sparkfun.com/commerce/product_info.php?products_id=10009

Modern Device link: http://www.moderndevice.com/products/usb-bub

On my Modern device 'BUB' I put two series resistors on the tx and rx lines for
current limiting. (300-330 ohm). So you need 3 lines, tx,rx and ground.
The BUB tx goes to Maple's rx and BUB rx to Maple's tx and ground to ground.
Make sure you set the 3.3VDC jumper on the BUB board!!!!

As for handshaking it might be a little more complex because both vendors have the signals for them but you need to do so hacking? Also, you probably need to provide the RTS & CTS signals from Comm x to the BUB board.

Also, ONLY Comm #2 and Comm #3 have hardware handshake lines! (RTS,CTS) - See GPIO docs.
The max baud on comm. 2 & 3 is 250K and on comm 1 450K <--------
If you really need the "high speed" comm #1 with handshaking you can use software NaK,ACK handshaking. You need to write your own driver.

Check with both vendors for the handshaking lines.

Note: Even though THe Maple's has hardware handshake lines for Comm #2 & #3 the question arises how do you enable them so the USART can use them? Leaflabs can help you on this.

Posted 5 years ago #
gbulmer
Moderator

StephenFromNYC - The max baud rates on the USARTS are 4.5Megabaud, and 2.5Megabaud, not 250k baud or 450k baud.

The COM port on your PC might not be able to match 2.5Mbaud; it is worth checking. Even if the port could do it, the OS might not be able to handle it.

Yes, you very would likely need a level translater if you want to talk to the COM port on your PC.

*Real* RS232 voltage limits are as big as +/-15V.
There are several companies which make parts for the job. Maxim is well known:
http://www.maxim-ic.com/datasheet/index.mvp/id/1798
Once you find where these are catalogued on your favourite electronics supplier, you'll likely find dozens of alternatives.

I belive that Maple Native is still an STM32F103/105, and hence is only a full-speed (12Mbits max) USB device. It is not a USB High-speed device.

FTDI say the FT232RL (the chip on the Sparkfun and modern devices boards) maximum baud rate is 3Megabaud:
http://www.ftdichip.com/Products/ICs/FT232R.htm

I don't know how fast the Maple transfers data (maybe I'll have a bit experiment later in the week), but it is theoretically capable of going faster than that.

A feasible option for a much higher transfer speed would be a USB High-speed adapter based on e.g. FT2232H

The spec says:

# USB to parallel FIFO transfer data rate up to 10Mbyte/sec (>80Megabaud)
# Single channel synchronous FIFO mode for transfers > 25 Mbytes/sec (>200Megabaud)

To talk to the FTDI USB chip at these rates, the Maple would be pushing a whole byte at a time into it (based on a brief skim of the datasheet).

How fast do you need to move data?

Posted 5 years ago #
leaflabsandy
Member

Quote: "StephenFromNYC - The max baud rates on the USARTS are 4.5Megabaud, and 2.5Megabaud, not 250k baud or 450k baud"

Reply: STMf130RB.PDF

Section 2.3.17

Quote: Universal synchronous/asynchronous receiver transmitter (USART)
One of the USART interfaces is able to communicate at speeds of up to 4.5 Mbit/s. The
other available interfaces communicate at up to 2.25 Mbit/s. <-----

Is there a difference between "Mega bits per second" and baud rate?

baud'' and`bps'' are perhaps one of the most misused terms in the computing and telecommunications field. Many people use these terms interchangeably, when in fact they are not! bps is simply the number of bits transmitted per second. The baud rate is a measure of how many times per second a signal changes (or could change).

http://www.sengpielaudio.com/calculator-transferrate.htm
http://web.forret.com/tools/bandwidth.asp?speed=4500000&unit=bps

4.5Mb/s = 4500000/10 = 450KBaud
2.25Mb/s = 2250000/10 = 225KBaud <--- whoops?

http://forums.leaflabs.com/topic.php?id=83

Posted 5 years ago #
gbulmer
Moderator

llandy - I was following the convention you adopted in your post

Also, ONLY Comm #2 and Comm #3 have hardware handshake lines! (RTS,CTS) - See GPIO docs.
The max baud on comm. 2 & 3 is 250K and on comm 1 450K <--------

