tinyclock002

Note

Work in progress, mostly for my own learning and experimentation with WCH, may take some time to update.

A compact, simple alarm clock / time display board built around the CH32V002 (CH32V002A4M6, RV32EmC MCU), with a multiplexed LED grid driven through a 74HC595 chain and a DS1302 RTC for timekeeping.

The design includes headers for UART and I²C access, input through a linear potentiometer + push button (not onboard), a piezo buzzer, and a speaker driver/header using the PAM8302AAS Class-D amplifier (pretty loud for an alarm).

The goal is to keep the board small (50×50mm), simple, and educational: a low-cost 2-layer RISC-V MCU board to practice PCB layout + bare-metal firmware. The final target is to be able to play MOD tracker files as an alarm sound through the speaker output, see ModPlayRISCV.

Design files

Design files are in /hw/. The project is made in KiCad 9.

Prototype boards were fabricated by PCBWay using their standard 2-layer service (matte black solder mask). Assembly was done by me.

For a tour of the design, you can find a schematic PDF in that same folder.

Functionality

• The CH32V002 runs at 48 MHz, with 4 kB RAM and 16 kB flash. Simple architecture, very cost-effective for the speed.
• Multiplexed LED circular display using a 74HC595 (structured as 4×3) for showing time + simple animations while using only a few GPIOs.
• DS1302 RTC for timekeeping, with support for setting time through UART commands.
  • The board allows either a supercap (trickle-charged by the DS1302) or a CR2032 holder.
• UART + I²C headers for debugging and potential future expansion (not currently used in firmware).
• Piezo buzzer for simple alarm sounds.
• Speaker output driven by PAM8302AAS + filter network.
  • Intended for more complex audio (PCM-over-PWM using TIM1, eventually MOD tracker audio).
• Linear potentiometer + push button for user input (not currently used in firmware).
  • A jumper allows powering the pot either from VCC (constant small draw), or from a 74HC595-controlled line to reduce idle consumption.

Limitations

The CH32V002 is tight: 4 kB RAM / 16 kB flash is not a lot for “clock + alarm + sound playback”.

Right now the firmware uses ~3/4 of flash and a small amount of RAM. There isn’t much room left unless with heavy size optimization. It might simply not be possible to fit a MOD player while keeping all the clock/alarm features on the same MCU...

Current measurements

With the current demo:

• Running LEDMUX demo, no audio: 2.3 mA
• CH32V002 in standby mode: 520 µA
  (TODO: IWDG might still be enabled, this number matches the typical “IWDG standby current” range.)

With the PAM8302AAS speaker driver installed:

• Idle current: depends on the speaker, ~7 mA @ 5V with an 8 Ω speaker connected
• Driving the speaker through TIM1 PWM (50% duty): 450 mA @ 5V
  (Tracks with PAM8302AAS rated maximum of 2.5 W.)

These are still estimates for the moment, as the firmware is still in early development and not optimized for low power.

Notes

• PAM8302AAS /SD (shutdown) is tied to the AUDIO_MUTE jumper, pulling it up to VCC in normal operation, so the amp is always enabled.
  • This can be fixed with a bodge wire to a spare GPIO output (possibly using LTHRU), but it’s not a goal right now.
  • If aiming for minimal current draw, this should be fixed.

Images

Video demo: media/running-ledmux.mp4