Replaced the 1.0mm 6P SM06B GPS port of V2.52 with MOLEX-4P I2C external compass port, combined MOLEX-5P GPS port and external I2C port together making it easier to connect. The function of MOLEX-5P GPS port is as same as SM06B GPS port, making it redundant.
Multifunctional MUX port is used to replace I2C port of V2.52 which makes it possible for the user to deploy functional output on bonding pad (optional: UART0,UART2,I2C,OSD default: OSD). Compared with V2.52, when data transmission and OSD are used at the same time, the improved scheme makes data transmission more stable using an isolation resistance.
V2.5.2's main power filter circuit uses high-frequency component filter circuits which consist of multiple 0.1uF capacitors. There wasn't a large capacitance to filter the ripple wave of the low frequency component. The new APM improved the stability of the power supply from APM onboard chip by adding a 100uF capacitor.
The AT45DB161D on V2.52 is a 3.3V powered flashrom while the main control chip ATMEGA2560 is 5V powered. In order to make the data level compatible and protect the AT45DB161D from the high level impact which comes from the 5V 2560, we used a dual power supplied TXB0104 as a level switch. But due to the failed design, port A for the low level was incorrectly connected to the high level 2560 while port B was connected to the low level---positive and negative were reversed. According to the datasheet for TXB0104, the trace level range of port A is 1.2V-3.6V. 2560 with 5V level is beyond its limit. The trace level range of port B is 1.65-5.5V, but it is 5V powered and connected to the low voltage AT45DB161, obviously disadvantageous to AT45DB161. This error has been corrected for the new APM.
When deciding to use an external compass on V2.5.2, the bonding pad which is connected to a internal compass has to be cut. Returning to the internal compass requires shorting the bonding pad again ---this is very inconvenient. We use jumpers to replace the bonding pad on the new APM as an improvement : plug the MAG Pin with the jumper to use the internal compass, unplug it to use the external one.
APM2.7 Board *1
3pin 2.54mm pitch 15mm connection leads *5
Case and 3M double tape (optional)