Reply: STMf130RB.PDF

Section 2.3.17

Quote: Universal synchronous/asynchronous receiver transmitter (USART)
One of the USART interfaces is able to communicate at speeds of up to 4.5 Mbit/s. The
other available interfaces communicate at up to 2.25 Mbit/s. <-----

Yes, that's where I got the figures from.
I apologise for my transcription error, I wrote 2.5Megabaud, rather than 2.25Megabaud.
I was probably reading your post where you wrote

max baud on comm. 2 & 3 is 250K

2.5Megabaud and 4.5megabaud, are 10x faster than

max baud on comm. 2 & 3 is 250K and on comm 1 450K

which seemed to be the important point for StephenFromNYC, who is asking using a USART to move data at high speed.

It kinda doesn't matter whether we are using bits/second (as used by ST) or baud, because close enough in this particular case.

baud'' and`bps'' are perhaps one of the most misused terms in the computing and telecommunications field. Many people use these terms interchangeably, when in fact they are not!

Strongly agree.

Baud rate is not

The baud rate is a measure of how many times per second a signal changes (or could change).

So how would you calculate the baud rate for the STM32F 2.25Mbits and 4.5Mbits USARTs, and what values do you get?

Actually, I dont believe that is correct.
Wikipedia has the definition which I was taught, Baud:

In telecommunications and electronics, baud (pronounced /ˈbɔːd/, unit symbol "Bd") is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes (signaling events) made to the transmission medium per second in a digitally modulated signal

My emphasis on symbols.

The term baud rate has sometimes incorrectly been used to mean bit rate, since these rates are the same in old modems as well as in the simplest digital communication links using only one bit per symbol, such that binary "0" is represented by one symbol, and binary "1" by another symbol. In more advanced modems and data transmission techniques, a symbol may have more than two states, so it may represent more than one bit (a bit (binary digit) always represents one of exactly two states).

So the difference between Baud and bps matters a lot on a system which can have more than two symbols on the line, but when there are only two symbols, voltage high and low, as on a STM32F USART, I believe it is close enough to be treated as equivalent (actually, I was taught it still isn't, but that is looking at small details).

There are other details to consider, but I think the important points are:
1. Baud ~= Bits/second in this case of USARTs
2. 2.25Megabaud and 4.5Megabaud are close enough to correct, and 'The max baud on comm. 2 & 3 is 250K and on comm 1 450K' is 10x too small.

Posted 5 years ago #
gbulmer
Moderator

llandy

4.5Mbps = 4.5 megabits/second (Mbps) = 4,500,000 bits/second = 4,500,000 Baud = 4.5Mbaud
on a USART (a system with only two symbols/unit transission time)

Posted 5 years ago #
leaflabsandy
Member

Quote: "So how would you calculate the baud rate for the STM32F 2.25Mbits and 4.5Mbits USARTs, and what values do you get?"

Reply: Again, from above:
4.5Mb/s = 4500000/10 = 450KBaud <10 is the frame # of bits 8 N 1>
2.25Mb/s = 2250000/10 = 225KBaud

Quote: "There are other details to consider, but I think the important points are:
1. Baud ~= Bits/second in this case of USARTs
2. 2.25Megabaud and 4.5Megabaud are close enough to correct, and 'The max baud on comm. 2 & 3 is 250K and on comm 1 450K' is 10x too small."

Reply: Sorry, I totally disagree with 1 & 2

From the link: http://forums.leaflabs.com/topic.php?id=83

Perry H Quote: "4. You can set any rate up to the max of the uart. They aren't required to be fixed, the clock divisors are computed from the entered baud rate. You listed many common rates above. At the moment (as Bryan noted), they're limited to 225000, which is a bug that I didn't catch. There's a fix coming soon."

So Perry H inferred 225000 is the baud rate which is the same as 225 KBaud which is the same as 2.25 Mbit/s.

He goes on to say "Error checking is performed, you can't set a baud rate higher than the max baud rate."
5. USART1 tops out at 4.5Mbits/s = 4500000 baud
6. USART2 and USART3 are clocked from the slower APB1 bus, so they top out at 2.5Mbits/s = 2250000 baud.
--------------------------------------------------------------------------------------
The above 5 & 6 statements are wrong. He computed the baud rate wrong by 10

Edit: Note: Both of Perry's statements contradict each other.

