BTW, what do you think of hardware mods?
I understand these mods won't be done by everybody, as some fine soldering skills are required - but it should provide a great deal of extra functionality.
So, I'd like to know your opinion, firmware-wise, is it worth it?
As for hardware - I could (or at least I could try to) implement this in my dm1801.
For now I'm exploring the following options:
1. To use MK22 built-in RTC.
As far as I can see in my dm1801, pins 28, 29 and 30 (XTAL32, EXTAL32 and VBAT raspectively) are left floating, as per datasheet requirement when unused.
The mod is to add a small 32K crystal, a bunch of capacitors, and something to power VBAT directly from the radio battery - i.e. a HT7133 voltage regulator with it's 5uA quiescent current. We should go well under 1mA in power-off mode after this mod.
For reference: now my dm1801 draws about 16mA in suspend mode, compared to average of 24mA in standby at Eco level 3 - which makes the suspend quite useless in terms of power saving.
2. GPS receiver integration.
As for the receiver itself, the only viable option I've found at the moment is sim33ela. It has an embedded antenna and it's pretty small, so I hope it will be easier to physically integrate it into the radio.
As for MCU connection, the bare minimum is only UART_RX pin (because software UART is not the thing you want to do). The combination of UART_RX, UART_TX and one GPIO to suspend the GPS should work for almost any possible config. The question is if there any unused UART pins are available.