Flashing DIY Frsky 8CH RX - AC810 / F801 … by David HK September 2016

Flashing DIY Frsky 8CH RX - AC810 / F801 … by David HK September 2016

I recently bought the Eachine Tiny QX80 without a receiver from China Taobao (long story short, Banggood does not delivery items with battery to Hong Kong, so I need to order from Taobao, but the QX80 in Taobao only comes with either DSM receiver or no receiver!), and originally I tried to buy the small Frsky compatible RX from Banggood QX90/QX80 spare parts, but it was in back order for at least 3 weeks, so I searched Taobao and came across this KS-Servo AC810 DIY Frsky RX.  This DIY RX is also being sold in HobbyKing, at around US$12-13 plus shipping.
Also selling in Banggood as "DIY FRSKY 8CH Receiver RX PPM Output For X9D(PLUS) XJT DJTDFT DHT" under US$12, quite a bargain!

I searched further and came across the below RCgroups discussion link on DIY Frsky TX/RX, which provides the firmware upgrade for this AC810/F801 (also AC820/F802) DIY Frsky RX.

So I bought this AC810 from Taobao for around US$10 plus shipping, and here it is, small size, weight 5g with all the pins, I later trimmed out the pins & part of the ‘L’ shape board and it weights less than 3g!

I am new to this quad hobby (less than 1 year), and just learned that to flash the RX, I need to buy something called USBASP programmer, and need to use 6 wires to connect some of the pins between the RX and the USBASP 10pins connector! Anyway, after 1 to 2 days searching and reading from the web, I managed successfully flashed the latest firmware from the above URL to the AC810 RX, so I want to share my experience to others who are new to this hobby.  Here are the steps:

1.       Prepare the USBASP hardware and wiring:

You can get the USBASP from Banggood, for less than US$4.  Note that it supports either 3.3V or 5V output, we need 3.3V for this flashing, important! 

The one I got is default to 5V, so I need to remove the R5 resistor at the back bottom of the USBASP.  Just use a hot iron to strip it away, like this, in the photo,

you can see the resistor is still attached at one end.  I used a multi-meter to check the VCC output pin just to make sure it is indeed 3.3V, otherwise 5V will fry the RX!  So this is important.

There is an excellent site in below URL that talks about how to use this 10pins USBASP to program AVR chip, from there you will find out which pins are the required 6 to use (note that the top red colour wire is the VCC, so this will give you some hints on the orientation of the 10 pins socket in the diagram)

http://www.learningaboutelectronics.com/Articles/Program-AVR-chip-using-a-USBASP-with-10-pin-cable.php

From above site, I learned the 6 required pins and their meaning, per below (extracted from the above site)

Pin Name	Description	Comment
MOSI	Master Out Slave In	This allows the master device (the USBASP) to send data to the slave device (target AVR being programmed).
MISO	Master In Slave Out	This allows the slave device (target AVR being programmed) to send information to the master device (the USBASP programmer).
SCK	Serial Clock	This is the mutual clock shared between the master and slave device for synchronized communication.
Reset	Target AVR MCU Reset	The reset pin for the AVR chip being programmed must be put in active low in order for programming to occur.
Vcc	Power	The master and slave device both need power in order to operate.
GND	Common Ground	The master and slave device must share a common power ground for operation.

To map these pins to the AC810 / F801 DIY Frsky RX, I learned it from below site:

https://nathan.vertile.com/blog/2016/02/09/airwolf-diy-frsky-rx-sbus/#flashing

So here is the complete mapping I wrote down to a piece of paper for easy reference while doing the wiring. (the AC801 F810 should be AC810 F801 :)

And here are the connected wires.  Note that for the reset pin, I did not solder it to the board; I just used my hand holding it to the reset point while doing the flashing like above site.

2.       Install driver

To use the USBASP, I need to install the USB driver (note that I am running win7), to do that, use zadig_2.2.exe (from below URL) which will install the driver while connected the USBASP and select usbasp driver to install.

http://zadig.akeo.ie/

for more details visit this link:  http://www.fischl.de/usbasp/

(note that I need 2 attempts to install the driver before my PC can recognize the USBASP device, I don’t know why!)

3.       Actual flashing

Get the latest RX firmware (by midelic) from below link, I used FrskyRx_F801_TELEMETRY_PFS_SBUS_240816.hex from below link:

http://www.rcgroups.com/forums/showpost.php?p=32959023&postcount=1147

I used the AVR Burn-O-Mat GUI to do the actual flashing, download from below link.  It requires Java runtime.

http://avr8-burn-o-mat.aaabbb.de/avr8_burn_o_mat_avrdude_gui_en.php

Steps:

1.       Click Settings -> AVRDUDE,

a.       set the avrdude.exe & avrdude.conf location (I’ve avrdude installed already when I installed the eepskye GUI for my Turnigy XR9 Pro tx radio)

b.      Programmer set to usbasp (http://www.fischl.de/usgasp/)

c.       Port set to your connected USB com port or just usb

2.       In main screen,

a.       set AVR type = ATmega328p

b.      under Flash, select the FrskyRx hex file that was downloaded from the RC group  (ie in my case, it is FrskyRx_F801_TELEMETRY_PFS_SBUS_240816.hex)

c.       under Flash, click the Write button - - this will do the actual flashing of the firmware!

It takes around 10 seconds, you will see Reading …. Writing …. Reading… where … are a few lines of notification of what it was doing, at the end you will see “avrdude.exe done. Thank you”.  Note that during the flash, you must touch the reset pin against the reset point in the RX board. 

There is a youtube video from SeByDocKy on how to flash this RX as well, but he was using a different GUI. 

After flashed the firmware, I get RSSI reading in my 9XR PRO radio immediately as I have setup a voice warning on RSSI! 

I will work on how to get voltage telemetry from the RX next (it requires soldering 2 resistors into the RX). [see below 20160918 update]

Trimming the RX pins and PCB:

Initially I just cut out the pins and soldered 3 wires into the bottom of the 'L' shape PCB, 

but later I saw some photos that people actually cut out the bottom part of the 'L' shape PCB, making this RX even smaller and lighter.  So this is exactly what I did :)

After the bottom 'L' part of the PCB is cut out, remember the 8 'S' pins are at the bottom, while on the top, you can see 9 pins instead of 8, all are Vcc+ pins except the middle pin, which is the GND.  You can use some clear nail polish as insulator to cover all the Vcc+ pins.

I installed the RX on top of the flight controller board of the QX80, this is what it looks like.

2060919 update:
Adding Voltage Telemetry:

I managed to get voltage telemetry from the RX. Here is the diagram I followed to create the voltage divider and soldered it to channel 8 pin. (diagram from the same "DIY Frsky TX/RX module" RCgroups, and thank you the brilliant work from midelic)

I am using the 10k / 3.3k resistors as advised. Here is my wiring, I grounded to the GND pin on the other side of the AC810.

In my 9XR Pro, I need to set the below screens in order to get the A1 voltage reading.

COM port 2, baudrate 56700

Also in telemetry screen, changed the 1st v value under A1 so that the reported voltage on the right is nearest to your lipo voltage (as measured by a multimeter)

Then you can set up a voice alarm as follows:

Here is a homepage screen that displays both the RSSI and flight voltage. Done! :)