Posted 5 years ago #
gbulmer
Moderator

10 is the frame # of bits 8 N 1

I exactly agree that the protocol frame (start, stop(s) and parity), are not part of the information transmission. That's what I was hinting at when I wrote

(actually, I was taught it still isn't, but that is looking at small details).

In this case, I read that as 8 data bits, no-parity, 1 stop bit (there is always 1 start bit), so for every 10 bits signalled on the 'transmission medium', there are 8 bits of data.

So, I am very happy to derate the 4.5Mbps to 4,500,000 *(8/10) = 3.6Mbaud
and similarly the 2.25mbps to 2Mbaud.

Posted 5 years ago #
gbulmer
Moderator

llandy - your edits crossed with my previous response again.

The above 5 & 6 statements are wrong. He computed the baud rate wrong by 10

No. No worse than by a factor of (8/10).
Baud is a measure of the number of distinct symbols changes on the line/second (4.5MHz = 4,500,000 symbol changes in the fastest case, 2.25MHz otherwise), and the number of symbols, which for a USART is 2 (it is not a multi-tone modem). I was taught to subtract the protocol framing (2 bits in your example) to avoid misleading folks.

So Perry H inferred 225000 is the baud rate which is the same as 225 KBaud which is the same as 2.25 Mbit/s.

Well, a baud rate of 225000 baud is 225KBaud (ignoring framing, which I think we agree can be excluded from the measurment of transmitted information), but I'd have to wait for Perry to explain what he was inferring, it is not the same as 2.25Mbit/s.

Posted 5 years ago #
leaflabsandy
Member

From HardwareSerial.cpp:
IDE 0.0.7

HardwareSerial Serial1(USART1, 4500000UL, GPIOA_BASE, 9, 10, TIMER1, 2);
HardwareSerial Serial2(USART2, 2250000UL, GPIOA_BASE, 2, 3, TIMER2, 3);
HardwareSerial Serial3(USART3, 2250000UL, GPIOB_BASE, 10, 11, 0, 0);
// TODO: High density device ports

HardwareSerial::HardwareSerial(uint8 usart_num,
uint32 max_baud,
GPIO_Port *gpio_port,
uint8 tx_pin,
uint8 rx_pin,
uint8 timer_num,
uint8 compare_num) {
this->usart_num = usart_num;
this->max_baud = max_baud;
this->gpio_port = gpio_port;
this->tx_pin = tx_pin;
this->rx_pin = rx_pin;
this->timer_num = timer_num;
this->compare_num = compare_num;

-----------------------------------------------------------------------
"Never believe a person for what they say but what they do"
-----------------------------------------------------------------------

It seems that in the above core code 4500000 & 2250000 are the bauds?

Gbulmer ... a new summary is needed.

Posted 5 years ago #
gbulmer
Moderator

Okay, so according to the code
usart_num=USART1, max_baud=4500000UL = 4,500,000 = 4.5Mbaud
usart_num=USART2, max_baud=2250000UL = 2,250,000 = 2.25Mbaud
usart_num=USART3, max_baud=2250000UL = 2,250,000 = 2.25Mbaud

not 450kbaud, or 250/225kbaud.
Agreed?

Posted 5 years ago #
leaflabsandy
Member

Code does not lie.

Posted 5 years ago #
gbulmer
Moderator

llandy - sorry, my post, and your edits crossed again.

I understand where they come from, those values are the underlying clock rates.

I'm not sure what the "new summary" needs to say. I think I wrote it in my first summary.

Posted 5 years ago #
gbulmer
Moderator

Okay, so you have read the code, and you are now agreeing with my intial Summary?

There are other details to consider, but I think the important points are:
1. Baud ~= Bits/second in this case of USARTs
2. 2.25Megabaud and 4.5Megabaud are close enough to correct, and 'The max baud on comm. 2 & 3 is 250K and on comm 1 450K' is 10x too small.

('Night all)

Posted 5 years ago #
leaflabsandy
Member

Summary is 100% correct.

BTW ... That is twice in 2 weeks that you got teflon llandy.

Posted 5 years ago #

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