求助，ble蓝牙模块最大传输单元为20字节怎么处理

采取了分包发送似乎也不行，监听返回的特征值一直显示错误指令

/**  
     * 向低功耗蓝牙设备特征值中写入二进制数据  
     * @param deviceId 蓝牙设备UUID  
     * @param serviceId 蓝牙特征值对应服务的 uuid  
     * @param characteristicId 蓝牙特征值的 uuid  
     * @param msg 写入的内容  
     */  
    function write(deviceId, serviceId, characteristicId, msg) {  
        console.log({ deviceId, serviceId, characteristicId, msg })  
        return new Promise((resolve, reject) => {  
            const buffer = hexStringToArrayBuffer(msg);  
            console.log(buffer)  
            let bytes = buffer.byteLength;  
            let pos = 0;  

            while (bytes > 0) {  
                let tmpBuffer;  
                if (bytes > 20) {  
                    tmpBuffer = buffer.slice(pos, pos + 20);  
                    pos += 20;  
                    bytes -= 20;  
                    uni.writeBLECharacteristicValue({  
                        deviceId,  
                        serviceId,  
                        characteristicId,  
                        value: tmpBuffer,  
                        success(res) {  
                            console.log('分包发送成功', res)  
                        },  
                        fail(err) {  
                            console.log('分包发送失败', err)  
                        }  
                    })  
                } else {  
                    tmpBuffer = buffer.slice(pos, pos + bytes);  
                    pos += bytes;  
                    bytes -= bytes;  
                    uni.writeBLECharacteristicValue({  
                        deviceId,  
                        serviceId,  
                        characteristicId,  
                        value: tmpBuffer,  
                        success(res) {  
                            console.log('write指令发送成功', res)  
                                                       uni.onBLECharacteristicValueChange((res) => {  
                                               console.log(res)  
                                               let resHex = ab2hex(res.value)  
                                               console.log(resHex)  
                                               console.log(hexCharCodeToStr(resHex))  
                                               })  
                            resolve(res);  
                        },  
                        fail(err) {  
                            console.log('write指令发送失败', err)  
                            reject(err);  
                        }  
                    })  
                }  
            }  
        })  
    }